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Q idioms

Wiki page converted to Markdown; some k sections removed

After Eugene McDonnell’s k idiom list for k2. See article by Simon Garland.

1. Ascending ordinals (ranking, shareable)

q)x:11 17 12 13 13 13 13 18
q)x[iasc x] ?/: x
0 6 1 2 2 2 2 7
q)x[iasc x]?x
0 6 1 2 2 2 2 7
q)asc[x]?x
0 6 1 2 2 2 2 7
/k)<x
/q)iasc x
/?/: find eachright

2. Max scan x partition y

q)x:1 1 0 0 0 1 0 0 1 1
q)y:3 4 8 2 5 6 9 4 5 4
q)raze maxs each (where x)_y
3 4 8 8 8 6 9 9 5 4

3. Min scan x partitions y

q)x:1 1 0 0 0 1 0 0 1 1
q)y:3 4 8 2 5 6 9 4 5 4
q)raze mins each (where x)_y
3 4 4 2 2 6 6 4 5 4

4. Are x and y permutations of each other

q)x:15 16 13 18 14 11 12
q)y:15 16 13 19 14 11 12
q)(asc x)~asc y
0b
q)y:15 16 13 14 18 12 11
q)(asc x)~asc y
1b

5. Sort subvectors ascending

5a. Sort ascending

q)sa:{asc x}
q)sa 30 10 40 20
`s#10 20 30 40
q)sa"popfly"
`s#"floppy"

5b. Indexes from lengths

q)x:10 30 20 50 60 40 5
q)y:4 3
q)il:{(count x)#sums 0,x}
q)il 4 3
0 4
q)raze sa each (il y) _ x
10 20 30 50 5 40 60

6. Subvector minima

q)x:1 1 0 0 0 1 1 0 0 1
q)y:3 4 8 2 5 6 9 4 5 4
q)min each (where x)_y
3 2 6 4 4

7. Subvector grade up

(See 15 for down.)

q)x:1 0 0 1 0 0 1 0
q)y:14 12 18 16 13 15 11 17
q){raze x +' iasc each x _ y}[where x; y]
1 0 2 4 5 3 6 7

8. Sort rows ascending

q)sa:{asc x}
q)x:(6 3 3 9 7;9 4 4 7 9;9 4 7 8 9)
q)sa each x
3 3 6 7 9
4 4 7 9 9
4 7 8 9 9

9.

See 8.

10. Replicate at depth

Add new dimension y-fold after dimension x of array z

10a. Depth

(n is depth at which to apply f)

q)d:{[n;f] $[n;d[n-1;f]';f]}
q)h:{d[x;y#,:]z}
q)z:2 5#"abcdefghij"
q)z
"abcde"
"fghij"
q)q:h[0;3;z]
"abcde" "fghij"
"abcde" "fghij"
"abcde" "fghij"
q)r:h[1;3;z]
q)r
"abcde" "abcde" "abcde"
"fghij" "fghij" "fghij"
q)s:h[2;3;z]
q)s
"aaa" "bbb" "ccc" "ddd" "eee"
"fff" "ggg" "hhh" "iii" "jjj"

Is application at depth not given by the syntax of Apply?

11. Mesh

Merge x, y, z, under control of g.

q)x:"abcd"
q)y:"123456789"
q)z:"zz"
q)g:1 0 1 1 2 1 2 1 1 0 1 0 1 0 1
q)(x,y,z)rank g
"1a23z4z56b7c8d9"
q)k)(x,y,z)[<<g]
"1a23z4z56b7c8d9"

12.

See 11.

13. Ascending ordinals

(ranking, all different)

q)x:15 16 13 18 15 12 13
q)rank x
3 5 1 6 4 0 2

14. Subvector maxima

q)x:1 1 0 0 0 1 1 0 0 1
q)y:3 4 8 2 5 6 9 4 5 4
q)max each (where x) _ y
3 8 6 9 4

15. Subvector grade down

See 7 for up.

q)x:1 0 0 1 0 0 1 0
q)y:14 12 18 15 13 16 11 17
q){raze x +' idesc each x _ y}[where x;y]
2 0 1 5 3 4 7 6

16. Merge x and y by g

q)x:5 9 8 7 4 3
q)y:10 20 30 40
q)g:1 0 1 1 0 0 1 0 1 1
q)(x,y)[iasc idesc g]
5 10 9 8 20 30 7 40 4 3

17. Descending ordinals

Ranking, all different.

q)x:15 16 13 18 14 11 12
q)iasc idesc x
2 1 4 0 3 6 5

18. Sort ascending by internal alphabet

q)w:("once";"more";"into")
q)w[iasc `$w]
"into"
"more"
"once"
q)string asc `$w
"into"
"more"
"once"

19. Sort character matrix ascending

q)m:("scion";"icons";"coins")
q)m[iasc m]
"coins"
"icons"
"scion"

20. Is x a permutation?

q)x:4 0 2 1 5 3 6
q)x~rank x
1b
q)x:4 3 3 6 0 5 4
q)x~rank x
0b

21. Rotate infixes of y determined by boolean x to the left one place

q)y:"abcdefghij"
q)x:1 0 1 0 0 1 1 0 0 0
q)y[iasc x + sums x]
"badecfhijg"

22. Index of first occurrence of minimum of x

q)show x:5+13?100
14 12 10 9 6 12 6 11 8 12 6 13 6
q)first iasc x
4
q)x?min x
4

23. Index of first occurrence of maximum of x

q)show x:5+13?100
11 10 8 8 8 7 5 9 12 12 10 12 8
q)x?max x
8
q)first idesc x
8

24. Median

q)show x:10?100
61 20 51 12 31 51 29 35 17 89
q).5*sum over x[(iasc x) (neg floor t;floor neg t:.5*1-count x)]
33f

25. Doubling quotes

q)f:{ssr[x;"\"";"\"\""]}
q)a:"Did he say, \"Hello\"?"
q)f a
"Did he say, \"\"Hello\"\"?"
q)ssr[a;"\"";2#] /alternative solution emphasizing doubling
"Did he say, \"\"Hello\"\"?"

26. Insert y "*" after "=" in x

q)x:"abc=,d=,fgh="
q)show g:where x="="
3 6 11
q)y:5
q)(x,"*")[(count x)&iasc (til count x),(y*count g)#g]
"abc=*****,d=*****,fgh=*****"

27. Insert 0 after indexes y of x

q)x:"abc,def,gh"
q)show y:(where x=","),#x
3 7 10
q)(count x)>(iasc (til count x),y)
1111011110110b

28. Insert g elements h after indexes y of x

q)x:"abcd=,def=,gh="
q)show y:where x="="
4 9 13
q)g:4
q)h:"x"
q)show a:g*count y
12
q)(x,a#h)[iasc (til count x),a#y]
"abcd=xxxx,def=xxxx,gh=xxxx"

29. Insert g elements h before indexes y of x

q)x:"1234,234,34"
q)y:0 5 9
q)g:5
q)h:"*"
q)a:g*count y
q)((a#h),x)[iasc (a#y),til count x]
"*****1234,*****234,*****34"

30. Grade up x according to key y

q)x:"fig lime"
q)y:" abcdefghijklmn"
q)y?/:x
6 9 7 0 12 9 13 5
q)iasc y?/:x
3 7 0 2 1 5 4 6
q)x[iasc y?/:x]
" efgiilm"

31. Merge

q)x:"egg"
q)y:"mrin"
q)g:1 0 1 0 1 1 0
q)(x,y)[rank g]
"merging"

32. Sort ascending indexes x according to data y

q)x:2 3 4 5 0 1 8 7 6
q)y:79 74 78 76 77 75 73 72 71
q)x[iasc y[x]]
8 7 6 1 5 3 4 2 0

33. Sort matrix x on column y

q)show x:5 6#30?100
37 41 41 72 60 0
91 59 5  19 17 26
24 90 28 63 42 56
75 67 45 14 38 49
85 11 23 61 64 44
q)y:2
q)x[;y]
41 5 28 45 23
q)iasc x[;y]
1 4 2 0 3
q)x[iasc x[;y]]
91 59 5  19 17 26
85 11 23 61 64 44
24 90 28 63 42 56
37 41 41 72 60 0
75 67 45 14 38 49

34. Choose grading direction

q)show x:10?100
30 45 97 77 35 49 87 82 79 8
q)y:1
q)iasc x*1 -1[y]
2 6 7 8 3 5 1 4 0 9
q)y:0
q)iasc x*1 -1[y]
9 0 4 1 5 3 8 7 6 2
q)iasc x*1 -1[1]
2 6 7 8 3 5 1 4 0 9
q)iasc x*1 -1[0]
9 0 4 1 5 3 8 7 6 2
q)parse "x*1 -1[1]"
*
`x
(1 -1;1)
q)parse "iasc x*1 -1[0]"
k){$[0h>@x;'`rank;<x]}
(*;`x;(1 -1;0))

/1 -1[0] is 1 and 1 -1[1] is -1 or (1;-1)[0] and (1;-1)[1].

35. Sort ascending

q)show x:10?100
84 63 31 42 95 58 9 37 84 39
q)iasc x
6 2 7 9 3 5 1 0 8 4
q)x[iasc x]
9 31 37 39 42 58 63 84 84 95
q)asc x
`s#9 31 37 39 42 58 63 84 84 95

36. Sort y on x

q)show x:6?10
q)x
9 2 3 1 9 3
q)show y:6?20
7 8 17 11 16 6
q)y[iasc x]
11 8 17 6 7 16

37. Invert permutation (the inverse puts y in ascending order)

q)show x:-7?7
q)x
5 3 2 0 6 4 1
q)iasc x
3 6 2 1 5 0 4
q)x[iasc x]     / check
0 1 2 3 4 5 6

38. Sort matrix descending

q)x:5 6#30?3
q)x
0 0 0 1 0 1
1 0 2 2 0 1
0 2 1 1 1 1
0 0 0 1 1 2
1 2 2 0 2 1
q)x[iasc x]
0 0 0 1 0 1
0 0 0 1 1 2
0 2 1 1 1 1
1 0 2 2 0 1
1 2 2 0 2 1
q)show y:"abcde"[5 6#30?5]
q)y
"dcdbed"
"aaaaab"
"dcdbdc"
"baaace"
"eedbec"
   y[iasc y]
"aaaaab"
"baaace"
"dcdbdc"
"dcdbed"
"eedbec"

39. Reverse infixes in x of lengths y

q)il:{(count x)#sums 0,x}
q)show x:11+til 8
q)y:3 3 2
q)x[reverse idesc sums(til count x)in il y]
13 12 11 16 15 14 18 17

40. Reverse infixes in x starting at indexes y

q)x:1 0 1 0 0 1 0 0 0 1
q)y:1 2 3 4 5 6 7 8 9 10
q)+\x
1 1 2 2 2 3 3 3 3 4
q)idesc+\x
9 5 6 7 8 2 3 4 0 1
q)reverse idesc+\x
1 0 4 3 2 8 7 6 5 9
q)y[reverse idesc sums x]
2 1 5 4 3 9 8 7 6 10

41. Indexes of 1s in boolean vector x

q)x:0 0 1 0 1 0 0 0 1 0
q)where x
2 4 8

42. Move all blanks to end of text

q)x:" sign if i cant "
q)x[iasc " "=x]
"significant     "

43. Move elements of x with characteristic y to beginning

q)x:"mjinase"
q)y:0 1 0 0 1 1 0
q)x[idesc y]
"jasmine"

44. Sort descending

q)show x:8?10
3 5 0 4 5 2 4 5
q)x[idesc x]
5 5 5 4 4 3 2 0
q)desc x
5 5 5 4 4 3 2 0

45. Binary representation of positive integer

q)2 vs 16
1 0 0 0 0
q)2 vs 20
1 0 1 0 0
q)2 sv 1 0 0 0 0
16
q)0b vs 16
0000000000000000000000000000000000000000000000000000000000010000b
q)0b vs 16h
0000000000010000b
q)0x0 vs 16
0x00000010
q)0x0 vs 16h
0x0010

46. Transposed formatted integers 1 through x

q)show q:10 vs 1+til 15
0 0 0 0 0 0 0 0 0 1 1 1 1 1 1
1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
q)"0123456789"[q]
"000000000111111"
"123456789012345"

47. Polynomial with roots x

q)x:1 -6 8
q){(x,0)-y*0,x} over 1,x
1 -3 -46 48

q)x:2 4
q){(x,0)-y*0,x} over 1,x
1 -6 8
q)x:1 2
q){(x,0)-y*0,x} over 1,x
1 -3 2
q)x:1 2 3
q){(x,0)-y*0,x} over 1,x
1 -6 11 -6

48. Saddle-point indexes

q)x
4 2 4 4 2 4
5 3 5 5 3 5
4 2 4 4 2 4
1 2 4 4 2 4
5 3 5 5 3 5
4 2 4 4 2 4

48a. Row minimum

q)rn:{x=' min each x}
q)rn x
010010b
010010b
010010b
100000b
010010b
010010b

48b. Column maximum

q)cx:{x=\:max x}
q)cx x
000000b
111111b
000000b
000000b
111111b
000000b

48c. Minmax of rows and columns

q)minmax:{(rn x)&(cx x)}
q)minmax x
000000b
010010b
000000b
000000b
010010b
000000b

48d. Locate 1s in ravel of Boolean matrix

q)ones:{where raze minmax x}
q)ones x
7 10 25 28

48e. Saddle-point indexes

q)rc:{(div;mod).\:(y;count first x)}   / row-col indexes of raze[x][y]
q)sp:{rc[x;]ones x}
q)sp x
1 1 4 4
1 4 1 4

49. Hexadecimal from decimal

q)hex:"0123456789abcdef"
q)hd:{hex[16 vs/:x]}
q)hd 10
,"a"
q)hd 20
"14"
q)x:10 12 19 1 28 100
q)hd x
,"a"
,"c"
"13"
,"1"
"1c"
"64"
q)y:10 12 19 1 28 300
q)hd y
,"a"
,"c"
"13"
,"1"
"1c"
"12c"

50. Connectivity list from connectivity matrix

q)show m:(1 0 1;1 0 1)
1 0 1
1 0 1
q)raze m
1 0 1 1 0 1
q)where raze m
0 2 3 5
q)rc[m;] where raze m
0 0 1 1
0 2 0 2
q)lm:{rc[x;]where raze x}
q)lm m
0 0 1 1
0 2 0 2

51. Indexes

The depth of a list is the number of nesting levels at which it is rectangular. Its shape is a vector of its count at each level at which it is rectangular, and corresponds to the left argument of Take.

q)depth:{$[type[x]<0; 0; "j"$sum(and)scan{1=count distinct count each x}each(raze\)x]}
q)shape:{$[0=d:depth x; 0#0j; d#{first(raze/)x}each(d{each[x;]}\count)@\:x]}
q)ix:('[{x vs til prd x};shape])
q)ix til 6
0 1 2 3 4 5
q)ix 2 3#til 6
0 0 0 1 1 1
0 1 2 0 1 2
q)ix 2 3 2#til 12
0 0 0 0 0 0 1 1 1 1 1 1
0 0 1 1 2 2 0 0 1 1 2 2
0 1 0 1 0 1 0 1 0 1 0 1

52. Truth table of order x

q)tt:{2 vs til "j"$2 xexp x}
q)tt 1
0 1
q)tt 2
0 0 1 1
0 1 0 1
q)tt 3
0 0 0 0 1 1 1 1
0 0 1 1 0 0 1 1
0 1 0 1 0 1 0 1

53. Decimal digits from integer

q)dd:10 vs
q)dd 2001
2 0 0 1
q)dd 123456789
1 2 3 4 5 6 7 8 9
/not quite the same
q){("i"$string x)-48} 2001  /C. Skelton
2 0 0 1
q){("i"$string x)-48} 1234567890
1 2 3 4 5 6 7 8 9 0

54. Represent y in base x

q)16 vs 256
1 0 0
q)2 vs 36
1 0 0 1 0 0
q)10 vs 123
1 2 3
q)0x0 vs 256
0x00000100
q)0b vs 36
00000000000000000000000000100100b
q)0b vs 36h
0000000000100100b

55. Indexes in x containing items in y

q)x:(3 7 8;2 5 9)
q)y:(3 1 6;8 9 2)
q)shape[x] vs where raze[x] in raze y
0 0 1 1
0 2 0 2

56. Hexadecimal from decimal characters

q)hex:"0123456789abcdef"
q)x:" abcdef"
q)"i"$x
32 97 98 99 100 101 102
q)16 vs "i"$x
2 6 6 6 6 6 6
0 1 2 3 4 5 6
q)hex 16 vs "i"$x
"2666666"
"0123456"
q)flip hex 16 vs "i"$x
"20"
"61"
"62"
"63"
"64"
"65"
"66"
q)" ",'flip hex 16 vs "i"$x
" 20"
" 61"
" 62"
" 63"
" 64"
" 65"
" 66"
q)raze" ",'flip hex 16 vs "i"$x
" 20 61 62 63 64 65 66"
q)16 vs/: "i"$x
2 0
6 1
6 2
6 3
6 4
6 5
6 6
q)hex 16 vs/: "i"$x
"20"
"61"
"62"
"63"
"64"
"65"
"66"
q)" ",'hex 16 vs/: "i"$x
" 20"
" 61"
" 62"
" 63"
" 64"
" 65"
" 66"
q)raze " ",'hex 16 vs/: "i"$x
" 20 61 62 63 64 65 66"
q)x:"GOLDEN"
q)raze" ",'flip hex 16 vs "i"$x
"47 4f 4c 44 45 4e"
q)raze " ",'hex 16 vs/: "i"$x
" 47 4f 4c 44 45 4e"

57. Vector from date

q)100000 100 100 vs 19980522
1998 5 22
/not quite the same as above
q)dt:gtime .z.Z
q)(dt.year;dt.mm;dt.dd)
2007 7 21

58. Pair each element of !x with each element of !y

q)x:3
q)y:4
q)(x,y) vs til x*y
0 0 0 0 1 1 1 1 2 2 2 2
0 1 2 3 0 1 2 3 0 1 2 3
q)flip til[x] cross til y
0 0 0 0 1 1 1 1 2 2 2 2
0 1 2 3 0 1 2 3 0 1 2 3

59. Truth table of order x

See 52.

60. Truth table of order x

See 52.

61. Cyclic counter, repeating 1 through n

q)x:1+til 10
q)y:8
q)1+x mod y
2 3 4 5 6 7 8 1 2 3

62. Integer and fractional parts of positive x

q)x:12.3 23.4 5.33 8.999
q)iif:{(floor x),'x-floor x}
q)iif x
12 0.3
23 0.4
5  0.33
8  0.999

iff is an iffy name.

63. Represent x in radix 10 100 1000

q)10 100 1000 _vs 123456
1 23 456
q) 10 100 1000 _vs 123456789
4 56 789

64. Represent integer time hhmmss as character time, items separated by colons

64a. Local time as integer of form hhmmss

q)ltime .z.p
2019.01.01D16:02:05.763168000
q)"i"$"v"$ltime .z.p
57739i
q)100 sv 60 vs "i"$"v"$ltime .z.p
160224

65. Represent integer date in form yyyymmdd as character date, parts separated by "/"

q)1000 sv 10000 100 100 vs 20190101
2019001001
q)string 1000 sv 10000 100 100 vs 20190101
"2019001001"
q){@[x;4 7;:;"/"]}string 1000 sv 10000 100 100 vs 20190101
"2019/01/01"
/not quite the same as above
q)@[s;(s:string "d"$.z.Z)ss".";:;"/"]
"2007/07/21"
q)s
"2007.07.21"

66. Selection by encoded list

q)"abcdefgh"[2 sv 1 0 1]
"f"
q)"abcdefgh"[2 _sv 0 0 0]
"a"
q)"abcdefgh"[2 _sv 1 1 1]
"h"

67. Extrapolated value of abscissa x and ordinate y at g

q)x:-1 0 1
q)y:1 0 1.0 / y ~ x^2
q)g:10
q)reverse key count x
2 1 0
q)x xexp/: reverse key count x
1  0 1
-1 0 1
1  1 1
q)(enlist y) lsq x xexp/: reverse key count x
1 0 4.440892e-16
q)g sv raze(enlist y) lsq x xexp/: reverse key count x
100f
q)g:5
q)g sv raze(enlist y) lsq x xexp/: reverse key count x
25f

68. Not relevant

69. Value of ascending polynomial coefficients y at points x

q)x:-1 0 2
q)y: 4 0 5 1
q)x sv\:y
-8 1 43

70. Remove duplicate rows

q)x:("to";"be";"or";"not";"to";"be")
q)distinct x
"to"
"be"
"or"
"not"

71. Connectivity matrix from connectivity list

q)y:(0 1;0 2;1 0;1 2;2 2)
q)x:3
q)x sv/:y
1 2 3 5 8
q)(til 9)in x sv/:y
011101001b

72. Encode date as integer

q)s:string "d"$.z.Z
q)s
"2007.07.21"
q)s[where not s="."]
"20070721"
q)"i"$s[where not s="."]
50 48 48 55 48 55 50 49
q)"I"$s[where not s="."]
20070721

73. Remove trailing blanks

q)x:"trailing blanks    "
q)nctb:{neg sum mins reverse " "=x} /negative count of trailing blanks
q)nctb x
-4
q)rtb:{(nctb x)_x}
q)rtb x
"trailing blanks"

74. Number of days in month x of Gregorian year y (ly from 463)

q)ly:{(sum 0=x mod/:4 100 400)mod 2}
q)x:1
q)$[2=x;28+ly y;(0,12#7#31 30)[x]]
31
q)x:2
q)$[2=x;28+ly y;(0,12#7#31 30)[x]]
29

75. Decimal from hexadecimal

q)x:("ff";"a9";"8ac";"ffff")
q)x
("ff"
"a9"
"8ac"
"ffff")
q)"0123456789abcdef"?/:"ff"
15 15
q)"0123456789abcdef"?/:/:x
15 15
10 9
8 10 12
15 15 15 15
q)16 sv/: "0123456789abcdef"?/:/:x
255 169 2220 65535
/not quite the same
q)16 sv'hex?/:x
255 169 2220 65535

76. Justify right

q)nctb:{neg sum mins reverse " "=x}  / negative count of trailing blanks
q)rj:{(nctb x)rotate x}
q)x:"trailing blanks    "
q)rj x
"    trailing blanks"
q)x:("a ";"bc ";"d e ";"fg h ";"ij kl ";"mnopqr")
q)rj'x
''
q)rj'[x]
" a"
" bc"
" d e"
" fg h"
" ij kl"
"mnopqr"

77. Present value of cash flows c at times t and discount rate d

Example: a 3-year bond with an annual 10% coupon and discount rate of 0.9

q)c:0.1 0.1 1.1
q)t:1 2 3
q)d:0.9
q){[c;t;d]sum c*d xexp t}
q)pv[c;t;d]
0.9729

78. Number from alphanumeric

q)x:"1998 51"
q)parse x
1998 51
q)3+parse x
2001 54

79. Index (1-origin) of first non-blank, counting from rear

q)x:"blanks at end   "
q)x=" "
0000001001000111b
q)1+reverse[x=" "]?0b
4

Historical note

Ancestral languages APL and k support the boolean vector as left argument (encoding system) of what in q is sv. Reversing the boolean vector could thus be omitted. sv does not support that.

80. Scattered indexing

q)show x:2 3 4#.Q.a
"abcd" "efgh" "ijkl"
"mnop" "qrst" "uvwx"
q)x ./:(0 0 0;1 1 3;1 2 2)
"atw"

81. Raveled index from general index

q)x:2 3 4#.Q.a
q)x[1;1;3]
"t"
q)shape x
2 3 4
q)2 3 4 sv 1 1 3
19
q)raze/[x]
"abcdefghijklmnopqrstuvwx"
q)raze/[x] 19
"t"

82. Future value of cash flows x at interest rate y

q)x:10 15 20 25
q)y:5
q)sum x*xexp[1+y%100;reverse til count x]
74.11375
q)sum x*(1+y%100)xexp(reverse til count x)
74.11375
q)fv:{sum x*(1+y%100)xexp(reverse til count x)}
q)fv[x;y]
74.11375

84. Scalar from boolean vector

q)2 sv 1 0 0 1 1 1 0 1
157

85. Is matrix x antisymmetric?

q)show x:(0 -7 1; 7 0 -4; -1 4 0)
0  -7 1
7  0  -4
-1 4  0
q)x~neg flip x
1b

86. Is matrix x symmetric?

q)show x:(0 4 7 1; 4 8 6 4; 7 6 2 0; 1 4 0 6)
0 4 7 1
4 8 6 4
7 6 2 0
1 4 0 6
q)x~flip x
1b

q)show x:4 4#16?10
6 6 3 3
9 7 9 4
4 7 9 9
4 7 8 9
q)x~flip x
0b

87. Number of decimals (maximum 7)

q)nd:{$[1=count x;0;-2+count x]}
q)ff:{string x-floor x}
q)ff 6.567
"0.567"
q)nd ff 1.234
3
q)nd ff 1234
0
q)nd ff 78.1234567
7
q)nd ff 78.12345678
7

88. Name variable according to x

q)x:"test"
q)y:2 3#til 6
q)eval parse "var",x,":y"
0 1 2
3 4 5
q)x:123
q)eval parse "var",string[x],":y"
0 1 2
3 4 5

93. Numbers from alphanumeric matrix

q)show x:4 3#" 1 12 0.5"
" 1 "
"12 "
"0.5"
" 1 "
q)x=" "
110b
100b
111b
000b
q)min each x=" "
0010b
q)
q)where min each x=" "
,2
q)z:.:'x
''
q)z:parse x
'type
q)z:parse each x
q)z
1
12
::
0.5
q)@[z;,2;:;0]
',
q)@[z;enlist 2;:;0]
1
12
0
0.5
q)@[parse each x;where min each x=" ";:;0]
1
12
0
0.5

But there may be no all-blank rows

q)show y:4 3#"  1 123450.5"
"  1"
" 12"
"345"
"0.5"
q)@[parse each y;where min each y=" ";:;0]
1
12
345
0.5
q)na:{@[parse each x;where min each x=" ";:;0]}
q)na y
1
12
345
0.5

94. Number from alphanumeric x, default y

q)x:""
q)y:"-1"
   .((x~"")#"y"),x
"-1"
q)x:"234.5"
   .((x~"")#"y"),x
234.5
q)na:{[x;y]$[x~"";parse y;parse x]}
q)na["";"-1"]
-1
q)na["123";"-1"]
123

95. Numeric from proper alphanumeric non-negative integer

q)x:"123 438"
q)eval parse each  " "vs x
123 438

96. Conditional execution

q)@[til; 6; :]
0 1 2 3 4 5
q)@[til; "a"; :]
:["type"]

98. Execute rows of character matrix

q)x1:4
q)x2:9
q)show x:2 5#"y1:x1y2:x2"
"y1:x1"
"y2:x2"
q)parse each x
: `y1 `x1
: `y2 `x2
q)('[eval;parse])each x
4 9
q)(y1;y2)
4 9

99. Numeric vector from evaluating rows of character matrix

q)show x:2 5#"1+2 41+3 6"
"1+2 4"
"1+3 6"
q)parse each x
+ 1 2 4
+ 1 3 6
q)('[eval;parse])each x
3 5
4 7
q)raze('[eval;parse])each x
3 5 4 7

100. Indexing arbitrary rank array

q)x:2 3 4 5#til 120
q)x[1]
60 61 62 63 64      65 66 67 68 69      70 71 72 73 74      75 76 77 78 79
80 81 82 83 84      85 86 87 88 89      90 91 92 93 94      95 96 97 98 99
100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119
q)x . enlist 1
60 61 62 63 64      65 66 67 68 69      70 71 72 73 74      75 76 77 78 79
80 81 82 83 84      85 86 87 88 89      90 91 92 93 94      95 96 97 98 99
100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119
q)x . 0 0
0  1  2  3  4
5  6  7  8  9
10 11 12 13 14
15 16 17 18 19
q)x . 0 0 0
0 1 2 3 4
q)x . 0 0 0 0
0
q)x[0;0;0;0]
0

101. Sum numbers in character matrix

q)show x:string til 5
,"0"
,"1"
,"2"
,"3"
,"4"
q)sum parse each x
10

104. Date in ascending format

q){x 9 10 0 6 7 0 1 2 3 4}"/",string"d"$.z.p
"01/01/2019"
q)"/"sv reverse 0 4 6_ except[;"."]string"d"$.z.p
"01/01/2019"

Not quite the same as above:

q)show dt:.z.p
2019.01.01D22:31:55.416124000
q)"."sv string (dt.dd;dt.mm; dt.year)
"1.1.2019"
q)raze string each(dt.year;".";dt.mm;".";dt.dd)
"2019.1.1"

105. Current time of 12-hour clock (am & pm)

q)a:01:58:57
q)p:13:59:59
q)prd 12 60 60  / seconds in 12 hours
43200
q)hm:{p:x>11:59:59; string[x-43200*p]," ","AP"[p],"M"}
q)hm a
"01:58:57 AM"
q)hm p
"01:59:59 PM"

106. Leading zeros for positive integers x in field width y

q)y:3
q)show x:10?40
37 36 17 38 29 4 31 12 35 25
q)z:string x+(y-1){x*10}/10
q)z
"1037"
"1036"
"1017"
"1038"
"1029"
"1004"
"1031"
"1012"
"1035"
"1025"
u:1_'z
q)u
"037"
"036"
"017"
"038"
"029"
"004"
"031"
"012"
"035"
"025"

107. Current date, American format

q)dt:2018.09.15
q)raze string (dt.mm;"/";dt.dd;"/";dt.year)
"9/15/2018"

The dot notation above does not work with local variables, i.e. within functions. The following does.

q){"/"sv string parse each 1 rotate"."vs string x}dt
"9/15/2018"

111. Count of format of x

q)cf:{count string x}
q)cf 12.345
6
q)cf -1
2
q)cf 1e-12
5
q)string 1e-12
"1e-12"

115, 116, 117. Case structure

$[c0;t0;f]
$[c0;t0;c1;t1;f]
$[c0;t0;c1;t1;c2;t2;f]
$[c0;t0;c1;t1;c2;t2;c3;t3;f]

Et cetera. In the first case, if c0 is nonzero, the result is t0; otherwise f. In all cases, the result is the t corresponding to the first non-zero c. If all the c are zero the result is f.

== DROP Again, not an idiom, just how Cond works. ==

121. Y-shaped array of numbers from x[0] to x[1]-1

q)x:4 9
q)y:3 4
q)first[x]+y#prd[y]?neg(-/)x
5 8 7 4
8 7 5 8
8 7 7 5

122. Y objects selected with replacement from !x (Roll)

q)y:3 5
q)x:7
q)y?x
6 2 1 2 2
4 4 6 3 0
6 3 4 5 1
q)3 5#7?7
5 1 2 1 2
0 3 5 1 2
1 2 0 3 5

== DROP Not an idiom; just how Roll works ==

123. Y objects selected without replacement from !x (Deal)

q)y:2 3
q)x:7
q)y#prd[y]?-x
1 6 4
3 5 2
q)6?-6
3 0 1 5 4 2

== DROP Not an idiom; just how Deal works ==

123.1 Normal deviates (from interval (0,1))

q)\P 4
q)4?1.
0.7418 0.007865 0.4953 0.1869

== DROP Not an idiom; just how Roll works. Or explain relation to “normal deviates” ==

124. Predicted values of exponential fit

q)x:64 70 77 82 92
q)y:56 60 66 70 78
q)a:x xexp/:0 1
q)a
1  1  1  1  1
64 70 77 82 92
q)log y
4.025352 4.094345 4.189655 4.248495 4.356709
q)lsq[enlist log y;a]
3.261029 0.01197249
q)
q)exp[flip mmu[flip a;flip lsq[enlist log y;a]]]
56.10745 60.28622 65.55641 69.60062 78.45289

125. Predicted values of best linear fit (least squares)

q)x:64 70 77 82 92 107 125 143 165 189f
q)y:56 60 66 70 78  88 102 118 136 155f
q)a:x xexp/:0 1
q)flip mmu[flip a;flip lsq[enlist y;a]]
55.32371 60.08021 65.62945 69.59319 77.52068 89.41191 103.6814 117.9509 135.3913 154.4173
q)flip (flip a) flip mmu (enlist y) lsq a
55.32371 60.08021 65.62945 69.59319 77.52068 89.41191 103.6814 117.9509 135.3913 154.4173

126. G-degree polynomial fit of points (x,y)

q)g:3
q)x:64 70 77 82 92 107 125 143 165 189
q)show y:(5*x xexp 3)+(-1*x xexp 2)+(4*x)+182
1307062 1710562 2277226 2750626 3885526 6114376 9750682 1.460134e+07 2.243424..
q)reverse raze enlist[y] lsq x xexp/:til g+1
5 -1 4 182f

127. Coefficients of exponential fit of points (x,y)

q)x:64 70 77 82 92 107 125 143 165 189
q)y:56 60 66 70 78 88 102 118 136 155
q)a:lsq[enlist log y;x xexp/:0 1]
q)a
3.563398 0.00817742
q)a[0]:exp[a[0]]
q)a
35.2829 0.00817742
q)a[0]*exp[x*a[1]]
59.54624 62.54071 66.22511 68.98898 74.86758 84.63791 98.05964 113.6098 136.0..
q)y
56 60 66 70 78 88 102 118 136 155

128. Coefficients of best linear fit of points (x,y) (least squares)

q)x:64 70 77 82 92 107 125 143 165 189f
q)y:56 60 66 70 78  88 102 118 136 155f
q)lsq[enlist y;x xexp/:0 1]
4.587803 0.7927486

129. Arctangent y%x

q)x:sqrt 3
q)y:1
q)atan y%x
0.5235988

131. Complementary angle (arccos sin x)

q)x:0.25
q)acos sin x
1.320796
q)x+acos sin x / should be 0.5*pi, approximately
1.570796
q)2*x+acos sin x
3.141593

132. Rotation matrix for angle x (in radians) counter-clockwise

q)x:0.25
q)((cos x;neg sin x);(sin x;cos x))
0.9689124 -0.247404
0.247404  0.9689124

133. Degrees from radians

q)x:0.5
q)57.295779513082323*x
28.64789

134. Radians from degrees

q)x:0.5
q)z:57.295779513082323*x
q)z
28.64789
q)0.017453292519943295*z
0.5

135. Number of permutations of n objects taken k at a time

q)fac:{[n]$[n>1;n*fac[n-1];1]}
q)binn:{[n;k]fac[n]%fac[n-k]*fac[k]}
q)pn:{[n;k]fac[k]*binn[n;k]}
q)pn[5;3]
60f

136. Pascal's triangle of order x (binomial coefficients)

See 1007.

== DROP ==

137. Taylor series with coefficients y at point x

q)fac:{[n]$[n>1;n*fac[n-1];1]}
q)x:3
q)y:1 1 1
q)a:til count y
q)sum y*(x xexp a)%fac each a
8.5
q)y:30#1
q)x:1
q)a:til count y
q)sum y*(x xexp a)%fac each a
2.718282

139. Beta function

See gamma in appendix.

Beta:{((gamma x)*gamma y)%gamma[x+y]}

== DROP. WTF? ==

142. Number of combinations of n objects taken k at a time

q)fac:{[n]$[n>1;n*fac[n-1];1]}
q)binn:{[n;k]fac[n]%fac[n-k]*fac[k]}
q)binn[12;7]
792f
q)binn[10;4]
210f

143. Indexes of distinct items

q)x:"ajhajhja"
q)group x
a| 0 3 7
j| 1 4 6
h| 2 5
q)value group x
0 3 7
1 4 6
2 5

144. Histogram

q)x:8 3 11 9 9 4 6 6 3 3 9 7 9
q)h:{@[(1+max x)#0;x;+;1]}
q)h x
0 0 0 3 1 0 2 1 1 4 0 1
q)b:h x
q)c:(1+max b)-b
q)c
5 5 5 2 4 5 3 4 4 1 5 4
q)d:reverse flip(b#\:1),'(c#\:0)
q)d
0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 1 0 0
0 0 0 1 0 0 0 0 0 1 0 0
0 0 0 1 0 0 1 0 0 1 0 0
0 0 0 1 1 0 1 1 1 1 0 1
q)" *"[d]
"            "
"         *  "
"   *     *  "
"   *  *  *  "
"   ** **** *"

145. Count of x between endpoints (l,h)

q)x:(66 8 6 4 86;82 91 52 53 89;43 0 62 3 17;0 26 81 2 12;37 41 41 72 60)
q)l:10
q)h:80
q)xl:x>l
q)xl
10001b
11111b
10101b
01101b
11111b
q)xh:x<h
q)xh
11110b
00110b
11111b
11011b
11111b
q)xbetween:xl&xh
q)xbetween
10000b
00110b
10101b
01001b
11111b
q)sum each xbetween
1 2 3 2 5

146. Compound interest for principals y at percentages g for periods x

q)x:1 2 3 4
q)y:1 2 3 4
q)g:0.5 1 1.5 2
q)z:y*\:(1+g%100)xexp\:x
q)\P 5
q)z
1.005 1.01   1.0151 1.0202 1.01  1.0201 1.0303 1.0406 1.015 1.0302 1.0457 1.0..
2.01 2.02   2.0302 2.0403  2.02 2.0402 2.0606 2.0812  2.03 2.0604 2.0914 2.12..
3.015 3.0301 3.0452 3.0605 3.03  3.0603 3.0909 3.1218 3.045 3.0907 3.137  3.1..
4.02 4.0401 4.0603 4.0806  4.04 4.0804 4.1212 4.1624  4.06 4.1209 4.1827 4.24..

147. Locations of string x in string y (including overlaps)

q)x:"**"
q)y:"*abcugj**jy***kmhix**12"
q)sss:{z[where y[z+\:til count x]~\:x]}
q)sss[x;y;where ((1-count x)_y)=first x]
7 11 12 19
q)y[7 11 12 19+\:0 1]
"**"
"**"
"**"
"**"

148. Node matrix from connection matrix

(Inverse to 157.)

q)show x:(1 1 0 0 0; 0 -1 0 1 1; -1 0 1 -1 0; 0 0 -1 0 -1)
1  1  0  0  0
0  -1 0  1  1
-1 0  1  -1 0
0  0  -1 0  -1

Each column in x represents a path between 2 nodes:

  • From node 0 to node 2
  • From node 0 to node 1
  • From node 2 to node 3
  • From node 1 to node 2
  • From node 1 to node 3
q)show a:1 -1=\:x
11000b 00011b 00100b 00000b
00000b 01000b 10010b 00101b
q)mul:{(mmu\:) . "f"$(flip each x;y)}
q)show b:mul[a;til count x]
0 0 2 1 1
2 1 3 2 3
q)nc:{mul[1 -1=\:x;til count x]}
q)nc x
0 0 2 1 1
2 1 3 2 3

149. Number of decimals in x, maximum y

q)nd:{sum not 0=(neg floor neg x*10 xexp y)mod/:neg floor neg(10 xexp y)*10 xexp neg til y+1}
q)x:1.4321 1.21 10
q)y:3
q)nd[x;y]
3 2 0

150. Sum items of x given by y

q)show x:_log[1+!5]
0 0.6931472 1.098612 1.386294 1.609438
q)y:1 4 1 4 1
q)show a:value group y
0 2 4
1 3
q)sum each x[a]
2.70805 2.079442
q)sum x[0 2 4]
2.70805
q)sum x[1 3]
2.079442
q)sum each x[value group y]
2.70805 2.079442

151. Efficient execution of f x where x has repeated values

q)x:1 2 3 2 3 2 1
q)show u:distinct x
1 2 3
q)u?x
0 1 2 1 2 1 0
q)f:{10*x}
q)f u
10 20 30
q)(f u)[0 1 2 1 2 1 0]
10 20 30 20 30 20 10
q)(f u)[(u:distinct x)?x]
10 20 30 20 30 20 10
q).Q.fu[f;x]
10 20 30 20 30 20 10

152. Sum items of y according by ordered codes g in x

q)y:1+til 20
q)z:"abcde"
q)x:"dcbbbaceeaeccbecbaea"
q)xz:z,x
q)yz:((count z)#0),y
q)yz
0 0 0 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
q)xz
"abcdedcbbbaceeaeccbecbaea"
q)sum each yz[group xz]
a| 54
b| 43
c| 50
   d| 1
e| 62
q)sum each yz[value group xz]
54 43 50 1 62
q)sc:{sum each(((count z)#0),y)[value group z,x]}
q)sc[x;y;z]
54 43 50 1 62

153. Index of rows of y in corresponding rows of x

q)show x:1+3 4#til 12
1 2  3  4
5 6  7  8
9 10 11 12
q)q)show y:(1 0 3 0;0 6 0 8;9 0 0 12)
1 0 3 0
0 6 0 8
9 0 0 12
q)x ?/:'y
0 4 2 4
4 1 4 3
0 4 4 3

Same as x?'y.

Column-by-column find.

154. Range (nub; remove duplicate items)

q)x:"wirlsisl"
q)distinct x
"wirls"
q)distinct (1 2 3;4 5;1 2 3;4 5;1 2 3)
1 2 3
4 5

== DROP Not an idiom ==

155. Greatest common divisor of list x

q)x:6 9 12
q)gcd:{last 1+where min each 0=x mod/:1+til min x}
q)gcd x
3
q)min x
6
q)til min x
0 1 2 3 4 5
q)1+til min x
1 2 3 4 5 6
q)x mod/:1+til min x
0 0 0
0 1 0
0 0 0
2 1 0
1 4 2
0 3 0
q)0=x mod/:1+til min x
111b
101b
111b
001b
000b
101b
q)min each 0=x mod/:1+til min x
101000b
q)where min each 0=x mod/:1+til min x
0 2
q)last 1+where min each 0=x mod/:1+til min x
3

156. Classify y into x classes between min and max y

q)/ Normalize y so minimum value is 0
q)sm:{x-min x}
q)sm y
256 412 14 23 261 332 0
q)/ Normalize again so values are in range 0 le y and y lt x
q)nr:{y*x%max y}
q)nr[x;sm[y]]
6.213592 10 0.3398058 0.5582524 6.334951 8.058252 0
q)/ Compare against interior range boundaries, 1+til x-1
q)not nr[x;sm[y]]</:1+til x-1
1100110b
1100110b
1100110b
1100110b
1100110b
1100110b
0100010b
0100010b
0100000b
q)/ Count the number of boundaries passed by each
q)sum not nr[x;sm[y]]</:1+til x-1  
6 9 0 0 6 8 0

157. Connection matrix from node matrix (inverse to 148)

Node matrix top and bottom rows give from and to nodes.

q)show x:(0 0 2 1 1;2 1 3 2 3)
0 0 2 1 1
2 1 3 2 3
q)/ Enumerate count of range
q)key count distinct raze x
0 1 2 3
q)/ Where is x equal to each of it
q)x=/:key count distinct raze x
11000b 00000b
00011b 01000b
00100b 10010b
00000b 00101b
q)/ Subtract "to" matrix from "from" matrix
q)(-/)each x=/:key count distinct raze x
1  1  0  0  0
0  -1 0  1  1
-1 0  1  -1 0
0  0  -1 0  -1

158.

== DROP ==

See 20

159. Is range of x 1 (are all items of x equal)

q)x:1 1 1 1 1
q)1=count distinct x
1b
q)y:1 1 0 1 1
q)1=count distinct y
0b

160. Move blanks in x to end of list

q)x:"sign if i cant"
q)x[iasc x=" "]
"significant   "
q)be:{x[iasc x=" "]}
q)y:("yo ho ho";"and a bottle";"of rum")
q)be each y
"yohoho  "
"andabottle  "
"ofrum "

161. Is y upper triangular?

q)show x:(1 0 0 1;0 2 1 0;0 0 1 2;0 0 0 0)
1 0 0 1
0 2 1 0
0 0 1 2
0 0 0 0
q)slt:{(key x)>\:key x}
q)slt[count x]
0000b
1000b
1100b
1110b
q)x*slt[count x]
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
q)zm:{(x,x)#0}
q)zm[count x]
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
q)iut:{zm[count x]~x*slt[count x]}
q)iut x
1b
q)flip x
1 0 0 0
0 2 0 0
0 1 1 0
1 0 2 0
q)iut[flip x]
0b

162. Is x lower triangular?

q)show x:(1 0 0 0;0 2 0 0;0 1 1 0;1 0 2 0)
1 0 0 0
0 2 0 0
0 1 1 0
1 0 2 0
q)sut:{(key x)<\:key x}
q)sut count x
0111b
0011b
0001b
0000b
q)zm:{(x,x)#0}
q)ilt:{zm[count x]~x*sut count x}
q)ilt x
1b
q)ilt flip x
0b

163. Polynomial product

q)x:1 2 1
q)y:1 3 3 1
q)y*/:x
1 3 3 1
2 6 6 2
1 3 3 1
q)1 _ 'zm count x
0 0
0 0
0 0
q)(1 _' zm count x),'y*/:x
0 0 1 3 3 1
0 0 2 6 6 2
0 0 1 3 3 1
q)(til count x) rotate' (1 _' zm count x),'y*/:x
0 0 1 3 3 1
0 2 6 6 2 0
1 3 3 1 0 0
q)sum (til count x) rotate' (1 _' zm count x),'y*/:x
1 5 10 10 5 1
q)pm:{sum(til count x)rotate'(1_'zm count x),'y*/:x}
q)pm[x;y]
1 5 10 10 5 1

164. Divisors

q)dv:{where 0=x mod/:key 1+x}
q)x:363
q)dv x
1 3 11 33 121 363
q)x:365
q)dv x
1 5 73 365
q)dv 367
1 367
q)dv each 1 2 3 4 5 6 7 8 9 10
,1
1 2
1 3
1 2 4
1 5
1 2 3 6
1 7
1 2 4 8
1 3 9
1 2 5 10

165. List of x zeros preceded by (y-x) ones

q)x:5
q)y:9
q)zo:{((y-x)#1),x#0}
q)zo[x;y]
1 1 1 1 0 0 0 0 0

166. Barchart of integer list x, down the page

q)x:2 5 7 4 9 3 6
q)xl:{(x#1),(y-x)#0}
q)" X"[reverse flip xl\:[x;max x]]
"    X  "
"    X  "
"  X X  "
"  X X X"
" XX X X"
" XXXX X"
" XXXXXX"
"XXXXXXX"
"XXXXXXX"
bd:{" X"[|+xl\:[x;|/x]]}
bd[x]
(" X "
" X "
" X X "
" X X X"
" XX X X"
" XXXX X"
" XXXXXX"
"XXXXXXX"
"XXXXXXX")
q)x:2 5 7 4 9 3 6
q)xl:{(x#1),(y-x)#0}
q)" X"[reverse flip xl\:[x;max x]]
"    X  "
"    X  "
"  X X  "
"  X X X"
" XX X X"
" XXXX X"
" XXXXXX"
"XXXXXXX"
"XXXXXXX"

167. List of x ones preceded by (y-x) zeros

q)x:3
q)y:9
q)xr:{((y-x)#0),x#1}
q)xr[x;y]
0 0 0 0 0 0 1 1 1
q)x:2 5 7 4 9 3 6
q)xr\:[x;y]
0 0 0 0 0 0 0 1 1
0 0 0 0 1 1 1 1 1
0 0 1 1 1 1 1 1 1
0 0 0 0 0 1 1 1 1
1 1 1 1 1 1 1 1 1
0 0 0 0 0 0 1 1 1
0 0 0 1 1 1 1 1 1

168. List of x zeros followed by (y-x) ones

q)x:3
q)y:9
q)(x#0),(y-x)#1
0 0 0 1 1 1 1 1 1
q)zl:{(x#0),(y-x)#1}
q)zl[x;y]
0 0 0 1 1 1 1 1 1
q)x:2 5 7 4 9 3 6
q)zl\:[x;y]
0 0 1 1 1 1 1 1 1
0 0 0 0 0 1 1 1 1
0 0 0 0 0 0 0 1 1
0 0 0 0 1 1 1 1 1
0 0 0 0 0 0 0 0 0
0 0 0 1 1 1 1 1 1
0 0 0 0 0 0 1 1 1

169.

See 172.

== DROP ==

170. Horizontal barchart of x with maximum z, normalized to length y

q)x:2 8 5 6 3 1 7 7 10 4
q)xl:{(x#1),((y-x)#0)}
q)ad:{floor x*y%z}
q)ad[2;5;10]
1
q)ad[23;50;80]
14
q)y:5
q)z:10
q)xl\:[ad[x;y;z];y]
1 0 0 0 0
1 1 1 1 0
1 1 0 0 0
1 1 1 0 0
1 0 0 0 0
0 0 0 0 0
1 1 1 0 0
1 1 1 0 0
1 1 1 1 1
1 1 0 0 0
q)" X"[xl\:[ad[x;y;z];y]]
"X    "
"XXXX "
"XX   "
"XXX  "
"X    "
"     "
"XXX  "
"XXX  "
"XXXXX"
"XX   "

171. Horizontal barchart of integer values x

Compare bh here with xl in #172.

q)bh:{@[y#0;til x;:;1]}
q)" X"[bh\:[x;max x]]
"XX        "
"XXXXXXXX  "
"XXXXX     "
"XXXXXX    "
"XXX       "
"X         "
"XXXXXXX   "
"XXXXXXX   "
"XXXXXXXXXX"
"XXXX      "

172. List of x ones followed by (y-x) zeros

q)x:5
q)y:9
q)(x#1),(y-x)#0
1 1 1 1 1 0 0 0 0
q)xl:{(x#1),(y-x)#0}
q)xl[x;y]
1 1 1 1 1 0 0 0 0
q)xl\:[x;y]
1 1 1 1 1 0 0 0 0
q)x:2 5 7 4 9 3 6
q)xl\:[x;y]
1 1 0 0 0 0 0 0 0
1 1 1 1 1 0 0 0 0
1 1 1 1 1 1 1 0 0
1 1 1 1 0 0 0 0 0
1 1 1 1 1 1 1 1 1
1 1 1 0 0 0 0 0 0
1 1 1 1 1 1 0 0 0

173. Assign x to one of y classes of width h, starting with g

q)f:{[x;y;g;h] value -1+ -1 _ count each group(1+til y),neg floor neg(x-g)%h}
q)x:32 56 36 48 36 24 28 44 52 32
q)y:4
q)h:10
q)g:10
q)f[x;y;g;h]
0 2 4 2

174. Move x into first quadrant

q)sm:{x-min x}
q)show x:(1 6 4;3 4 7;7 8 6)
1 6 4
3 4 7
7 8 6
q)sm each x
0 5 3
0 1 4
1 2 0

175. Primes to n

q)n:10
q)show x:1+til n
1 2 3 4 5 6 7 8 9 10
q)x mod/:x:1+til n
0 0 0 0 0 0 0 0 0 0
1 0 1 0 1 0 1 0 1 0
1 2 0 1 2 0 1 2 0 1
1 2 3 0 1 2 3 0 1 2
1 2 3 4 0 1 2 3 4 0
1 2 3 4 5 0 1 2 3 4
1 2 3 4 5 6 0 1 2 3
1 2 3 4 5 6 7 0 1 2
1 2 3 4 5 6 7 8 0 1
1 2 3 4 5 6 7 8 9 0
q)sum 0=x mod/:x:1+til n
1 2 2 3 2 4 2 4 3 4
q)2=sum 0=x mod/:x:1+til n
0110101000b
q)0b,2=sum 0=x mod/:x:1+til n
00110101000b
q)where 0b,2=sum 0=x mod/:x:1+til n
2 3 5 7
q)pn:{[n]where 0b,2=sum 0=x mod/:x:1+til n}
q)pn 30
2 3 5 7 11 13 17 19 23 29
q)/Classic:
q)p:{x[where not (x in distinct raze x*/:\:x:2_ key x)]}
q)p 10
2 3 5 7
q)p 30
2 3 5 7 11 13 17 19 23 29

177. Ordinal of word in x pointed at by y

q)ow:{sum not y<1+where x=" "}
q)x:"ordinal of word in x pointed at by y"
q)ow[x;5]
0
q)ow[x;6]
0
q)ow[x;7]
0
q)ow[x;8]
1
q)ow[x;26]
5

177. Indexes of start of string x in string y

q)x:"st"
q)y:"indexes of start of string x in string y"
q)y ss x
11 20 32

== DROP Not an idiom ==

178. Index of first occurrence of string x in string y

q)x:"st"
q)y:"index of first occurrence of string x in string y"
q)first y ss x
12

== DROP Not an idiom ==

179. Contour levels y at points with altitude x

q)cl:{y[-1+sum not y>x]}
q)y:-100 0 10 100 1000 10000
q)cl[-5;y]
-100
q)cl[0;y]
0
q)cl[99;y]
10
q)cl[9;y]
0
q)cl[10;y]
10

180. Is x in range [y)

q)x:19 20 21 39 40 41
q)y:20 40
q)not x<\:y
00b
10b
10b
10b
11b
11b
q)hc:{10b~/:not x<\:y}
q)hc[x;y]
011100b

181. Which class of y x belongs to

q)cl:{-1+sum x>/:y}
q)x:87 9 931 7 27 92 654 1416 7 911
q)y:0 50 100 1000
q)sum x>/:y
2 1 3 1 1 2 3 4 1 3
q)x>/:y
1111111111b
1010011101b
0010001101b
0000000100b
q)-1 sum x>/:y
'type
q)-1+ sum x>/:y
1 0 2 0 0 1 2 3 0 2
q)cl[x;y]
1 0 2 0 0 1 2 3 0 2

182. Indexes of consecutive repeated elements

q)x:"aaabccccdeee"
q)group x
a| 0 1 2
b| ,3
c| 4 5 6 7
d| ,8
e| 9 10 11
q)count group x
5
q)count each group x
a| 3
b| 1
c| 4
d| 1
e| 3
q)where 1<count each group x
"ace"
q)value 1<count each group x
10101b
q)where value 1<count each group x
0 2 4

183. Maximum table

q)x:til 10
q)x&\:x
0 0 0 0 0 0 0 0 0 0
0 1 1 1 1 1 1 1 1 1
0 1 2 2 2 2 2 2 2 2
0 1 2 3 3 3 3 3 3 3
0 1 2 3 4 4 4 4 4 4
0 1 2 3 4 5 5 5 5 5
0 1 2 3 4 5 6 6 6 6
0 1 2 3 4 5 6 7 7 7
0 1 2 3 4 5 6 7 8 8
0 1 2 3 4 5 6 7 8 9

184. Right-justify fields x of length y to length g

q)x:"abcdefghij"
q)y:2 3 4 1
q)g:5
q)a:sums 0,-1_y
q)a
0 2 5 9
q)a _ x
"ab"
"cde"
"fghi"
,"j"
q)(g#" "),/:a _ x
"     ab"
"     cde"
"     fghi"
"     j"
q)b:(neg g)#/:(g#" "),/:a _ x
q)b
"   ab"
"  cde"
" fghi"
"    j"
q)raze b
"   ab  cde fghi    j"
q)rj:{[x;y;g]raze(neg g)#/:(g#" "),/:(sums 0,-1_y) _ x}
q)rj[x;y;g]
"   ab  cde fghi    j"

185. Left-justify fields x of length y to length g

q)x:"abcdefghij"
q)y:2 3 4 1
q)g:5
q)a:sums 0,-1_y
q)a
0 2 5 9
q)a _ x
"ab"
"cde"
"fghi"
,"j"
q)((sums 0,-1_y)_x),\:g#" "
"ab     "
"cde     "
"fghi     "
"j     "
q)g#/:((sums 0,-1_y)_x),\:g#" "
"ab   "
"cde  "
"fghi "
"j    "
q)ljust:{[x;y;g]raze g#/:((sums 0,-1_y)_x),\:g#" "}
q)ljust[x;y;g]
"ab   cde  fghi j    "

186. Annuity coefficient for x periods at interest rate y%

q)x:10 15 20 25
q)y:8 9 10 15
q)flip 1-xexp\:[(1+y%100);neg x]
0.5368065 0.5775892 0.6144567 0.7528153
0.6847583 0.725462  0.760608  0.8771055
0.7854518 0.8215691 0.8513564 0.9388997
0.8539821 0.8840322 0.907704  0.9696224
q)\P 3
q)flip 1-xexp\:[(1+y%100);neg x]
0.537 0.578 0.614 0.753
0.685 0.725 0.761 0.877
0.785 0.822 0.851 0.939
0.854 0.884 0.908 0.97
q)ac:{(y%100)%/:flip 1-xexp\:[(1+y%100);neg x]}
q)ac[x;y]
0.149  0.156 0.163 0.199
0.117  0.124 0.131 0.171
0.102  0.11  0.117 0.16
0.0937 0.102 0.11  0.155

187. Direct matrix product

q)x:1+3 2#til 6
q)y:1+2 4#til 8
q)flip each x*\:\:y
1 2 3 4     2 4 6 8     5  6  7  8  10 12 14 16
3 6 9  12   4 8 12 16   15 18 21 24 20 24 28 32
5 10 15 20  6 12 18 24  25 30 35 40 30 36 42 48
q)dp:{flip each x*\:\:y}

188. Shur product

q)show x:3 2#til 6
1 2
3 4
5 6
q)show y:2 4#1+til 8
1 2 3 4
5 6 7 8
q)((last shape x)#x) * (first shape y)#'y
1  4
15 24

189. Add x to each row of y

q)x:1+til 4
q)show y:3 4#2+til 12
2  3  4  5
6  7  8  9
10 11 12 13
q)x+/:y
3  5  7  9
7  9  11 13
11 13 15 17

190.

See 189.

== DROP ==

191. Shur sum

q)show x:3 2#til 6
1 2
3 4
5 6
q)show y:2 4#1+til 8
1 2 3 4
5 6 7 8
q)((last shape x)#x) + (first shape y)#'y
2 4
8 10

192. Add x to each column of y

q)x:1+til 2
q)show y:2 5#3+til 10
3 4 5  6  7
8 9 10 11 12
q)x+'y
4  5  6  7  8
10 11 12 13 14

== DROP Not an idiom; just how Each works ==

193. See 192

== DROP ==

195. Upper triangular matrix of order x

q)x:5
q){not x>\:x} til x
11111b
01111b
00111b
00011b
00001b
q)ut:{{not x>\:x}til x}
q)ut 5
11111b
01111b
00111b
00011b
00001b

196. Lower triangular matrix of order x

q)lt:{{not x<\:x}til x}
q)lt 5
10000b
11000b
11100b
11110b
11111b

197. Identity matrix of order x

q)id:{(2#x)#1,x#0}
q)id 5
1 0 0 0 0
0 1 0 0 0
0 0 1 0 0
0 0 0 1 0
0 0 0 0 1

198. Hilbert matrix of order x

q)hm:{reciprocal 1+(til x)+/:til x}
q)hm 5
1         0.5       0.3333333 0.25      0.2
0.5       0.3333333 0.25      0.2       0.1666667
0.3333333 0.25      0.2       0.1666667 0.1428571
0.25      0.2       0.1666667 0.1428571 0.125
0.2       0.1666667 0.1428571 0.125     0.1111111

199. Multiplication table of order x

q)mt:{{x*\:x}1+til x}
q)mt 5
1 2  3  4  5
2 4  6  8  10
3 6  9  12 15
4 8  12 16 20
5 10 15 20 25

200. Replicating a dimension of rank-3 array x y-fold

Three copies along second axis.

q)show x:2 3 3#1+til 18
1 2 3    4 5 6    7 8 9
10 11 12 13 14 15 16 17 18
q)y:3
q)x[;raze(y#1)*\:til(shape x)1;]
1 2 3    4 5 6    7 8 9    1 2 3    4 5 6    7 8 9    1 2 3    4 5 6    7 8 9
10 11 12 13 14 15 16 17 18 10 11 12 13 14 15 16 17 18 10 11 12 13 14 15 16 17 18

201. Moving index y-wide for x

q)x:"abcdef"
q)y:3
q)(til (count x)-y-1)+\:y
3 4 5 6

202. Indexes of infixes of length y

q)x:4+til 5
q)y:3
q)x+\:til 3
4 5 6
5 6 7
6 7 8
7 8 9
8 9 10

203. One-column matrix from numeric list

q)x:34 31 51 29 35 17 89
q)flip enlist x
34
31
51
29
35
17
89

204. Numeric array and its negative

q)show x:3+3 4#til 12
3  4  5  6
7  8  9  10
11 12 13 14
q)x,''neg x
3 -3   4 -4   5 -5   6 -6
7  -7  8  -8  9  -9  10 -10
11 -11 12 -12 13 -13 14 -14

205. Remove trailing blank rows

q)show x:flip 5 9#"abc de   "
"aaaaa"
"bbbbb"
"ccccc"
"     "
"ddddd"
"eeeee"
"     "
"     "
"     "
q)x~\:(count flip x)#" "
000100111b
q)not x~\:(count flip x)#" "
111011000b
q)reverse maxs reverse not x~\:(count flip x)#" "
111111000b
q)where reverse maxs reverse not x~\:(count flip x)#" "
0 1 2 3 4 5
q)rtr:{x where reverse maxs reverse not x~\:(count flip x)#" "}
q)rtr x
"aaaaa"
"bbbbb"
"ccccc"
"     "
"ddddd"
"eeeee"

206. Remove duplicate rows

q)show x:("abc";"def";"abc";"ghi";"jkl";"abc";"ghi";"abc")
"abc"
"def"
"abc"
"ghi"
"jkl"
"abc"
"ghi"
"abc"
q)distinct x
"abc"
"def"
"ghi"
"jkl"

== DROP Not an idiom ==

207. Indexes in matrix x of rows of matrix y

q)show x:flip 3 8#"abcdefgh"
"aaa"
"bbb"
"ccc"
"ddd"
"eee"
"fff"
"ggg"
"hhh"
q)show y:flip 3 4#"afmc"
"aaa"
"fff"
"mmm"
"ccc"
q)x?y
0 5 8 2

== DROP Not an idiom ==

209. Remove trailing blank columns

We can convert this to to the trailing blank rows of #205 and use rtr.

q)rtr:{x where reverse maxs reverse not  x~\:(count flip x)#" "}
q)show x:3 9#"abc de   "
"abc de   "
"abc de   "
"abc de   "
q)rtr x
"abc de   "
"abc de   "
"abc de   "
q)rtr flip x
"aaa"
"bbb"
"ccc"
"   "
"ddd"
"eee"
q)flip rtr flip x
"abc de"
"abc de"
"abc de"

Or.

q)x=" "
000100111b
000100111b
000100111b
q)min x=" "
000100111b
q)reverse min x=" "
111001000b
q)mins reverse min x=" "
111000000b
q)sum mins reverse min x=" "
3i
q)neg[sum mins reverse min x=" "]_'x
"abc de"
"abc de"
"abc de"

210. Remove leading blank columns

Again we can convert this to the trailing blanks of #205 and use rtr.

q)rtr:{x where reverse maxs reverse not  x~\:(count flip x)#" "}
q)show x:3 9#"   ed cha"
"   ed cha"
"   ed cha"
"   ed cha"
q)flip reverse rtr reverse flip x
"ed cha"
"ed cha"
"ed cha"

Or.

q)x=" "
111001000b
111001000b
111001000b
q)min x=" "
111001000b
q)mins min x=" "
111000000b
q)(mins min x=" ")?0b
3
q)((mins min x=" ")?0b)_'x
"ed cha"
"ed cha"
"ed cha"

211. Remove leading blank rows

Again we can convert this to the trailing blanks of #205 and use rtr.

q)rtr:{x where reverse maxs reverse not  x~\:(count flip x)#" "}
q)show x:reverse flip 3 9#"abc de   "
"   "
"   "
"   "
"eee"
"ddd"
"   "
"ccc"
"bbb"
"aaa"
q)reverse rtr reverse x
"eee"
"ddd"
"   "
"ccc"
"bbb"
"aaa"

Or.

q)x=" "
111b
111b
111b
000b
000b
111b
000b
000b
000b
q)min each x=" "
111001000b
q)(min each x=" ")?0b
3
q)((min each x=" ")?0b)_ x
"eee"
"ddd"
"   "
"ccc"
"bbb"
"aaa"

213. Maxima of infixes of x specified by boolean list y

q)x:-17 7 30 12 5 2 -5 6 -3 -19
q)show y:10#1 1 0
1 1 0 1 1 0 1 1 0 1
q)max x where y
12

214.

See 159. == DROP ==

215.

See 159. == DROP ==

216. Rows of matrix x starting with y

q)x:("sit";"sat";"sin";"tin")
q)y:"si"
q)y in/:x
11b
10b
11b
01b
q)min each y in/:x
1010b
q)where min each y in/:x
0 2
q)rb:{x where min each y in/:x}
q)rb[x;y]
"sit"
"sin"

217. Index of last non-blank in string

q)show x:("love's not ";"time's fool ";"though rosy ")
q)ln:{last where not " "=x}
q)ln each x
9 10 10

218. Single blank row from multiple

Remove multiple blanks from string.

q)s:"  a bc  def    g"
q)a:not b:s=" "
q)a
0010110011100001b
q)b
1101001100011110b
q)show c:(>':)b
1001001000010000b
q)show d:a|c
1011111011110001b
q)where d
0 2 3 4 5 6 8 9 10 11 15
q)s where d
" a bc def g"

Remove multiple blanks rows from matrix.

q)show x:flip 3 10#"a  b  c  d"
"aaa"
"   "
"   "
"bbb"
"   "
"   "
"ccc"
"   "
"   "
"ddd"
q)show a:x~\:(count flip x)#" "
0110110110b
q)b:not a
q)show c:(>':)a
0100100100b
q)show d:b|c
1101101101b
q)x where d
"aaa"
"   "
"bbb"
"   "
"ccc"
"   "
"ddd"
q)rs:{x where{not[x]|(>':)x}x~\:count[flip x]#" "}

219.

See 147. == DROP ==

220. Remove duplicate blank columns

q)show x:3 9#"a b c   d"
"a b c   d"
"a b c   d"
"a b c   d"
q)flip rs flip x
"a b c d"
"a b c d"
"a b c d"

221. Is x an integer in interval [ g,h )

q)g:6
q)h:12
q)x:18
q)
q)(x=floor x)&(not x<g)&(x<h)
0b
q)x:7
q)(x=floor x)&(not x<g)&(x<h)
1b
q)x:7.1
q)(x=floor x)&(not x<g)&(x<h)
0b

222. Maximum of x with weights y

q)x:1 2 3 4 5
q)y:5 4 3 2 1
q)max x*y
9

223. Minimum of x with weights y

q)x:1 2 3 4 5
q)y:5 4 3 2 1
q)min x*y
5

224. Extend distance table to next leg

q)show x:(0 50 80 20 999; 50 0 20 40 30; 80 20 0 999 30; 20 40 999 0 10; 999 30 30 10 0)
0   50 80  20  999
50  0  20  40  30
80  20 0   999 30
20  40 999 0   10
999 30 30  10  0

Notice x[0;2] is 80 while x[0;1]+x[1;2] is 70.

q)x('[min;+])\:x
0  50 70 20 30
50 0  20 40 30
70 20 0  40 30
20 40 40 0  10
30 30 30 10 0

225. Remove blank rows

q)show x:("aaa";"bbb";"   ";"ccc";"   ")
"aaa"
"bbb"
"   "
"ccc"
"   "
q)x except enlist count[first x]#" "
"aaa"
"bbb"
"ccc"

Or.

q)x<>" "
111b
111b
000b
111b
000b
q)max each x<>" "
11010b
q)x where max each x<>" "
"aaa"
"bbb"
"ccc"

226. Remove blank columns

Convert to the blank rows of #225.

q)show x:flip 4 4#"xxxx    hhhh  ii"
"x h "
"x h "
"x hi"
"x hi"
   flip(flip x)except enlist count[x]#" "
"xh "
"xh "
"xhi"
"xhi"

Or.

q)x<>" "
1010b
1010b
1011b
1011b
q)max x<>" "
1011b
q)x[;where max x<>" "]
"xh "
"xh "
"xhi"
"xhi"

227.

See 69. == DROP ==

228. Is y a row of x?

q)show x:("xxx";"yyy";"zzz";"yyy")
q)"yyy" in x
1b
q)"abc" in x
0b

== DROP Keyword, not idiom ==

229.

See 228. == DROP ==

230. Extend a transitive binary relation

== FIXME ==

q)show x:(0 1 0 0;0 0 1 1;1 0 0 0;0 0 1 0)
0 1 0 0
0 0 1 1
1 0 0 0
0 0 1 0
q)x(|/[&])\:x  / but how to parse that?
0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 1 0 0 0 0 0 1 0
0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0
q)x &\:x  / did I miss something?
0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 1 0 0 0 0 0 1 0
0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0

231. Which rows of x contain elements different from y?

q)show x:("aaa";"bbb";"ooo";"pop")
q)y:"o"
q)x<>y
111b
111b
000b
101b
q)max each x<>y
1101b

232. Is y a row of x

q)show x:("aaa";"bbb";"ooo";"ppp";"kkk")
q)y:"ooo"
q)y in x
1b

== DROP Keyword not idiom ==

233. Is x within range [ y )

q)x:9
q)show y:(1 9;9 16;5 7;10 20;6 10)
1  9
9  16
5  7
10 20
6  10
q)x<y
00b
01b
00b
11b
01b
q)(</)each x<y
01001b

234. Is x within the range ( y ]

q)show y:(1 9;9 16;5 7;10 20;6 10)
1  9
9  16
5  7
10 20
6  10
q)x<=y
01b
11b
00b
11b
01b
q)(</)each x<=y
10001b

235.

See 234. == DROP ==

236. Number of occurrences of x in y

q)show y:3+7?6
6 4 7 7 6 6 4
q)x:7
q)sum x=y
2

237. Average (mean) of x weighted by y

q)y:78 80 90 88 72
q)x:20 15 20 22 19
q)x*y
1560 1200 1800 1936 1368
q)sum x*y
7864
q)(sum x*y)%count x
1572.8

239. Sum reciprocal series

q)x:10 9 10 7 8
q)y:80 63 70 63 64
q)sum y%x
39f

240. Matrix product

q)show x:`float$(1 2 3;4 5 6)
1 2 3
4 5 6
q)show y:`float$(1 2;3 4;5 6)
1 2
3 4
5 6
q)x mmu y
22 28
49 64

== DROP Keyword not idiom ==

241. Sum over subsets of x specified by y

q)show x:`float$1+3 4#til 12
1 2  3  4
5 6  7  8
9 10 11 12
q)show y:`float$4 3#1 0
1 0 1
0 1 0
1 0 1
0 1 0
q)x mmu y
4  6  4
12 14 12
20 22 20

242. Sum squares of x

q)x:1 2 3 4 5
q)sum x*x
55

== DROP Trivial ==

243. Dot product of vectors

q)x:1 2 3 4 5
q)y:10 20 30 40 50
q)sum x*y
550

== DROP Trivial ==

244. Product over subsets of x specified by y

q)show x:1+3 4#til 12
1 2  3  4
5 6  7  8
9 10 11 12
q)show y:4 3#1 0
1 0 1
0 1 0
1 0 1
0 1 0
q)x('[prd;xexp])\:y
3  8   3
35 48  35
99 120 99

245. Randomize the random seed

q)\S
-314159
q)\S -1154371779
q)\S
-1154371779

== DROP Not an idiom ==

246.

See 242. == DROP ==

247. Interlace x[i]#1 and y[i]#0

q)x:1 3
q)y:2 4
q)raze x,'y
1 2 3 4
q){count[x]#1 0}raze x,'y
1 0 1 0
q){x#'count[x]#1 0}raze x,'y
,1
0 0
1 1 1
0 0 0 0
q)raze{x#'count[x]#1 0}raze x,'y
1 0 0 1 1 1 0 0 0 0

248. Center text x in line of width y

q)x:"1234567890"
q)y:16
q)y#x,y#" "
"1234567890      "
q)y-count x
6
q)floor(y-count x)%2
3
q)neg[floor(y-count x)%2]rotate y#x,y#" "
"   1234567890   "

249. Offset enumeration

q)x:10
q)y:3
q)x+til y
10 11 12

q)x:10 20 30
q)y:3 4 2
q)raze x+til each y
10 11 12 20 21 22 23 30 31

250. Replicate y x times

q)x:3 4 2
q)y:10 20 30
q)x#'y
10 10 10
20 20 20 20
30 30
q)raze x#'y
10 10 10 20 20 20 20 30 30

251.

See 250. == DROP ==

252. X alternate takes of 1s and 0s

q)x:1 2 3 4 5
q)(count x)#1 0
1 0 1 0 1
q)x#'(count x)#1 0
,1
0 0
1 1 1
0 0 0 0
1 1 1 1 1
q)raze x#'(count x)#1 0
1 0 0 1 1 1 0 0 0 0 1 1 1 1 1

253.

See 250. == DROP ==

254. Running parity of infixes of y indicated by x

q)x:1 0 0 0 0 1 0 0 0 0 1 0 0 0
q)y:1 0 0 1 1 1 0 0 1 0 1 1 0 0
q)where x
0 5 10
q)where[x] _ y
1 0 0 1 1
1 0 0 1 0
1 1 0 0
q)sums each where[x] _ y
1 1 1 2 3
1 1 1 2 2
1 2 2 2
q)(sums each where[x] _ y)mod 2
1 1 1 0 1
1 1 1 0 0
1 0 0 0
q)raze(sums each where[x] _ y)mod 2
1 1 1 0 1 1 1 1 0 0 1 0 0 0

255. Running sum of infixes of y indicated by x

q)x:1 0 0 0 1 0 0 0 1
q)y:1 2 3 4 5 6 7 8 9
q)where x
0 4 8
q)where[x] _ y
1 2 3 4
5 6 7 8
,9
q)sums each where[x] _ y
1 3 6 10
5 11 18 26
,9
q)raze sums each where[x] _ y
1 3 6 10 5 11 18 26 9

256. Groups of 1s in y pointed at by x

q)y:0 0 0 1 1 1 0 1 1 1 0 1 1 1 1 1
q)x:0 0 0 1 0 1 0 0 0 0 0 1 0 0 0 1
q)x:0 0 0 1 0 0 0 0 1 0 0 1 0 0 0 1
q)a:sums >':[y]
q)a
1 1 1 2 2 2 2 2 2 2 2 3 3 3 3 3
q)a:sums >':[0,y]
q)a
1 1 1 1 2 2 2 2 2 2 2 2 3 3 3 3 3
q)a:+\>':[0,y]
'\
q)a:sums >':[0,y]
q)a:sums >':[y]
q)a
1 1 1 2 2 2 2 2 2 2 2 3 3 3 3 3
q)a-1
0 0 0 1 1 1 1 1 1 1 1 2 2 2 2 2
q)a:a-1
q)y&a=maxs x*a
0 0 0 1 1 1 1 1 1 0 0 1 1 1 1 1

== DROP What is this about? y is just <>\[x] and the final result is simply y. How are the groups in y ‘pointed at’ by x? ==

257. Sums of infixes of x determined by lengths y

q)x:1+til 10
q)y:2 3 2 3
q)a:sums 0,-1 _ y
q)a
0 2 5 7
q)a _ x
1 2
3 4 5
6 7
8 9 10
q)sum each a _ x
3 12 13 27
q)sum each sums[0,-1_ y] _ x
3 12 13 27

259. Removing leading and trailing blanks

q)x:" abcd e fg "
q)x:"   abcd e  fg   "
q)a:not x=" "
q)a
0001111010011000b
q)(maxs a) and reverse maxs reverse a
0001111111111000b
q)where (maxs a) and reverse maxs reverse a
3 4 5 6 7 8 9 10 11 12
q)x[where (maxs a) and reverse maxs reverse a]
"abcd e  fg"
q)lt:{x[where (maxs a) and reverse maxs reverse a]}
q)lt x
"abcd e  fg"

260. First 10 figurate numbers of order x

   fg:{x+\/10#1}
   fg 0
1 1 1 1 1 1 1 1 1 1
   fg 1
1 2 3 4 5 6 7 8 9 10
   fg 2
1 3 6 10 15 21 28 36 45 55
   fg 3
1 4 10 20 35 56 84 120 165 220
   fg 4
1 5 15 35 70 126 210 330 495 715
q)fg:{x+\/10#1}
q)fg 0
1 1 1 1 1 1 1 1 1 1
q)fg 1
1 2 3 4 5 6 7 8 9 10
q)fg 2
1 3 6 10 15 21 28 36 45 55
q)fg 3
1 4 10 20 35 56 84 120 165 220
q)fg 4
1 5 15 35 70 126 210 330 495 715

261. First group of 1s

q)x:1 1 1 0 1 0 1
   x&&\x=|\x
1 1 1 0 0 0 0
q)x:0 0 0 1 1 0 1
   x&&\x=|\x
0 0 0 1 1 0 0
q)x:0 1 0 1 0 1 0
   x&&\x=|\x
0 1 0 0 0 0 0

262. Value of saddle point (see 48)

q)x:(5 4 6 4 12 5
> 16 2 4 5 16 18
> 8 18 7 12 16 11
> 20 17 16 14 16 20
> 16 8 12 9 17 13)
   rn:{x='&/'x}
   cx:{x=\:|/x}
   minmax:{(rn x)&(cx x)}
   minmax x
(0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 1 0 0
0 0 0 0 0 0)
   ones:{&,/minmax x}
   ones x
,21
   (,/x)[ones[x]]
,14
q)x:(5 4 6 4 12 5;16 2 4 5 16 18;8 18 7 12 16 11;20 17 16 14 16 20;16 8 12 9 17 13)
q)rn:{x=' min each x}
q)cx:{x=\:max x}
q)minmax:{(rn x)&(cx x)}
q)minmax x
(000000b;000000b;000000b;000100b;000000b)
q)ones:{where raze minmax x}
q)ones x
,21
q)(raze x)[ones[x]]
,14

264. Insert x[i] blanks after y[g[i]]

b:(0,g)_ y
b
("ab"
"cd"
"ef"
,"g")
c:b,'(x,0)#\:" "
c
("ab "
"cd "
"ef "
,"g")
,/c
"ab cd ef g"
ib:{[x;y;g],/((0,g)_ y),'(x,0)#\:" "}
ib[x;y;g]
"ab cd ef g"

265. Insert x[i] zeroes after i-th infix of y

q)y:0 0 1 0 1 0 1 1
q)x:1 2 2 1
   &y
2 4 6 7
q)a:@[&#y;&y;:;x]
q)a
0 0 1 0 2 0 2 1
q)b:1+a
q)b
1 1 2 1 3 1 3 2
q)d:(!(#y)++/x)
q)d
0 1 2 3 4 5 6 7 8 9 10 11 12 13
q)d:(1_!(#y)++/x)
q)d
1 2 3 4 5 6 7 8 9 10 11 12 13 14
   d _lin c
1 1 0 1 1 0 0 1 1 0 0 1 0 1

266. Remove trailing blanks

q)x:" phrase 266 "
q)a:~x=" "
q)a
0 0 1 1 1 1 1 1 0 1 1 1 0 0 0
q)b:||\|a
q)b
1 1 1 1 1 1 1 1 1 1 1 1 0 0 0
   x[&b]
" phrase 266"
q)x:"  phrase 266   "
q)a:not x=" "
q)b:reverse maxs reverse a
q)b
111111111111000b
q)x[where b]
"  phrase 266"

267. Remove leading blanks

q)x:" phrase 267 "
q)a:~x=" "
q)a
0 0 1 1 1 1 1 1 0 1 1 1 0 0
q)b:|\a
q)b
0 0 1 1 1 1 1 1 1 1 1 1 1 1
   x[&b]
"phrase 267 "
q)x[where b]
"  phrase 266"
q)x:"  phrase 266   "
q)b:maxs a
q)b
001111111111111b
q)x[where b]
"phrase 266   "

268. Is x in ascending order

q)x:2 5 7 9 6 8 3
   x~x[<x]
0
q)x:0 1 1 1 7 8 9
   x~x[<x]
1
q)x:2 5 7 9 6 8 3
q)x~x[iasc x]
0b
q)x:0 1 1 1 7 8 9
q)x~x[iasc x]
1b

269. See 248

270. See 268

271. Slight variation of 264

272. See 266

273. Join scalar to each list item

q)x:"a"
q)y:"01234"
   x,/:y
("a0"
"a1"
"a2"
"a3"
"a4")
q)x:("01234";"56789")
q)y:("abcde";"fghij")
   x,y
("01234"
"56789"
"abcde"
"fghij")
   x,'y
("01234abcde"
"56789fghij")
q)x:"a"
q)y:"01234"
q)x,/:y
"a0"
"a1"
"a2"
"a3"
"a4"
q)x:("01234";"56789")
q)y:("abcde";"fghij")
q)x,y
"01234"
"56789"
"abcde"
"fghij"
q)x,'y
"01234abcde"
"56789fghij"

274. See 273

275. See 76

276. See 185

277. End indicators from lengths

q)x:1 2 3 4 5
   +\x
1 3 6 10 15
   -1++\x
0 2 5 9 14
   +/x
15
   @[&+/x;-1++\x;:;1]
1 0 1 0 0 1 0 0 0 1 0 0 0 0 1
q)x:1 2 3 4 5
q)sums x
1 3 6 10 15
q)-1+sums x
0 2 5 9 14
q)sum x
15
q)@[(sum x)#0;-1+sums x;:;1]
1 0 1 0 0 1 0 0 0 1 0 0 0 0 1

278. Start indicators from lengths

q)x:1 2 3 4 5
   (!+/x) _lin\: +\0,x
1 1 0 1 0 0 1 0 0 0 1 0 0 0 0
q)x:1 2 3 4 5
q)(til sum x) in\: sums 0,x
110100100010000b

279. Variation of 265

280. See 41

281. Value of taylor series with coefficients y at x

     x:12
q)y:7 5 6 6
   1+!-1+#y
1 2 3
   1.0,x%1+!-1+#y
1 12 6 4.0
   *\1.0,x%1+!-1+#y
1 12 72 288.0
   y**\1.0,x%1+!-1+#y
7 60 432 1728.0
   +/y**\1.0,x%1+!-1+#y
2227.0
q)x:12
q)y:7 5 6 6
q)1+til -1+count y
1 2 3
q)1+til -1+count y
1 2 3
q)1.0,x%(1+til -1 +count y)
1 12 6 4f
q)prds 1.0,x%(1+til -1 +count y)
1 12 72 288f
q)y*prds 1.0,x%(1+til -1 +count y)
7 60 432 1728f
q)sum y*prds 1.0,x%(1+til -1 +count y)
2227f

282. Index of first blank

q)x:"ab c d"
   x?" "
2
q)x:("ab c d";" a bc";"abcd ")
q)x
("ab c d"
" a bc"
"abcd ")
   x?\:" "
2 0 4
q)x:"ab c d"
q)x?" "
2
q)x:("ab c d";" a bc";"abcd ")
q)x
"ab c d"
" a bc"
"abcd "
q)x?\:" "
2 0 4

283. Locate field y of fields beginning with first of x

q)x:"abcabbbaccccaddd"
q)y:2
   y=+\x=*x
0 0 0 1 1 1 1 0 0 0 0 0 0 0 0 0
   x[&y=+\x=*x]
"abbb"
q)y:4
   x[&y=+\x=*x]
"addd"
q)x:"abcabbbaccccaddd"
q)y:2
q)y=sums x=first x
0001111000000000b
q)x[where y=sums x=first x]
"abbb"
q)y:4
q)x[where y=sums x=first x]
"addd"

284. Sum items of x marked by y

q)x:1 2 3 4 5 6 7
q)y:1 1 1 2 2 3 3
   =y
(0 1 2
3 4
5 6)
   x[=y]
(1 2 3
4 5
6 7)
   +/'x[=y]
6 9 13
q)y:1 2 1 3 3 2 1
   +/'x[=y]
11 8 9
q)x:1 2 3 4 5 6 7
q)y:1 1 1 2 2 3 3
q)group y
1| 0 1 2
2| 3 4
3| 5 6
q)x[group y]
1| 1 2 3
2| 4 5
3| 6 7
q)sum each x[group y]
1| 6
2| 9
3| 13
q)y:1 2 1 3 3 2 1
q)sum each x[group y]
1| 11
2| 8
3| 9
q)value group y
0 2 6
1 5
3 4
q)value x[group y]
1 3 7
2 6
4 5
q)value sum each x[group y]
11 8 9

285. Moving sum

q)y:3
q)x:1 2 3 4 5
   +\x
1 3 6 10 15
   (-y)
-3
   (-y)_ a:+\x
1 3
   0,(-y)_ a
0 1 3
   (y-1)_ a
6 10 15
   ((y-1)_ a)-0,(-y)_ a
6 9 12
   ms:{((y-1)_ a)-0,(-y)_ a:+\x}
   ms[x;y]
6 9 12
q)y:3
q)x:1 2 3 4 5
q)sums x
1 3 6 10 15
q)neg y
-3
q)neg y _ a:sums x
-10 -15
q)
q)(neg y) _ a:sums x
1 3
q)0,(neg y)_a
0 1 3
q)(y-1)_a
6 10 15
q)((y-1)_a)-0,(neg y)_ a
6 9 12
q)ms:{((y-1)_a)-0,(neg y)_ a:sums x}
q)ms[x;y]
6 9 12

286. Fifo stock y decremented with x units

q)x:5
q)y:1 2 3 4 5
   (+\y)-x
-4 -2 1 5 10
q)g:0|(+\y)-x
q)g
0 0 1 5 10
   -':0,g
0 0 1 4 5
   ff:{-':0,0|(+\y)-x}
   ff[x;y]
0 0 1 4 5
q)(sums y)
1 3 6 10 15
q)(sums y)-x
-4 -2 1 5 10
q)g:0|(sums y)-x
q)g
0 0 1 5 10
q)deltas 0,g
0 0 0 1 4 5
q)1_ deltas 0,g
0 0 1 4 5
q)ff:{1_deltas 0,0|(sums y)-x}
q)ff[x;y]
0 0 1 4 5

289. Or-scan of infixes of y indicated by x

q)y:1 0 0 1 0 1 0 0
q)x:1 0 1 0 0 0 1 0
q)a:&x
q)b:a _ y
q)b
(1 0
0 1 0 1
0 0)
q)c:|\'b
q)c
(1 1
0 1 1 1
0 0)
   ,/c
1 1 0 1 1 1 0 0
q)y:1 0 0 1 0 1 0 0
q)x:1 0 1 0 0 0 1 0
q)a:where x
q)a
0 2 6
q)b:a _ y
q)b
1 0
0 1 0 1
0 0
q)c:max each b
q)c
1 1 0
q)c:maxs each b
q)c
1 1
0 1 1 1
0 0
q)raze c
1 1 0 1 1 1 0 0

290. And-scan of infixes of y indicated by x

q)y:1 0 0 1 0 1 0 0
q)x:1 0 1 0 0 0 1 0
q)a:&x
q)b:a _ y
q)b
(1 0
0 1 0 1
0 0)
q)c:&\'b
q)c
(1 0
0 0 0 0
0 0)
   ,/c
1 0 0 0 0 0 0 0
q)y:1 0 0 1 0 1 0 0
q)x:1 0 1 0 0 0 1 0
q)a:where x
q)b:a _ y
q)b
1 0
0 1 0 1
0 0
q)c:mins each b
q)c
1 0
0 0 0 0
0 0
q)raze c
1 0 0 0 0 0 0 0

291. Sums of infixes of y indicated by x

q)y:1 2 3 4 5
q)x:1 0 1 0 1
q)a:&x
q)b:a _ y
q)b
(1 2
3 4
,5)
q)c:+/'b
q)c
3 7 5
q)y:1 2 3 4 5
q)x:1 0 1 0 1
q)a:where x
q)b:a _ y
q)b
1 2
3 4
,5
q)c:sum each b
q)c
3 7 5

292. Groups of 1s in y pointed to by x

q)y:1 1 1 0 0 1 1
q)x:0 1 0 1 0 0 0
   -1 _ 0,y
0 1 1 1 0 0 1
   +\y>-1 _ 0,y
1 1 1 1 1 2 2
   x&y
0 1 0 0 0 0 0
q)a:+\y>-1 _ 0,y
q)a
1 1 1 1 1 2 2
   a[&x&y]
,1
   a _lin ,1
1 1 1 1 1 0 0
   y&a _lin ,1
1 1 1 0 0 0 0
   go:{y&a _lin(a:+\y>-1 _ 0,y)[&x&y]}
   go[x;y]
1 1 1 0 0 0 0
q)y:1 1 1 0 0 1 1
q)x:0 1 0 1 0 0 0
q)-1 _ 0,y
0 1 1 1 0 0 1
q)sums y > -1 _ 0,y
1 1 1 1 1 2 2
q)y > -1 _ 0,y
1000010b
q)x&y
0 1 0 0 0 0 0
q)a:sums y > -1 _ 0,y
q)a
1 1 1 1 1 2 2
q)a[where x&y]
,1

293. Locate quotes and text between them

q)x:"abc\"de\"f"
   #x
8
q)a:x="\""
q)a
0 0 0 1 0 0 1 0
q)b:&a
q)b
3 6
q)c:(*b)+!1+--/b
q)c
3 4 5 6
   @[&#x;c;:;1]
0 0 0 1 1 1 1 0
q)x:"abc\"de\"f"
q)count x
8
q)a:x="\""
q)a
00010010b
q)b:where a
q)b
3 6
q)c:(first b)+til 1+ neg -/[b]
q)c
3 4 5 6
q)@[(count x)#0;c;:;1]
0 0 0 1 1 1 1 0

294. Locate text between quotes

q)x:"abc\"de\"f"
   #x
8
q)a:x="\""
q)a
0 0 0 1 0 0 1 0
q)b:&a
q)b
3 6
q)c:b+1 -1
q)c
4 5
q)d:(*c)+!1+--/c
q)d
4 5
   @[&#x;d;:;1]
0 0 0 0 1 1 0 0 c
q)x:"abc\"de\"f"
q)count x
8
q)a:x="\""
q)a
00010010b
q)b:where a
q)b
3 6
q)c:b+1 -1
q)c
4 5
q)d:(first c)+til 1+neg -/[c]
q)d
4 5
q)@[(count x)#0;d;:;1]
0 0 0 0 1 1 0 0

295. Depth of parentheses

q)x:"a(b((c)de)f)g(h)"
   dp:{+\("("=x)--1 _ 0,")"=x}
   dp x
0 1 1 2 3 3 3 2 2 2 1 1 0 1 1 1
q)x:"a(b((cde)f)g(ki)"
   dp x
0 1 1 2 3 3 3 3 3 2 2 1 2 2 2 2
q)x:"ab((c)de)f)g(ki)"
   dp x
0 0 1 2 2 2 1 1 1 0 0 -1 0 0 0 0
q)x:"a(b((c)de)f)g(h)"
   dp x
0 1 1 2 3 3 3 2 2 2 1 1 0 1 1 1
q)x:"a(b((c)de)f)g(h)"
q)dp:{sums ("("=x)- -1_ 0,")"=x}
q)dp x
0 1 1 2 3 3 3 2 2 2 1 1 0 1 1 1
q)x:"a(b((cde)f)g(ki)"
q)dp x
0 1 1 2 3 3 3 3 3 2 2 1 2 2 2 2
q)x:"ab((c)de)f)g(ki)"
q)dp x
0 0 1 2 2 2 1 1 1 0 0 -1 0 0 0 0
q)x:"a(b((c)de)f)g(h)"
q)dp x
0 1 1 2 3 3 3 2 2 2 1 1 0 1 1 1

296. Starting positions of infixes from lengths x

q)x:2 3 1 5
   +\-1 _ 0,x
0 2 5 6
   sl:{+\-1 _ 0,x}
   sl[x]
0 2 5 6
q)x:2 3 1 5
q)sums -1 _0,x
0 2 5 6
q)sl:{sums -1 _ 0,x}
q)sl x
0 2 5 6

297. Spread marked field heads right

q)x:"abcdef"
q)y:1 1 0 0 1 0
q)a:&y
q)a
0 1 4
q)b:#:'a _ x
q)b
1 3 2
q)c:b#'a
q)c
(,0
1 1 1
4 4)
q)d:,/c
q)d
0 1 1 1 4 4
   x[d]
"abbbee"
   x[,/(#:'a _ x)#'a:&y]
"abbbee"
   sh:{x[,/(#:'a _ x)#'a:&y]}
   sh[x;y]
"abbbee"
q)x:"abcdef"
q)y:1 1 0 0 1 0
q)a:where y
q)a
0 1 4
q)a _ x
,"a"
"bcd"
"ef"
q)count a _ x
3
q)count each a _ x
1 3 2
q)
q)b:count each a _ x
q)c:b#'a
q)c
,0
1 1 1
4 4
q)d:raze c
q)d
0 1 1 1 4 4
q)x[d]
"abbbee"
q)x[raze(count each a _ x)$'a:where y]
'type
q)x[raze(count each a _ x)#'a:where y]
"abbbee"
q)sh:{x[raze(count each a _ x)#'a:where y]}
q)sh[x;y]
"abbbee"

298. See 266

299. See 267

300. Gth infix of y marked by x

q)x:1 0 0 1 0 1 0 0 0 1 0
q)y:"abcdefghijk"
q)g:2
q)a:&x
q)a
0 3 5 9
q)b:a _ x
q)b
("abc"
"de"
"fghi"
"jk")
b[g]
"fghi"
((&x)_ y)[g]
"fghi"
q)x:1 0 0 1 0 1 0 0 0 1 0
q)y:"abcdefghijk"
q)a:where x
q)a
0 3 5 9
q)b:a _ y   /original says x but must be y
q)b
"abc"
"de"
"fghi"
"jk"
q)b 2
"fghi"
q)((where x)_ y)[g]
"fghi"

301. Alternating sum series

q)x:1+!10
q)x
1 2 3 4 5 6 7 8 9 10
q)a:((#x)#1 -1)
q)a
1 -1 1 -1 1 -1 1 -1 1 -1
q)b:x*a
   +\b
1 -1 2 -2 3 -3 4 -4 5 -5
   as:{+\x*(#x)#1 -1}
   as[x]
1 -1 2 -2 3 -3 4 -4 5 -5
q)x:1+til 10
q)x
1 2 3 4 5 6 7 8 9 10
q)a:((count x)#1 -1)
q)a
1 -1 1 -1 1 -1 1 -1 1 -1
q)b:x*a
q)b
1 -2 3 -4 5 -6 7 -8 9 -10
q)sums b
1 -1 2 -2 3 -3 4 -4 5 -5
q)as:{sums x*(count x)#1 -1}
q)as[x]
1 -1 2 -2 3 -3 4 -4 5 -5

302. X first triangular numbers

q)x:6
   +\!x
0 1 3 6 10 15
q)x:6
q)sums til x
0 1 3 6 10 15

303. Smearing 1s between pairs of 1s

q)x:0 1 0 0 1 0 1 0 1 0 1 1 0
q)a:(+\x)!2
q)a
1 1 1 0 0 0 0 1 1 0 0
   x|a
1 1 1 1 0 0 0 1 1 1 0
   x|(+\x)!2
1 1 1 1 0 0 0 1 1 1 0
q)x:0 1 0 0 1 0 1 0 1 0 1 1 0
q)a:(sums x)mod 2
q)a
0 1 1 1 0 0 1 1 0 0 1 0 0
q)x or a
0 1 1 1 1 0 1 1 1 0 1 1 0
q)x|a
0 1 1 1 1 0 1 1 1 0 1 1 0
q)x|(sums x)mod 2
0 1 1 1 1 0 1 1 1 0 1 1 0

304. Invert 0s following 1st 1

q)x:0 0 1 0 0 1 1
   |\x
0 0 1 1 1 1 1
q)x:0 0 1 0 0 1 1
q)maxs x
0 0 1 1 1 1 1

305. Invert fields marked by pairs of 1s

q)x:1 0 1 0 0 1 0 0 1
q)a:(+\x)!2
q)a
1 1 0 0 0 1 1 1 0
   ~x
0 1 0 1 1 0 1 1 0
   (~x)&a
0 1 0 0 0 0 1 1 0
   (~x)&(+\x)!2
0 1 0 0 0 0 1 1 0
q)x:1 0 1 0 0 1 0 0 1
q)a:(sums x)mod 2
q)a
1 1 0 0 0 1 1 1 0
q)not x
010110110b
q)(not x)&a
0 1 0 0 0 0 1 1 0
q)(not x)&(sums x)mod 2
0 1 0 0 0 0 1 1 0

306. Invert all 1s after 1st 0

q)x:1 1 0 1 0 1 0
   &\x
1 1 0 0 0 0 0
q)x:1 1 0 1 0 1 0
q)mins x
1 1 0 0 0 0 0

307. Invert all 1s after 1st 1

q)x:0 0 1 1 0 1
   &#x
0 0 0 0 0 0
   x?1
2
@[&#x;x?1;:;1]
0 0 1 0 0 0

q)x:0 0 1 1 0 1 q)(count x)#x 0 0 1 1 0 1 q)(count x)#0 0 0 0 0 0 0 q)x?1 2 q)@[(count x)#0;x?1;:;1] 0 0 1 0 0 0

308. Invert all 0s after 1st 0

q)x:1 0 0 1 1 0
q)a:x?0
q)a
1
   !#x
0 1 2 3 4 5
q)b:(a+1)_!#x
q)b
2 3 4 5
   @[x;b;:;1]
1 0 1 1 1 1
   @[x;(1+x?0)_!#x;:;1]
1 0 1 1 1 1
q)x:1 0 0 1 1 0
q)a:x?0
q)a
1
q)til count x
0 1 2 3 4 5
q)b:(a+1)_til count x
q)b
2 3 4 5
q)@[x;b;:;1]
1 0 1 1 1 1
q)@[x;(1+x?0)_til count x;:;1]
1 0 1 1 1 1

309. Running parity

q)x:0 1 1 1 1 0 1 0 0
   (+\x)!2
0 1 0 1 0 0 1 1 1
q)x:0 1 1 1 1 0 1 0 0
q)(sums x)mod 2
0 1 0 1 0 0 1 1 1

310. Running sum

q)x:1 20 300 4000
+\x
1 21 321 4321
q)sums x
1 21 321 4321

311. See 159

312. Maximum separation of items of x

q)x:10+7 _draw 9
q)x
17 14 14 17 14 17 18
   (|/x)-(&/x)
4
q)x:17 14 14 17 14 17 18
q)(max x)-min x
4

313. Value of two-by-two determinant

   det: {-/y*|x}
q)x:(13 21;34 55)
q)x
(13 21
34 55)
   (13 * 55) - (34 * 21)
1
   13 21 * 55 34
715 714
   -/13 21 * 55 34
1
   det x
,1
q)det:{-/[x[0]*reverse x[1]]}

314. See 313

315. See 159

316. See 159

317. See 159

318. Area of triangle with sides x (heron's rule)

q)x:3 4 5
   hr:{(*/(+/x%2)-0,x)^0.5}
   hr[x]
6.0
q)hr:{sqrt (prd (sum x%2)-0,x)}
q)hr x
6f

319. Standard deviation

q)x:44 77 48 24 28 36 17 49 90 91
std:{((+/(x-(+/x)%#x)^2)%#x)^0.5}
std[x]
25.48411
q)std:{mean:sum x%count x; sqr:{x*x}; sqrt sum sqr[x-mean]%count x}
q)std x
25.48411

320. Variance (dispersion)

q)x:44 77 48 24 28 36 17 49 90 91.0
   var:{(+/(x-(+/x)%#x)^2)%#x}
   var[x]
649.44
(var[x])^0.5
25.48411
q)x:44 77 48 24 28 36 17 49 90 91.0
q)var x
649.44
q)var
k){avg[x*x]-a*a:avg x:"f"$x}
q)varr:{mean:sum x%count x; sqr:{x*x}; sum sqr[x-mean]%count x}
q)varr x
649.44

321. Y-th moment of x

q)x:44 77 48 24 28 36 17 49
   ym:{(+/(x-(+/x)%#x)^y)%#x}
   ym[x;2]
309.23
   ym[x;0]
1.0
   ym[x;1]
4.4409e-016
   ym[x;3]
3889.9
q)x:44 77 48 24 28 36 17 49
q)ym:{(sum(x-sum x%count x)xexp y)%count x}
q)ym[x;2]
309.2344
q)ym[x;0]
1f
q)ym[x;1]
4.440892e-16
q)ym[x;3]
3889.934

322.

323. See 248

324. See 159

325. Average (mean)

   av:{(+/x)%#x}
   av[1 10 100]
37.0
q)av:{(sum x)%count x}
q)av[1 10 100]
37f

326. See 325

327. See 70

328. Number of items

   #"abcd"
4
   #(1;2 3;4 5 6)
3
   #2 3 4#!24
2
q)count "abcd"
4
q)count(1;2 3;4 5 6)
3
q)count 2 3 4#til 24
'length

329. Mask from positive integers in x

q)x:-5+7 _draw 10
q)x
2 3 3 -2 4 4 -1
   (!1+|/x) _lin x
0 0 1 1 1
q)x:2 3 3 -2 4 4 -1
q)(til 1+max x) in x
00111b

330. Index of 1st occurrence of maximum item of x

q)x:5 3 7 0 5 7 2
   x?|/x
2
q)x:5 3 7 0 5 7 2
q)x?max x
2

331. Identity for floating point maximum, negative infinity -0i

-1e100|-0i
-1e+100
identity for integer maximum, negative infinity -0I
-123456789|-0I
q)-1e100|-0w
-1e+100
q)-1e100|-0W
-2.147484e+09
q)-123456789|-0w
-1.234568e+08
q)-123456789|-0W
-123456789

332. See 326

333. Quick membership for nonnegative integers

q)x:5 3 7 2
q)y:8 5 2 6 1 9
   &1+|/x,y
0 0 0 0 0 0 0 0 0 0
q)a:&1+|/x,y
   @[a;y;:;1]
0 1 1 0 0 1 1 0 1 1
   (@[a;y;:;1])[x]
1 0 0 1
   @[&1+|/x,y;y;:;1][x]
1 0 0 1
q)x:5 3 7 2
q)y:8 5 2 6 1 9
q)max x,y
9
q)(1+max x,y)#0
0 0 0 0 0 0 0 0 0 0
q)a:(1+max x,y)#0
q)a
0 0 0 0 0 0 0 0 0 0
q)@[a;y;:;1]
0 1 1 0 0 1 1 0 1 1
q)(@[a;y;:;1])[x]
1 0 0 1
q)@[(1+max x,y)#0;y;:;1][x]
1 0 0 1

334. Nonnegative maximum

q)x:1 2 3 4 5
   |/x,0
5
q)x:-1 -2 -3 -4 -5
   |/x,0
0
q)x:!0
q)x
   !0
   |/x,0
0
q)x:0n
q)max x,0
0f
/XXX doublecheck

335. Maximum

q)x:5 3 7 2
   |/x
7
q)max x
7

336. Index of first occurrence of minimum

q)x:5 3 7 2 5 3
   x?&/x
3
q)x?min x
3

337. Identity for floating point minimum, positive infinity 0i

   1e100&0i
1e+100
identity for integer minimum, positive infinity 0I
   123456789&0I
123456789
q)1e100&0w
1e+100
q)1e100&0W
2.147484e+09
q)123456789&0w
1.234568e+08
q)123456789&0W
123456789

338. Locate first occurrence in x of an item of y

q)x:"abcdef"
q)y:"dbf"
   &/x?/:y
1
   x[1]
"b"
q)x:"abcdef"
q)y:"dbf"
q)min x?/:y
1
q)x[1]
"b"

339. Minimum

q)x:5 3 7 2
   &/x
2
q)min x
2

340. See 159

341. See 159

342. Arabic from roman number

q)x:"MCMIX"
   "MDCLXVI"?/:x
0 2 0 6 4
q)a:0,1000 500 100 50 10 5 1["MDCLXVI"?/:x]
q)a
0 1000 100 1000 1 10
   a<1!a
1 0 1 0 1 0
   _ a*-1^a<1!a
0 1000 -100 1000 -1 10
   +/_ a*-1^a<1!a
1909
   ar:{+/_ a*-1^a<1!a:0,1000 500 100 50 10 5 1["MDCLXVI"?/:x]}
   ar[x]
1909
q)x:"MCMIX"
q)"MDCLXVI"?/:x
0 2 0 6 4
q)a:0,1000 500 100 50 10 5 1["MDCLXVI"?/:x]
q)a
0 1000 100 1000 1 10
q)a<1 rotate a
101010b
q)floor a*-1 xexp a<1 rotate a
0 1000 -100 1000 -1 10
q)sum floor a*-1 xexp a<1 rotate a
1909
q)ar:{sum floor a*-1 xexp a<1 rotate a:0,1000 500 100 50 10 5 1["MDCLXVI"?/:x]}
q)ar[x]
1909

343. See 159

344. Pairwise match

    x:("123";"123";"45";"45")
q)x
("123"
"123"
"45"
"45")
   ~':x
1 0 1
   (~':x),0
1 0 1 0
   pm:{(~':x),0}
   pm 1 1 1 1 2 2 3 4 4 4
1 1 1 0 1 0 0 1 1 0
q)x:("123";"123";"45";"45")
q)x
"123"
"123"
"45"
"45"
q)~':[x]
0101b
q)~':[x],0b
01010b
q)(~':[x]),0b
01010b
q)k)(~':[x]),0b
01010b
q)pm:{1 _ (~':[x]),0b}
q)pm 1 1 1 1 2 2 3 4 4 4
1110100110b
/each prior is different in k4/q?

345. Do ranges of x and y match

q)x:1 2 3
q)y:3 2 1 1
a:?x
q)x
1 2 3
q)a
1 2 3
q)b:?y
q)b
3 2 1
   a[<a]~b[<b]
1
   om:{x[<x]~y[<y]}
   om[?x;?y]
1
   om[?"bca";?"cabba"]
1

346. See 20

347. See 159

348. Do x and y have items in common

q)x:"aba"
q)y:"cdeac"
   x _lin\: y
1 0 1
   |/x _lin\: y
1
q)y:"edge"
   |/x _lin\: y
0
q)x:"aba"
q)y:"cdeac"
q)x in\:y
101b
q)max x in\:y
1b
q)y:"edge"
q)max x in\:y
0b

349. See 159'

350. Is x 1s and 0s only (boolean)

q)x:0 1 0 1
   &/x _lin\:0 1
1
q)x:1 1 1 1
   &/x _lin\:0 1
1
q)x:1 0 1 2
   &/x _lin\:0 1
0
q)x:0 1 0 1
q)min x in\:0 1
1b
q)x:1 1 1 1
q)min x in\:0 1
1b
q)x:1 0 1 2
q)min x in\:0 1
0b

351. Is x a subset of y

q)x:"abgk"
q)y:"abcdefghijkl"
   &/x _lin\:y
1
q)x:"abgx"
   &/x _lin\:y
0
q)x:"abgk"
q)y:"abcdefghijkl"
q)min x in\:y
1b
q)x:"abgx"
q)min x in\:y
0b

352. See 159

353. Are items unique

     x:"abcdefg"
   (#x)=(#?x)
1
q)x:"abcdefa"
   (#x)=(#?x)
0
q)x:"abcdefg"
q)(count x)=(count distinct x)
1b
q)x:"abcdefa"
q)(count x)=(count distinct x)
0b

354. See 159

355. None

q)x:&7
q)x
0 0 0 0 0 0 0
   ~|/x
1
q)x:7#0 1
q)x
0 1 0 1 0 1 0
   ~|/x
0
q)x:7#0
q)not max x
1b
q)x:7#0 1
q)x
0 1 0 1 0 1 0
q)not max x
0b

356. Any

q)x:&7
   |/x
0
q)x:7#0 1
   |/x
1
q)x:7#0
q)max x
0
q)any x
0
q)x:7#0 1
q)max x
1
q)any x
1

357. Does x match y

q)x:("abc";`sy;1 3 -7)
q)y:("abc";`sy;1 3 -7)
   x~y
1
q)x:1 2 3
q)y:1 4 3
   x~y
0
q)x:("abc";`sy;1 3 -7)
q)y:("abc";`sy;1 3 -7)
q)x~y
1b
q)x:1 2 3
q)y:1 4 3
q)x~y
0b

358. See 159

359. Locate blank rows

q)x:+5 6#"a bc d"
q)x
("aaaaa"
" "
"bbbbb"
"ccccc"
" "
"ddddd")
   x~\:(#+x)#" "
0 1 0 0 1 0
q)x:flip 5 6#"a bc d"
q)x
"aaaaa"
"     "
"bbbbb"
"ccccc"
"     "
"ddddd"
q)x~\:(count flip x)#" "
010010b

360. All

q)x:1 1 0 1
   &/x
0
q)x:1 1 1 1
   &/x
1
q)x:0 0 0 0
   &/x
0
q)x:1 1 0 1
q)min x
0
q)all x
0
q)x:1 1 1 1
q)min x
1
q)all x
1
q)x:0 0 0 0
q)min x
0
a)all x
0

361. Parity

q)x:2 _vs !8
q)x
(0 0 0 0 1 1 1 1
0 0 1 1 0 0 1 1
0 1 0 1 0 1 0 1)
   (+/x)!2
0 1 1 0 1 0 0 1

362. Count of occurrences of x in y

     x:"q"
q)y:"quaquaqua"
   +/x=y
3
q)x:"q"
q)y:"quaquaqua"
q)sum x=y
3

363. Solve quadratic

   qu:{(q%x),(z%q:qq[x;y;z])}
   qq:{-0.5*y+sg[y]*ds[x;y;z]}
   ds:{_sqrt[(y*y)-(4*x*z)]}
   sg:{(x>0)-(x<0)}
q)a:1
q)b:-1e30
q)c:1
   sg[b]
-1
   ds[a;b;c]
1e+030
   qq[a;b;c]
1e+030
   qu[a;b;c]
1e+030 1e-030
   qu[1;-8;15]
5 3.0
q)qu:{(q%x),(z%q:qq[x;y;z])}
q)qq:{-0.5*y+sg[y]*ds[x;y;z]}
q)ds:{sqrt[(y*y)-(4*x*z)]}
q)sg:{(x>0)-(x<0)}   /or use the builtin signum
q)a:1
q)b:-1e30
q)c:1
q)sg[b]
-1
q)ds[a;b;c]
1e+30
q)qq[a;b;c]
1e+30
q)qu[a;b;c]
1e+30 1e-30
q)qu[1;-8;15]
5 3f

364. See 325

365. See 325

366. Count of scalars

   cs:{#,//x}
   cs[1]
1
   cs[1 2]
2
   cs[(1 2;3 4 5)]
5
   cs[(1 2;(3 4;5))]
5
   cs[("ab";("cd";"efg"))]
7
   cs[!0]
0
q)cs:{count raze over x}
q)cs 1
1
q)cs[1 2]
2
q)cs[(1 2;3 4 5)]
5
q)cs[(1 2;(3 4;5))]
5
q)cs[("ab";("cd";"efg"))]
7
q)cs[til 0]
0

367. Alternating product

q)x:1 2 3 4 5
q)a:(#x)#1 -1
q)a
1 -1 1 -1 1
   */x^a
1.875
q)x:1 2 3 4 5
q)a:(count x)#1 -1
q)a
1 -1 1 -1 1
q)prd xexp[x;a]
1.875
q)xexp[x;a]
1 0.5 3 0.25 5

368. Product

q)x:1 2 3 4 5
   */x
120
q)x:1 2 3 4 5
q)prd x
120

369. Alternating sum

q)x:1+!10
q)x
1 2 3 4 5 6 7 8 9 10
q)a:((#x)#1 -1)
q)a
1 -1 1 -1 1 -1 1 -1 1 -1
q)b:x*a
   +/b
-5
   +/x*(#x)#1 -1
-5
q)x:1+til 10
q)x
1 2 3 4 5 6 7 8 9 10
q)a:((count x)#1 -1)
q)b:x*a
q)sum b
-5
q)sum x*(count x)#1 -1
-5

370. Count of 1s in boolean list

q)x:1 0 0 1 0 1 1
   +/x
4
q)x:1 0 0 1 0 1 1
q)sum x
4

371. Scalar from 1-item list

q)x:,5
q)x
,5
   +/x
5
q)a:,"a"
q)a
,"a"
   +/a
type error
   +/a
> \
   a[0]
"a"
q)x:enlist 5
q)x
,5
q)sum x
5
q)a:enlist "a"
q)a
,"a"
q)sum a
'type
q)a[0]
"a"

372. Sum columns of matrix

q)x:1+3 4# !12
q)x
(1 2 3 4
5 6 7 8
9 10 11 12)
   +/x
15 18 21 24
q)x:1+3 4#til 12
q)x
1 2  3  4
5 6  7  8
9 10 11 12
q)sum x
15 18 21 24

373. Sum rows of matrix

q)x:1+3 4# !12
q)x
(1 2 3 4
5 6 7 8
9 10 11 12)
   +/'x
10 26 42
q)x:1+3 4#til 12
q)x
1 2  3  4
5 6  7  8
9 10 11 12
q)sum each x
10 26 42

374. Sum

q)x:1 2 3 4 5
   +/x
15
q)x:1 2 3 4 5
q)sum x
15

375. Insert row x in matrix y after row g

q)y:4 3#1+!12
q)y
(1 2 3
4 5 6
7 8 9
10 11 12)
q)x:13 14 15
q)g:2
q)a:y,,x
q)a
(1 2 3
4 5 6
7 8 9
10 11 12
13 14 15)
q)b:<(!#y),g
q)b
0 1 2 4 3
   a[b]
(1 2 3
4 5 6
7 8 9
13 14 15
10 11 12)
   (y,,x)[<(!#y),g]
(1 2 3
4 5 6
7 8 9
13 14 15
10 11 12)
q)y:4 3#1+til 12
q)y
1  2  3
4  5  6
7  8  9
10 11 12
q)x:13 14 15
q)g:2
q)a:y,enlist x
q)a
1  2  3
4  5  6
7  8  9
10 11 12
13 14 15
q)b:iasc(til count y),g
q)b
0 1 2 4 3
q)a[b]
1  2  3
4  5  6
7  8  9
13 14 15
10 11 12
q)(y,enlist x)[iasc(til count y),g]
1  2  3
4  5  6
7  8  9
13 14 15
10 11 12

376. Append y at the bottom of matrix x

q)x:4 3#1+!12
q)x
(1 2 3
4 5 6
7 8 9
10 11 12)
q)y:13 14 15
   x,,y
(1 2 3
4 5 6
7 8 9
10 11 12
13 14 15)
q)x:4 3#1+til 12
q)x
1  2  3
4  5  6
7  8  9
10 11 12
q)y:13 14 15
q)x,enlist y
1  2  3
4  5  6
7  8  9
10 11 12
13 14 15

377. Fill x to length y with x's last item

q)x:"quiz"
q)y:9
q)a:(!y)&-1+#x
q)a
0 1 2 3 3 3 3 3 3
   x[a]
"quizzzzzz"
   x[(!y)&-1+#x]
"quizzzzzz"
or, an alternative way:
   y#x,y#*|x
"quizzzzzz"
q)x:"quiz"
q)y:9
q)a:(til y)&-1+count x
q)a
0 1 2 3 3 3 3 3 3
q)x[a]
"quizzzzzz"
q)x[(til y)&-1+count x]
"quizzzzzz"
q)y#x,y#last  x
"quizzzzzz"

378. [omitted]

379. Remove leading, multiple and trailing y's from x

q)x:0 0 1 2 0 0 3 4 0 5 0 0 0
q)y:0
q)a:x=y
q)b:~a&1!a
q)a
1 1 0 0 1 1 0 0 1 0 1 1 1
q)b
0 1 1 1 0 1 1 1 1 1 0 0 0
   x[&b]
0 1 2 0 3 4 0 5
   a[0]_ x[&b]
1 2 0 3 4 0 5
q)x:0 0 1 2 0 0 3 4 0 5 0 0 0
q)y:0
q)a:x=y
q)b:not a&1 rotate a
q)b
0111011111000b
q)a
1100110010111b
q)x[where b]
0 1 2 0 3 4 0 5
q)a[0]_ x[where b]
1 2 0 3 4 0 5

380. Change items of x with value y[0] to y[1]

q)x:"abcde"[15 _draw 5]
q)x
"ddaeecadbbcbedc"
q)y:"d "
   @[x;&x=*y;:;*|y]
" aeeca bbcbe c"
q)x:"ddaeecadbbcbedc"
q)y:"d "
q)@[x;where x=first y;:;last y]
"  aeeca bbcbe c"

381. Invert all but 1st 1 in group of 1s

q)x:0 0 1 1 1 0 1 1 0 1
   x>-1 _ 0,x
0 0 1 0 0 0 1 0 0 1
   m381:{x>-1 _ 0,x}
   m381[x]
0 0 1 0 0 0 1 0 0 1
q)x:"ddaeecadbbcbedc"
q)y:x:0 0 1 1 1 0 1 1 0 1
q)x
0 0 1 1 1 0 1 1 0 1
q)x>-1 _ 0,x
0010001001b
q)m381:{x>-1 _ 0,x}
q)m381[x]
0010001001b

382. Insert y in x after index g

q)x:1 2 3
q)y:10*1+!7
q)y
10 20 30 40 50 60 70
q)g:3
   ((g+1)#y),x,(g+1)_ y
10 20 30 40 1 2 3 50 60 70
q)x:1 2 3
q)y:10*1+til 7
q)y
10 20 30 40 50 60 70
q)g:3
q)((g+1)#y),x,(g+1)_ y
10 20 30 40 1 2 3 50 60 70

383. Pairwise difference

q)x:9 3 5 2 0
   --':x
6 -2 3 2
q)deltas x
9 -6 2 -3 -2
q)1_ deltas x
-6 2 -3 -2
q)1_ neg deltas x
6 -2 3 2

384. Drop 1st, postpend 0

q)x:3 4 5 6
   1 _ x,0
4 5 6 0
q)x:3 4 5 6
q)1_ x,0
4 5 6 0

385. Drop last, prepend 0

q)x:3 4 5 6
   -1 _ 0,x
0 3 4 5
q)x:3 4 5 6
q)-1 _ 0,x
0 3 4 5

386. Shift x right y, fill 0

q)x:1+!12
q)x
1 2 3 4 5 6 7 8 9 10 11 12
q)y:3
   @[(-y)!x;!y;:;0]
0 0 0 1 2 3 4 5 6 7 8 9
q)x:1+til 12
q)y:3
q)@[(neg y) mod  x; til y;:;0]
0 0 0 1 2 3 4 5 6 7 8 9

387. Shift x left y, fill 0

q)x:1+!12
q)x
1 2 3 4 5 6 7 8 9 10 11 12
q)y:3
   @[y!x;((#x)-y)+!y;:;0]
4 5 6 7 8 9 10 11 12 0 0 0
q)x:1+til 12
q)y:3
q)@[y rotate x;((count x)-y)+til y;:;0]
4 5 6 7 8 9 10 11 12 0 0 0

388. Drop y rows from top of matrix x

q)x:6 3# !18
q)x
(0 1 2
3 4 5
6 7 8
9 10 11
12 13 14
15 16 17)
q)y:2
   y _ x
(6 7 8
9 10 11
12 13 14
15 16 17)
q)x:6 3#til 18
q)x
0  1  2
3  4  5
6  7  8
9  10 11
12 13 14
15 16 17
q)y:2
q)y _ x
6  7  8
9  10 11
12 13 14
15 16 17

389. Playing order of x ranked players

   i:{1+2_sv'+|tt[-_-log2[x]]}
   j:{@[x;&x>y;:;0]}
   k:{j[i[x];x]}
   x:6
   i[x]
1 5 3 7 2 6 4 8
   j[i[x];x]
1 5 3 0 2 6 4 0
   k[x]
1 5 3 0 2 6 4 0

390. Conform table x rows to list y

   f390:{@[((1 0*+/^y)|^x)#0;!#x;:;x]}
   x:3 3#1+!9
   y:1 2 3 4
   f390[x;y]
(1 2 3
4 5 6
7 8 9
0 0 0)

391. Conform table y columns to list y

q)x:4 2 # 9
q)y:5#8
q)a:((0 1*+/^y)|^x)#0
q)a
(0 0 0 0 0
0 0 0 0 0
0 0 0 0 0
0 0 0 0 0)
   a[;!(^x)[1]]:x
q)a
(9 9 0 0 0
9 9 0 0 0
9 9 0 0 0
9 9 0 0 0)

392. Matrix from scalar or vector

   x:4
   x
   !0
   (1+~#^x),:/x
   ,,4
   (1+~#^x),:/x
1 1
   x:7 8
   (1+~#^x),:/x:7 8
1 2

393. See 248

394. [deferred]

395. [deferred]

396. Remove columns y from x

q)y
((1 2 3 4
5 6 7 8
9 10 11 12)
(13 14 15 16
17 18 19 20
21 22 23 24))
   y _di\:\: 0 2
((2 4
6 8
10 12)
(14 16
18 20
22 24))

397. See 73

398. Diagonals from columns

x:(1 2 3 4 5
6 7 8 9 10
11 12 13 14 15
16 17 18 19 20
21 22 23 24 25)
(-!5)!'x
(1 2 3 4 5
10 6 7 8 9
14 15 11 12 13
18 19 20 16 17
22 23 24 25 21)
q)x:5 5#1+til 25
q)x
1  2  3  4  5
6  7  8  9  10
11 12 13 14 15
16 17 18 19 20
21 22 23 24 25
q)(neg til 5)rotate' x
1  2  3  4  5
10 6  7  8  9
14 15 11 12 13
18 19 20 16 17
22 23 24 25 21

399. Columns from diagonals

      x
(1 2 3 4 5
10 6 7 8 9
14 15 11 12 13
18 19 20 16 17
22 23 24 25 21)
   (!5)!'x
(1 2 3 4 5
6 7 8 9 10
11 12 13 14 15
16 17 18 19 20
21 22 23 24 25)
q)x:(1 2 3 4 5;10 6 7 8 9;14 15 11 12 13;18 19 20 16 17;22 23 24 25 21)
q)(til 5)rotate'x
1  2  3  4  5
6  7  8  9  10
11 12 13 14 15
16 17 18 19 20
21 22 23 24 25

400. [deferred]

401. First word in string x

q)x:"twas brillig and the slith"
   x?" "
4
   (x?" ")#x
"twas"
   fw:{(x?" ")#x}
   fw x
"twas"
q)x:"twas brillig and the slith"
q)x?" "
4
q)(x?" ")#x
"twas"
q)fw:{(x?" ")#x}
q)fw x
"twas"

402. See 392

403. [omitted]

404. End points for x fields of length y

q)x:5
q)y:3
   @[&x*y;(y-1)+y*!x;:;1]
0 0 1 0 0 1 0 0 1 0 0 1 0 0 1
q)x:5
q)y:3
q)@[(x*y)#0;(y-1)+y*til x;:;1]
0 0 1 0 0 1 0 0 1 0 0 1 0 0 1

405. Start points for x fields of length y

q)x:5
q)y:3
   @[&x*y;y*!x;:;1]
1 0 0 1 0 0 1 0 0 1 0 0 1 0 0
q)x:5
q)y:3
q)@[(x*y)#0;y*til x;:;1]
1 0 0 1 0 0 1 0 0 1 0 0 1 0 0

406. Add y to last item of x

q)x:1 2 3 4 5
q)y:100
   @[x;-1+#x;+;y]
1 2 3 4 105
q)x:1 2 3 4 5
q)y:100
q)@[x;-1+count x;+;y]
1 2 3 4 105

407. Vector length y of x 1s, the rest 0s

q)x:5
q)y:12
   @[&12;!x;:;1]
1 1 1 1 1 0 0 0 0 0 0 0
q)x:5
q)x:12
q)@[12#0;til x;:;1]
1 1 1 1 1 0 0 0 0 0 0 0

408. Initial empty row to start matrix of x columns

q)x:15
   ,&x
,0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
   ,1.0*&x
,0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.0
   ,x#" "
," "
q)x:15
q)enlist x#0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
q)enlist 1.0*x#0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
q)enlist x#" "
"               "

409. [omitted]

410. Number of columns in matrix x

q)x:2 7#" "
q)x
2 7
   *|^x
7
/not quite the same
q)x:2 7#" "
q)count x
2
q)count each x
7 7
q)(count;first count each)@\:x
2 7

411. Number of rows in matrix x

q)x:2 7#" "
q)x
2 7
   #x
2
/not quite the same
q)x:2 7#" "
q)count x
2
q)count each x
7 7
q)(count;first count each)@\:x
2 7

412. See 184

413. Omitted

414. Ending indexes from field lengths

q)x:4 7 13 15 20
   (1+*x),-':x
5 3 6 2 5
q)x:4 7 13 15 20
q)(1+first x),-':[x]
5 4 3 6 2 5

415. Lengths of infixes of 1 in x

q)x:0 0 1 1 1 0 0 1 1 1 1 0 1
q)a:<':0,x,0
q)b:>':0,x,0
q)a
0 0 0 0 0 1 0 0 0 0 0 1 0 1
q)b
0 0 1 0 0 0 0 1 0 0 0 0 1 0
   &a
5 11 13
   &b
2 7 12
   (&a)-(&b)
3 4 1
   (&<':0,x,0)-&>':0,x,0
3 4 1
   m415:{(&<':0,x,0)-&>':0,x,0}
q)x:0 0 1 1 1 0 0 1 1 1 1 0 1
q)a:<':[0,x,0]
q)b:>':[0,x,0]
q)a
000000100000101b
q)b
100100001000010b
q)a:1_a
q)b:1_b
q)a
00000100000101b
q)b
00100001000010b
q)where a
5 11 13
q)where b
2 7 12
q)(where a)-(where b)
3 4 1
q)m415:{(where 1_<':[0,x,0])-(where 1_>':[0,x,0])}
q)m415 x
3 4 1

416. Omitted

417. End points of equal infixes

q)x:"baackkkegtt"
   (~(1 _ x)=-1 _ x),1
1 0 1 1 0 0 1 1 1 0 1
q)x:"baackkkegtt"
q)(not(1 _ x)=-1 _ x),1b
10110011101b

418. Starting indexes of equal item infix

q)x:"baackkkegtt"
   1,~(1 _ x)=-1 _ x
1 1 0 1 1 0 0 1 1 1 0
q)x:"baackkkegtt"
q)(not (1 _ x)=-1 _ x),1b
10110011101b

419. Pairwise ratios

q)x:2 10 50 100
   %':x
5 5 2.0
q)ratios x
2 5 5 2f
q)1_ratios x
5 5 2f

420. See 383

421. Deferred

422. See 154

423. Lengths from start indicator

q)x:1 0 1 0 0 1 0 0 0 1 0
   &x,1
0 2 5 9 11
   -':&x,1
2 3 4 2
q)1_deltas where x,1
2 3 4 2

424. Single blank from multiples

q)x:"a b c d"
   x[&a|1 _ 1!1,a:~" "=x]
"a b c d"
q)x:"a    b       c    d"
q)x[where a|1 _ 1 rotate 1b,a:not " "=x]
"a b c d"

425. See 380

426. Change all multiple infixes of y in x to single

q)x:"bccbceekl"
q)y:"c"
   x[&a|-1 _ 1,a:~x=y]
"bcbceekl"
q)x:"bccbceekl"
q)y:"c"
q)x[where a|-1 _ 1b,a:not x=y]
"bcbceekl"

427. Deferred

428. See 73

429. Matrix with diagonal x

q)x:5 9 6 7 2
   (2##x)#,/x,'(2##x)#0
(5 0 0 0 0
0 9 0 0 0
0 0 6 0 0
0 0 0 7 0
0 0 0 0 2)
q)x:5 9 6 7 2
q)(2#count x)#raze x,'(2#count x)#0
5 0 0 0 0
0 9 0 0 0
0 0 6 0 0
0 0 0 7 0
0 0 0 0 2

430. Polynomial derivative

q)x:1 2 3 4 5
   -1 _ x*|!#x
4 6 6 4
q)x:1 2 3 4 5
q)-1 _ x*reverse til count x
4 6 6 4

431. Does item differ from next one

q)x:"ceefffmeksc"
   (~=':x),1
1 0 1 0 0 1 1 1 1 1 1
q)x:"ceefffmeksc"
q)1_ differ x
1010011111b

432. Does item differ from previous one

q)x:"ceefffmeksc"
   1,~=':x
1 1 0 1 0 0 1 1 1 1 1
q)differ x
11010011111b

433. Replace last item of x with y

q)x:"abbccdefcdab"
q)y:"t"
   @[x;-1+#x;:;y]
"abbccdefcdat"
q)x:"abbccdefcdab"
q)y:"t"
q)@[x;-1+count x;:;y]
"abbccdefcdat"

434. Replace first item of x with y

q)x:"abbccdefcdab"
q)y:"t"
   @[x;0;:;y]
"tbbccdefcdab"
q)x:"abbccdefcdab"
q)y:"t"
q)@[x;0;:;y]
"tbbccdefcdab"

435. [deferred]

436. [deferred]

437. Remove leading zeros

q)x:"00002345600345000"
((x="0")?0)_ x
"2345600345000"
q)x:"00002345600345000"
q)((x="0")?0b) _ x
"2345600345000"

438. Index of 1st 1 following index y in x

q)x:1 0 0 1 1 0 1 1 0
q)y:3
   y+(y _ x)?1
3
   y+((y+1)_ x)?1
3
(y+1)+((y+1)_ x)?1
4
/ a more K-like alternative
   &x
0 3 4 6 7
   y<&x
0 0 1 1 1
   (&x)[*&y<&x]
4
q)x:1 0 0 1 1 0 1 1 0
q)y:3
q)y+(y _ x)?1
3
q)y+((y+1)_ x)?1
3
q)(y+1)+((y+1)_ x)?1
4
q)where x
0 3 4 6 7
q)y<where x
00111b
q)(where x)[first where y<where x]
4

439. Last 1s in groups of 1s

q)x:0 1 1 0 1 1 1 0 0 1
   x>1 _ x,0
0 0 1 0 0 0 1 0 0 1
q)x:0 1 1 0 1 1 1 0 0 1
q)x>1 _ x,0
0010001001b

440. 1st 1 in groups of 1s

q)x:0 1 1 0 1 1 1 0 0 1
   x>1 _ x,0
0 0 1 0 0 0 1 0 0 1
q)x:0 1 1 0 1 1 1 0 0 1
q)x>1 _ x,0
0010001001b
/440=439?

441. Comma separated list from table

q)x:("Swift";"Austen";"Dickens")
q)x
("Swift"
"Austen"
"Dickens")
",",'x
(",Swift"
",Austen"
",Dickens")
   ,/",",'x
",Swift,Austen,Dickens"
   1 _ ,/",",'x
"Swift,Austen,Dickens"
q)x:("Swift";"Austen";"Dickens")
q)",",x
","
"Swift"
"Austen"
"Dickens"
q)",",'x
",Swift"
",Austen"
",Dickens"
q)raze ",",'x
",Swift,Austen,Dickens"
q)1_ raze ",",'x
"Swift,Austen,Dickens"

442. 1st difference

q)x:+\1 2 3 4 5
q)x
1 3 6 10 15
   -':x
2 3 4 5
   (*x),(-':x)
1 2 3 4 5
q)x:sums 1 2 3 4 5
q)deltas x
1 2 3 4 5

443. Drop first y columns from matrix x

q)x:4 3# !12
q)x
(0 1 2
3 4 5
6 7 8
9 10 11)
q)y:2
   y _ x
(6 7 8
9 10 11)
q)x:4 3#til 12
q)y:2
q)y _ x
6 7  8
9 10 11

444. Drop first y rows from matrix x

q)x:4 3# !12
q)x
(0 1 2
3 4 5
6 7 8
9 10 11)
q)y:2
   y _' x
(,2
,5
,8
,11)
q)x:4 3#til 12
q)y:2
q)y _ 'x
2
5
8
11

445. Number of columns in matrix x

q)x:4 3# !12
q)x
(0 1 2
3 4 5
6 7 8
9 10 11)
   *|^x
3

446. Number of rows in matrix x

q)x:4 3# !12
q)x
(0 1 2
3 4 5
6 7 8
9 10 11)
   #x
4
q)x
0 1  2
3 4  5
6 7  8
9 10 11
q)x:4 3#til 12
q)count x
4

447. Conditional drop of y items from array x

q)x:4 3# !12
q)x
(0 1 2
3 4 5
6 7 8
9 10 11)
q)g:0
(y*g) _ x
(0 1 2
3 4 5
6 7 8
9 10 11)
q)g:1
(y*g) _ x
(6 7 8
9 10 11)
q)x:4 3#til 12
q)g:0
q)y:2
q)(y*g)_ x
0 1  2
3 4  5
6 7  8
9 10 11
q)g:1
q)(y*g)_ x
6 7  8
9 10 11

448. Conditional drop of last item of array x

q)x:4 3# !12
q)x
(0 1 2
3 4 5
6 7 8
9 10 11)
q)y:0
   (-y) _ x
(0 1 2
3 4 5
6 7 8
9 10 11)
q)y:1
   (-y) _ x
(0 1 2
3 4 5
6 7 8)
q)x:4 3#til 12
q)x
0 1  2
3 4  5
6 7  8
9 10 11
q)y:0
q)(neg y)_x
0 1  2
3 4  5
6 7  8
9 10 11
q)y:1
q)(neg y)_x
0 1 2
3 4 5
6 7 8

449. Limiting x between l and h, inclusive

q)x: 5 6 _draw 100
q)x
(58 9 37 84 39 99
60 30 45 97 77 35
49 87 82 79 8 30
46 61 20 51 12 34
31 51 29 35 17 89)
q)l:30
q)h:70
   l|h&x
(58 30 37 70 39 70
60 30 45 70 70 35
49 70 70 70 30 30
46 61 30 51 30 34
31 51 30 35 30 70)
q)x:(58 9 37 84 39 99;60 30 45 97 77 35;49 87 82 79 8 30;46 61 20 51 12 34;31 51 29 35 17 89)
q)l:30
q)h:70
q)l|h&x
58 30 37 70 39 70
60 30 45 70 70 35
49 70 70 70 30 30
46 61 30 51 30 34
31 51 30 35 30 70

450. Arithmetic precision of system in decimals

   log10:{(_log x)%_log 10}
   log10 3
0.47712125471966244
   _ _abs log10 _abs 1-3*%3
0I
This indicates that the precision is infinite (which isn't true)
and is a consequence of
3*%3
1.0
q)log10:{(log x)%log 10}
q)log10 3
0.4771213
q)\P 17
q)log10 3
0.47712125471966238
q)\P 0
q)log10 3
0.47712125471966238
q)abs log10 abs 1-3*reciprocal 3
0w
q)floor abs log10 abs 1-3*reciprocal 3
0W

451. Arithmetic progression from x to y with step g

   ap:{[x;y;g]x+g*!1+_(y-x)%g}
   ap[3;20;5]
3 8 13 18
   ap[3;-20;-5]
3 -2 -7 -12 -17
q)ap:{[x;y;g]x+g*til 1+ floor (y-x)%g}
q)ap[3;20;5]
3 8 13 18
q)ap[3;-20;-5]
3 -2 -7 -12 -17

452. Number of positions in integer x

   dp:{1+(x<0)+_ log10[_abs[x+0=x]]}
   dp 1234
4
   dp -1234
5
   dp 0
1
   dp 1
1
   dp 7
1
   dp 12345678
8
q)log10:{(log x)%log 10}
q)dp:{1+(x<0)+floor log10[abs[x+0=x]]}
q)dp 1234
4
q)dp -1234
5
q)dp 0
1
q)dp 7
1
q)dp 12345678
8

453. Round to nearest even integer

   re:{_ x+~1>x!2}
q)x:0.9 1 2.5 3.1 -0.2 -1.9
   re x
0 2 2 4 0 -2
q)re:{floor x+not 1>x mod 2}
q)x:0.9 1 2.5 3.1 -0.2 -1.9
q)re x
0 2 2 4 0 -2

454. Rounding, but to nearest even integer if fractional part is 0.5

   rn:{_ x+0.5*~0.5=x!2}
q)x:23.6 40.5 3.2 -14.02 3.5 4.5
   rn x
24 40 3 -14 4 4
q)rn:{floor x+0.5*not 0.5=x mod 2}
q)x:23.6 40.5 3.2 -14.02 3.5 4.5
q)rn x
24 40 3 -14 4 4

455. Number of digit positions in x

   nd:{1+_ log10(x=0)+x*1 -10[x<0]}
q)x:4 678 -21 -10854
   nd x
1 3 3 6
q)nd:{1+floor log10(x=0)+x*1 -10[x<0]}
q)x:4 678 -21 -10854
q)nd x
1 3 3 6

456. Number of digits in nonnegative integer x

   np:{1+_ log10 x+0=x}
q)x:0 13 523 16008
   np x
1 2 3 5
q)np:{1+floor log10 x+0=x}
q)x:0 13 523 16008
q)np x
1 2 3 5

457. Is x integral

   ii:{x=_ x}
q)x:67 -120 3.83 -5.5
   ii x
1 1 0 0
q)ii:{x=floor x}
q)x:67 -120 3.83 -5.5
q)ii x
1100b

458. [omitted]

459. Leading digit of numeric code abbb

   ld:{_ x%1000}
q)x:6 _draw 10000
q)x
1319 8629 6581 6988 790 9045
   ld x
1 8 6 6 0 9
q)ld:{floor x%1000}
q)x:1319 8629 6581 6988 790 9045
q)ld x
1 8 6 6 0 9

460. Round y to x decimals

   rn:{(10^-x)*_ 0.5+y*10^x}
q)y:3.3256789
q)x:3
   rn[x;y]
3.326
q)rn:{xexp[10;neg x]*`long$y*xexp[10;x]}
q)y:3.3256789
q)x:3
q)rn[x;y]
3.3259999999999978
q)\P 7
q)rn[x;y]
3.326
q)rn[2;123123123123.123123]
123123123123.12

461. Round to nearest hundredth

   rh:{0.01*_0.5+x*100}
q)x:3.1414 2.71828 -12.66666
   rh x
3.14 2.72 -12.67
q)rh:{0.01*floor 0.5+x*100}
q)x:3.1414 2.71828 -12.66666
q)rh x
3.14 2.72 -12.67

462. Round to nearest integer

   ri:{_0.5+x}
q)x:4.5 3.21 80.9 -2.4 -9.6
   ri x
5 3 81 -2 -10
q)ri:{floor 0.5+x}
q)x:4.5 3.21 80.9 -2.4 -9.6
q)ri x
5 3 81 -2 -10

463. Is x a leap year

   ly:{(+/0=x!/:4 100 400)!2}
q)x:1900 1901 1904 2000
   ly x
0 0 1 1
q)ly:{(sum 0=x mod/:4 100 400)mod 2}
q)x:1900 1901 1904 2000
q)ly x
0 0 1 1

464. Framing character matrix x

q)x:4 4#"abcdefghijklmnop"
q)x
("abcd"
"efgh"
"ijkl"
"mnop")
   +"-",'(+"|",'x,'"|"),'"-"
("------"
"|abcd|"
"|efgh|"
"|ijkl|"
"|mnop|"
"------")
q)x:4 4#"abcdefghijklmnop"
q)x
"abcd"
"efgh"
"ijkl"
"mnop"
q)flip"-",'(flip"|",'x,'"|"),'"-"
"------"
"|abcd|"
"|efgh|"
"|ijkl|"
"|mnop|"
"------"

465. Magnitude of fractional part

q)x:6.13 -6.13
   _abs[x]!1
0.13 0.13
q)abs[x] mod floor abs x
0.13 0.13

466. Remove every y-th item of x

q)x:4+!10
q)x
4 5 6 7 8 9 10 11 12 13
q)y:3
   !#x
0 1 2 3 4 5 6 7 8 9
   (!#x)!y
0 1 2 0 1 2 0 1 2 0
   ~0=(!#x)!y
0 1 1 0 1 1 0 1 1 0
   &~0=(!#x)!y
1 2 4 5 7 8
   x[&~0=(!#x)!y]
5 6 8 9 11 12
q)x:4+til 10
q)x
4 5 6 7 8 9 10 11 12 13
q)y:3
q)til count x
0 1 2 3 4 5 6 7 8 9
q)(til count x)mod y
0 1 2 0 1 2 0 1 2 0
q)0=(til count x)mod y
1001001001b
q)not 0=(til count x)mod y
0110110110b
q)where not 0=(til count x)mod y
1 2 4 5 7 8
q)x[where not 0=(til count x)mod y]
5 6 8 9 11 12

467. Select every y-th item of y

q)x:4+!10
q)x
4 5 6 7 8 9 10 11 12 13
q)y:3
   !#x
0 1 2 3 4 5 6 7 8 9
   (!#x)!y
0 1 2 0 1 2 0 1 2 0
   0=(!#x)!y
1 0 0 1 0 0 1 0 0 1
   &0=(!#x)!y
0 3 6 9
   x[&0=(!#x)!y]
4 7 10 13
q)x:4+til 10
q)x
4 5 6 7 8 9 10 11 12 13
q)y:3
q)til count x
0 1 2 3 4 5 6 7 8 9
q)(til count x)mod y
0 1 2 0 1 2 0 1 2 0
q)0=(til count x)mod y
1001001001b
q)where 0=(til count x)mod y
0 3 6 9
q)x[where 0=(til count x)mod y]
4 7 10 13

468. See dv in 164

469. Remove every second item

q)x:"abcdefghijklmn"
   (!#x)!2
0 1 0 1 0 1 0 1 0 1 0 1 0 1
   &(!#x)!2
1 3 5 7 9 11 13
   x[&(!#x)!2]
"bdfhjln"
q)x:"abcdefghijklmn"
q)(til count x)mod 2
0 1 0 1 0 1 0 1 0 1 0 1 0 1
q)where (til count x)mod 2
1 3 5 7 9 11 13
q)x[where (til count x)mod 2]
"bdfhjln"

470. Items of x divisible by y

q)x:10 _draw 100
q)x
95 33 64 10 78 1 47 20 92 95
q)y:4
   x!y
3 1 0 2 2 1 3 0 0 3
   0=x!y
0 0 1 0 0 0 0 1 1 0
   &0=x!y
2 7 8
   x[&0=x!y]
64 20 92
q)x:95 33 64 10 78 1 47 20 92 95
q)y:4
q)x mod y
3 1 0 2 2 1 3 0 0 3
q)0=x mod y
0010000110b
q)where 0=x mod y
2 7 8
q)x[where 0=x mod y]
64 20 92

471. Index of first occurrence of g in x (circularly) after y

q)x: 15 _draw 10
q)x
6 6 0 0 8 9 8 1 0 2 9 4 6 3 5
q)g:0 6 5
q)y:9
   (y+(y!x)?g)!#x
9
   (y+(y!x)?/:g)!#x
2 12 14
/not quite the same
q)x:6 6 0 0 8 9 8 1 0 2 9 4 6 3 5
q)g:0 6 5
q)y:9
q)(y+(y rotate x)?g)mod count x
2 12 14

472. Omitted

473. Is x even

q)x:1 2 3 4 5
   ~x!2
0 1 0 1 0
q)x:1 2 3 4 5
q)not x mod 2
01010b

474. Round x to zero if magnitude less than y

q)x:1e-4 -1e-8 -1e-12 1e-16
   x*~y>_abs x
0.0001 -1e-008 0 0
q)x:1e-4 -1e-8 -1e-12 1e-16
q)y:1e-9
q)x*not y>abs x
0.0001 -1e-08 -0 0
q)y>abs x
0011b
q)not y>abs x
1100b
q)x*not y>abs x
0.0001 -1e-08 -0 0
/y is unspecified and arbitrarily defined
/need to explain -0.

475. Increase absolute value without sign change

q)x:0 -1 2 -3 4 -5
   sg:{(x>0)-(x<0)}
q)y:10
   (sg x)*y+_abs x /problem with x=0
0 -11 12 -13 14 -15.0
q)x:0 -1 2 -3 4 -5
q)sg:{(x>0)-(x<0)}  /or use the builtin signum
q)y:10
q)(sg x)*y+abs x /problem with x=0
0 -11 12 -13 14 -15

476. Fractional part with sign

q)x:0.2 2.3 -0.2 -1.8 0 5 -7
   (sg x)*(_abs x)!1
0.2 0.3 -0.2 -0.8 0 0 0
q)sg:{(x>0)-(x<0)}      /or use the builtin signum
q)x:0.2 2.3 -0.2 -1.8 0 5 -7
q)(sg x)*(abs x)mod 1
0.2 0.3 -0.2 -0.8 0 0 -0

477. Square x retaining sign

q)x:0 -1 2 -3 4
   x*_abs x
0 -1 4 -9 16.0
q)x:0 -1 2 -3 4
q)x*abs x
0 -1 4 -9 16

478. Fractional part

q)x:0 1 -2 3.4 -5.6 -6.1
   x!1
0 0 0 0.4 0.4 0.9
q)x:0 1 -2 3.4 -5.6 -6.1
q)x mod 1
0 0 0 0.4 0.4 0.9

479. Last part of abbb

q)x:1234 5678 9012 345 6789
   x!1000
234 678 12 345 789
q)x:1234 5678 9012 345 6789
q)x mod 1000
234 678 12 345 789

480. Replace items of x in y by 0

q)x:1 2 3 4 5
q)y:2 4
   x _lin y
0 1 0 1 0
   x*~x _lin y
1 0 3 0 5
q)x:1 2 3 4 5
q)y:2 4
q)x in y
01010b
q)x*not x in y
1 0 3 0 5

481. Replace items of x not in y by 0

q)x:1 2 3 4 5
q)y:2 4
   x _lin y
0 1 0 1 0
   x*x _lin y
0 2 0 4 0
q)x:1 2 3 4 5
q)y:2 4
q)x in y
01010b
q)x*x in y
0 2 0 4 0

482. Merge x any under control of g

q)x:1 2 3 4 5
q)y:100 200 300 400 500
q)g:1 0 0 1 1 0 1 0 0 1
   (x,y)[<<g]
100 1 2 200 300 3 400 4 5 500
q)x:1 2 3 4 5
q)y:100 200 300 400 500
q)g:1 0 0 1 1 0 1 0 0 1
q)(x,y)[rank g]
100 1 2 200 300 3 400 4 5 500

483. See 481

484. Right to left scan

q)x:1 2 3 4 5
   |+\|x
15 14 12 9 5
q)x:1 2 3 4 5
q)reverse sums reverse x
15 14 12 9 5

485. Append empty row on matrix

   x:("ab";"cd";"ef")
   x
("ab"
"cd"
"ef")
   x,,(*|^x)#" "
("ab"
"cd"
"ef"
" ")

486. [omitted]

487. Insert empty row in x after row y

q)x:("ab";"cd";"ef")
q)x
("ab"
"cd"
"ef")
q)y:1
q)a:x,,(*|^x)#" "
q)a
("ab"
"cd"
"ef"
" ")
q)b:<(!#x),y
q)b
0 1 3 2
   a[b]
("ab"
"cd"
" "
"ef")
   (x,,(*|^x)#" ")[<(!#x),y]
("ab"
"cd"
" "
"ef")

488. Omitted

489. Make string y into table guided by marker x

q)y:"eachwordinarow"
q)x:1 0 0 0 1 0 0 0 1 0 1 1 0 0
q)a:&x
q)a
0 4 8 10 11
q)b:a _ y
q)b
("each"
"word"
"in"
,"a"
"row")
   (&x)_ y
("each"
"word"
"in"
,"a"
"row")
q)y:"eachwordinarow"
q)x:1 0 0 0 1 0 0 0 1 0 1 1 0 0
q)a:where x
q)a
0 4 8 10 11
q)b:a _ y
q)b
"each"
"word"
"in"
,"a"
"row"
q)(where x) _ y
"each"
"word"
"in"
,"a"
"row"

490. Insert spaces in text

q)x:"wider"
q)a:+,x
q)a
(,"w"
,"i"
,"d"
,"e"
,"r")
q)b:a,'" "
q)b
("w "
"i "
"d "
"e "
"r ")
   ,/b
"w i d e r "
   ,/(+,x),'" "
"w i d e r "
q)x:"wider"
q)a:flip enlist x
q)a
,"w"
,"i"
,"d"
,"e"
,"r"
q)b:a,'" "
q)b
"w "
"i "
"d "
"e "
"r "
q)raze b
"w i d e r "
q)raze(flip enlist x),'" "
"w i d e r "

491. Or-reduce infixes of y marked by x

q)x:1 0 0 1 0 0 0 1 0 0 0 0
q)y:0 0 0 0 1 0 0 0 0 0 1 0
q)a:(&x)_ y
q)a
(0 0 0
0 1 0 0
0 0 0 1 0)
   |/'a
0 1 1
q)x:1 0 0 1 0 0 0 1 0 0 0 0
q)y:0 0 0 0 1 0 0 0 0 0 1 0
q)a:(where x)_y
q)a
0 0 0
0 1 0 0
0 0 0 1 0
q)max'[a]
0 1 1

492. And-reduce infixes of y marked by x

q)x:1 0 0 1 0 0 0 1 0 0 0 0
q)y:0 1 1 1 1 1 1 1 1 1 1 0
   (&x)_ y
(0 1 1
1 1 1 1
1 1 1 1 0)
   &/'(&x)_ y
0 1 0
q)x:1 0 0 1 0 0 0 1 0 0 0 0
q)y:0 1 1 1 1 1 1 1 1 1 1 0
q)(where x)_y
0 1 1
1 1 1 1
1 1 1 1 0
q)min'[(where x)_y]
0 1 0

493. Choose x or y depending on boolean g

q)x:"abcdef"
q)y:"xyz"
q)g:0
   :[g;x;y]
"xyz"
q)g:1
   :[g;x;y]
"abcdef"
q)x:"abcdef"
q)y:"xyz"
q)g:0
q)$[g;x;y]
"xyz"
q)g:1
q)$[g;x;y]
"abcdef"

494. See 424

495. Indexes of all occurrences of y in x

q)x:"abcdefgab"
q)y:"afc*"
   x _lin y
1 0 1 0 0 1 0 1 0
   &x _lin y
0 2 5 7
q)x:"abcdefgab"
q)y:"afc*"
q)x in y
101001010b
q)where x in y
0 2 5 7

496. Remove punctuation characters

q)x:"oh! no, stop it. you will?"
q)y:",;:.!?"
   x _dvl y
"oh no stop it you will"
/not quite the same
q)x[where not x in y]
"oh no stop it you will"

497. Set union

q)x:"12345"
q)y:"4567890"
   y,x[&~x _lin y]
"4567890123"
q)x:"12345"
q)y:"4567890"
q)y,x[where not x in y]
"4567890123"

498. Set difference

q)x:"12345"
q)y:"4567890"
   x _dvl y
"123"
q)x:"12345"
q)y:"4567890"
q)x[where not x in y]
"123"

499. Rows of y starting with item of x

q)x:("abcd";"efgh";"ijkl";"mnop")
q)x
("abcd"
"efgh"
"ijkl"
"mnop")
q)y:"ai"
   x[;0] _lin y
1 0 1 0
   &x[;0] _lin y
0 2
   x[&x[;0] _lin y]
("abcd"
"ijkl")
q)x:("abcd";"efgh";"ijkl";"mnop")
q)x
"abcd"
"efgh"
"ijkl"
"mnop"
q)y:"ai"
q)x[;0] in y
1010b
q)where x[;0] in y
0 2
q)x[where x[;0] in y]
"abcd"
"ijkl"

500. Set intersection

q)x:"abcdefghijxyz"
q)y:"yacqwopzbx"
   x[&x _lin y]
"abcxyz"
q)x:"abcdefghijxyz"
q)y:"yacqwopzbx"
q)x[where x in y]
"abcxyz"

501.see 154

502. Deferred

503. Indexes of all occurrences of y in x

q)x:"abcdeabc"
q)y:"a"
   &x=y
0 5
q)x:"abcdeabc"
q)y:"a"
q)where x=y
0 5

504. Replace items of y satisfying x with g

q)x:1 0 0 0 1 0 1 1 0 1
q)y:"abcdefghij"
q)g:" "
   @[y;&x;:;g]
" bcd f i "
q)x:1 0 0 0 1 0 1 1 0 1
q)y:"abcdefghij"
q)g:" "
q)@[y;where x;:;g]
" bcd f  i "

505. See 154

506. See 41

507. Insert blank in y after mark in x

q)x:1 0 0 0 0 1 0 0
q)y:"abcdefgh"
   x#\:" "
(," "
""
""
""
""
," "
""
"")
   y,' x#\:" "
("a "
,"b"
,"c"
,"d"
,"e"
"f "
,"g"
,"h")
   ,/ y,' x#\:" "
"a bcdef gh"
q)x:1 0 0 0 0 1 0 0
q)y:"abcdefgh"
q)x#\:" "
," "
""
""
""
""
," "
""
""
q)y,' x#\:" "
"a "
,"b"
,"c"
,"d"
,"e"
"f "
,"g"
,"h"
q)raze y,' x#\:" "
"a bcdef gh"

508. Conditional text

q)x:0
   ,/((~x)#'"in"),"correct"
"incorrect"
q)x:1
   ,/((~x)#'"in"),"correct"
"correct"
q)x:0
q)raze((not x)#'"in"),"correct"
"incorrect"
q)x:1
q)raze((not x)#'"in"),"correct"
"correct"

509. Remove y from x

q)x:"abcdeabc"
q)y:"a"
   x _dv y
"bcdebc"
q)x:"abcdeabc"
q)y:"a"
q)x[where not  x in y]
"bcdebc"

510. Remove blanks

q)x:" bcde bc"
   x _dv " "
"bcdebc"
q)y:" "
q)x:" bcde bc"
q)x[where not  x in y]
"bcdebc"

511. Apply f over all of x

  x:2 3 4#1+!24
   x
((1 2 3 4
5 6 7 8
9 10 11 12)
(13 14 15 16
17 18 19 20
21 22 23 24))
   +//x
300
   ao:{[f;x]f//x}
   ao[+;x]
300
   ao[*;1.0*x]
6.204484e+023
   ao[+;-x]
-300

512. Select items of x according to markers in y

   x:2 3 4#1+!24
   x
((1 2 3 4
5 6 7 8
9 10 11 12)
(13 14 15 16
17 18 19 20
21 22 23 24))
q)y:1 0 0 1
   x[;;&y]
((1 4
5 8
9 12)
(13 16
17 20
21 24))

513. Empty matrix

   x:,!0
   x
,!0
   x
1 0

514. Apply to dimension 1 function defined on dimension 0

q)x:3 4#1+!12
q)x
(1 2 3 4
5 6 7 8
9 10 11 12)
   +/x
15 18 21 24
   +/'x
10 26 42
q)sum x
15 18 21 24
q)sum each x
10 26 42

515. Deferred

516. Multiply each column of x by y

q)x:(1 2 3 4 5 6
7 8 9 10 11 12)
q)y:10 100
   x*y
(10 20 30 40 50 60
700 800 900 1000 1100 1200)
   y*x
(10 20 30 40 50 60
700 800 900 1000 1100 1200)
q)x:(1 2 3 4 5 6;7 8 9 10 11 12)
q)y:10 100
q)x*y
10  20  30  40   50   60
700 800 900 1000 1100 1200
q)y*x
10  20  30  40   50   60
700 800 900 1000 1100 1200

517. Deferred

518. Transpose matrix x on condition y

q)x:2 3# !6
q)x
(0 1 2
3 4 5)
q)y:1
   y +:/x
(0 3
1 4
2 5)
q)y:0
   y +:/x
(0 1 2
3 4 5)
q)x:2 3#til 6
q)x
0 1 2
3 4 5
q)y:1
q)y +:/x
0 3
1 4
2 5
q)y:0
q)y+:/x
0 1 2
3 4 5

519. See 85

520. See 86

521. Matrix with x columns y

q)x:4
q)y:"abc"
   x#'y
("aaaa"
"bbbb"
"cccc")
q)x:4
q)y:"abc"
q)x#'y
"aaaa"
"bbbb"
"cccc"

522. See 80

523. Deferred

524. Deferred

525. Main diagonal

   x:(1 2 3 4
5 6 7 8
9 10 11 12)
   y:2#'!#x
   y
(0 0
1 1
2 2)
   x ./: y
1 6 11

526. See 525

527. Transpose planes of three-dimensional x

   x:2 2 2# !8
   x
((0 1
2 3)
(4 5
6 7))
   +:'x
((0 2
1 3)
(4 6
5 7))

528. Vector (cross) product

q)x:2 8 5 6 3 1 7 7 10 4
q)y:6 9 1 1 6 7 1 4 1 5
   ((1!x)*-1!y)-(-1!x)*1!y
4 28 46 -27 -41 39 45 3 -19 -58
q)x:2 8 5 6 3 1 7 7 10 4
q)y:6 9 1 1 6 7 1 4 1 5
q)((1 rotate x)*-1 rotate y)-(-1 rotate x)*1 rotate y
4 28 46 -27 -41 39 45 3 -19 -58

529. Markers for x at y

q)x:"abcdefghijklmn"
q)y:3 7 9
   @[&#x;y;:;1]
0 0 0 1 0 0 0 1 0 1 0 0 0 0
q)x:"abcdefghijklmn"
q)y:3 7 9
q)@[(count x)#0;y;:;1]
0 0 0 1 0 0 0 1 0 1 0 0 0 0

530. Index of last occurrence of y in x

q)x:10 _draw 5
q)x
3 0 4 3 1 4 4 3 3 1
q)y:4
   *|&x=y
6
q)y:3
   *|&x=y
8
q)x:3 0 4 3 1 4 4 3 3 1
q)y:4
q)last  where x=y
6
q)y:3
q)last where x=y
8

531. Replace each item of y with index of its last occurrence

q)x:"aabbbcccc"
q)y:x,"ddd"
q)x
"aabbbcccc"
q)y
"aabbbccccddd"
   (|x)?/:y
7 7 4 4 4 0 0 0 0 9 9 9
   #x
9
   (#x)-(|x)?/:y
2 2 5 5 5 9 9 9 9 0 0 0
   0|-1+(#x)-(|x)?/:y
1 1 4 4 4 8 8 8 8 0 0 0
q)x:"aabbbcccc"
q)y:x,"ddd"
q)x
"aabbbcccc"
q)y
"aabbbccccddd"
q)(reverse x)?/:y
7 7 4 4 4 0 0 0 0 9 9 9
q)count x
9
q)(count x)-(reverse x)?/:y
2 2 5 5 5 9 9 9 9 0 0 0
q)0|-1+(count x)-(reverse x)?/:y
1 1 4 4 4 8 8 8 8 0 0 0

532. Index of last occurrence of y in x, counted from the rear

q)x:8 4 9 1 5 7
q)y:8 2 3 4 9 5 7 1 10 6 8 2
   (|x)?/:y
5 6 6 4 3 1 0 2 6 6 5 6
q)x:8 4 9 1 5 7
q)y:8 2 3 4 9 5 7 1 10 6 8 2
q)(reverse x)?/:y
5 6 6 4 3 1 0 2 6 6 5 6

533. Reverse x on condition y

q)x:1 2 3 4 5
q)y:0
   y |:/x
1 2 3 4 5
q)y:1
   y |:/x
5 4 3 2 1

534. See 203

535. Avoiding parentheses using reverse

q)x:1 2 3 4 5
   (#x),1
5 1
   |1,#x
5 1
q)x:1 2 3 4 5
q)(count x),1
5 1
q)reverse 1,count x
5 1

536. Rotate rows left

q)x:3 4#1+!12
q)x
(1 2 3 4
5 6 7 8
9 10 11 12)
   1!'x
(2 3 4 1
6 7 8 5
10 11 12 9)
q)x:3 4#1+til 12
q)1 rotate 'x
2  3  4  1
6  7  8  5
10 11 12 9

537. Rotate rows right

q)x:3 4#1+!12
q)x
(1 2 3 4
5 6 7 8
9 10 11 12)
   -1!'x
(4 1 2 3
8 5 6 7
12 9 10 11)
q)x:3 4#1+til 12
q)-1 rotate 'x
4  1 2  3
8  5 6  7
12 9 10 11

538. Insert 0 in list of ones x after indexes y

q)x:1 1 1 1 1 1 1 1 1 1
q)y
1 3 7
   +,x
(,1
,1
,1
,1
,1
,1
,1
,1
,1
,1)
   @[+,x;y;{x,0}]
(,1
1 0
,1
1 0
,1
,1
,1
1 0
,1
,1)
   ,/@[+,x;y;{x,0}]
1 1 0 1 1 0 1 1 1 1 0 1 1
q)x:1 1 1 1 1 1 1 1 1 1
q)y:1 3 7
q)flip enlist x
1
1
1
1
1
1
1
1
1
1
q)@[flip enlist x;y;{x,0}]
,1
1 0
,1
1 0
,1
,1
,1
1 0
,1
,1
q)raze @[flip enlist x;y;{x,0}]
1 1 0 1 1 0 1 1 1 1 0 1 1

539. Boolean vector of length y with zeros in locations x

q)x:2 3 4 8
q)y:10
   method A
   ~(!y) _lin x
1 1 0 0 0 1 1 1 0 1
   method B
   ~@[&y;x;:;1]
1 1 0 0 0 1 1 1 0 1
q)x:2 3 4 8
q)y:10
q)not @[y#0;x;:;1]
1100011101b

540. Markers in boolean vector of length x at indexes y

q)x:10
q)y:1 3 7
   method A
  (!x) _lin y
0 1 0 1 0 0 0 1 0 0
   method B
   @[&x;y;:;1]
0 1 0 1 0 0 0 1 0 0
q)x:10
q)y:1 3 7
q)@[x#0;y;:;1]
0 1 0 1 0 0 0 1 0 0

541. Omitted

542. Omitted

543. See 540

544. Do x and y match

   x~y
q)x~y

545. Zero items of y not in x

q)y: 2 3 4 5 6 7 8 9 10 11
q)x:2 3 5 7 11
   y _lin x
1 1 0 1 0 1 0 0 0 1
   y*y _lin x
2 3 0 5 0 7 0 0 0 11
q)y: 2 3 4 5 6 7 8 9 10 11
q)x:2 3 5 7 11
q)y in x
1101010001b
q)y*y in x
2 3 0 5 0 7 0 0 0 11

546. Is count of atoms 1 (uses cs from 366)

   cs:({#,//x}
   co:{1=cs[x]}
   co[35]
1
   co[,35]
1
   co[1 1#35]
1
   co[1 1 1#35]
1
   co[1 2]
0
   co[!0]
0
q)cs:{count raze over x}
q)co:{1=cs[x]}
q)co[35]
1b
q)co[enlist 35]
1b
q)co[1 1#35]
1b
q)co[1 2]
0b
q)co[til 0]
0b

547. Is x vector

   iv:{1=#^x}
   iv[0]
0
   iv[1 2]
1
   iv[(1;2)]
1
   iv[(1 2;3 4)]
0
   iv[2 3#!6]
0
   iv[!0]
1

548. Test if empty

   ie:{0 _in ^x}
   ie 7
0
   ie 8 9
0
   ie 2 3#6
0
   ie (1 0#5)
1

549. Alphabetic comparison (depends on storage values)

   "a"<"b"
1
   "a">"b"
0
   _ic";"
59
   _ic"/"
47
   ";"<"/"
0
q)"a"<"b"
1b
q)"a">"b"
0b
q)"i"$";"
59
q)`int$"/"
47
q)";"<"/"
0b

550. See 37

551. Index of first differing item of x and y

q)x:3 1 4 1 6 0
q)y:3 1 4 1 5 9
   (~x=y)?1
4
q)(not x=y)?1b
4

552. Which items of x are not in y

q)x:2 3 4 5 6 7 8 9 10 11
q)y:2 3 5 7 11
   ~x _lin y
0 0 1 0 1 0 1 1 1 0
q)x:2 3 4 5 6 7 8 9 10 11
q)y:2 3 5 7 11
q)not x in y
0010101110b

553. See 37

554. Select from g based on index of x in y

q)g:("William Shakespeare"
> "John Milton"
> "Jonathan Swift"
> "Jane Austen"
> "John Keats"
> "Charles Dickens")
q)y:1564 1608 1667 1775 1795 1812
q)x:1775
   g[y?x]
"Jane Austen"
q)g:("William Shakespeare";"John Milton";"Jonathan Swift";"Jane Austen";"John Keats";"Charles Dickens")
q)y:1564 1608 1667 1775 1795 1812
q)x:1775
q)g[y?x]
"Jane Austen"

555. All axes of rectangular array x

q)x:2 2 2 2# !16
   !#^x
0 1 2 3

556. All indexes of vector x

q)x:2 2 2 2# !16
   !#^x
0 1 2 3

557. Arithmetic progression of y numbers from x with step g

q)x:5
q)y:8
q)g:100
   x+g*!y
5 105 205 305 405 505 605 705
   ap:{[x;g;y]x+g*!y}
   ap[x;g;y]
5 105 205 305 405 505 605 705
q)x:5
q)y:8
q)g:100
q)x+g*til y
5 105 205 305 405 505 605 705
q)ap:{[x;g;y]x+g*til y}
q)ap[x;g;y]
5 105 205 305 405 505 605 705

558. Consecutive integers from x to y

   ci:{x+!1+y-x}
   ci[5;10]
5 6 7 8 9 10
q)ci:{x+til 1+y-x}
q)ci[5;10]
5 6 7 8 9 10

559. Index of first marker in boolean x

q)x:0 0 1 0 1 0 0 1 1 0
   x?1
2
q)x?1
2

560. Omitted

561. Numeric code from character

q)x:" aA0"
   _ic[x]
32 97 65 48
q)x:" aA0"
q)"i"$x
32 97 65 48

562. Index of y in x

q)x:" abcdefgh"
q)y:"faded head"
   x?/:y
6 1 4 5 4 0 8 5 1 4
q)y:"deaf adder"
   x?/:y
4 5 1 6 0 1 4 4 5 9
q)x:" abcdefgh"
q)y:"faded head"
q)x?/:y
6 1 4 5 4 0 8 5 1 4
q)y:"deaf adder"
q)x?/:y
4 5 1 6 0 1 4 4 5 9

563. Empty vector

!0
!0
!0
,0
""
""
""
,0

564. Is x within range ( y[0],y[1] )

   ci:{(y[0]<x)&(x<y[1])}
q)y:3 8
q)x:5
   ci[x;y]
1
q)x:3
   ci[x;y]
0
q)x:2
   ci[x;y]
0
q)x:8
   ci[x;y]
0
q)x:9
   ci[x;y]
0
q)ci:{(y[0]<x)&(x<y[1])}
q)y:3 8
q)x:5
q)ci[x;y]
1b
q)x:3
q)ci[x;y]
0b
q)x:2
q)ci[x;y]
0b
q)x:8
q)ci[x;y]
0b
q)x:9
q)ci[x;y]
0b

565. Is x within range [ y[0],y[1] ]

   oi:{(~y[0]>x)&(~x>y[1])}
q)y:3 8
q)x:5
   oi[x;y]
1
q)x:3
   oi[x;y]
1
q)x:2
   oi[x;y]
0
q)x:8
   oi[x;y]
1
q)x:9
   oi[x;y]
0
q)oi:{(not y[0]>x)&(not x>y[1])}
q)y:3 8
q)x:5
q)oi[x;y]
1b
q)x:3
q)oi[x;y]
1b
q)x:2
q)oi[x;y]
0b
q)x:8
q)oi[x;y]
1b
q)x:9
q)oi[x;y]
0b

566. Zero all items of boolean x

q)x:0 1 0 1 1 0 0 1 1 1 0
0&x
0 0 0 0 0 0 0 0 0 0 0
q)x:0 1 0 1 1 0 0 1 1 1 0
q)0&x
0 0 0 0 0 0 0 0 0 0 0

567. Select x or y depending on g

q)x:`hot `white `short `old
q)y:`cold `black `tall `young
q)g:1 0 0 1
   x,'y
(`hot `cold
`white `black
`short `tall
`old `young)
   (!#g),'g
(0 1
1 0
2 0
3 1)
   (x,'y) ./: (!#g),'g
`cold `white `short `young
q)x:`hot`white`short`old
/short is a reserved name (but it is not a problem here)
q)y:`cold`black`tall`young
q)g:1 0 0 1
q)x,'y
hot   cold
white black
short  tall
old   young
q)(key count g),'g
0 1
1 0
2 0
3 1
q)(x,'y)./:(key count g),'g
`cold`white`short`young
q)(x,'y)@'g /more simple
`cold`white`short`young
q)?["b"$g;y;x] /or to use the builtin construct for it
`cold`white`short`young

568. Omitted

569. Change y to one if x

q)y:10 5 7 12 20.0
q)x:0 1 0 1 1
   y^~x
10 1 7 1 1.0
alternatively
   @[y;&x;:;1]
10 1 7 1 1
q)y:10 5 7 12 20.0
q)x:0 1 0 1 1
q)@[y;where x;:;1.]
10 1 7 1 1f
/note that 1. --otherwise 'type as 9h=type y

570. X implies y

q)x:0 1 0 1
q)y:0 0 1 1
   ~x>y
1 0 1 1
q)x:0 1  0 1
q)y:0 0 1 1
q)not x>y
1011b

571. X but not y

q)x:0 1 0 1
q)y:0 0 1 1
   x>y
0 1 0 0
q)x:0 1  0 1
q)y:0 0 1 1
q)x>y
0100b

572. Division by 0

   dz:{(~0=x)*y%x+x=0}
q)y:10 15 -20
q)x:2 0 0
   y%x
5 0i -0i
   dz[x;y]
5 0 0.0
q)dz:{(not 0=x)*y%x+x=0}
q)y:10 15 -20
q)x:2 0 0
q)y%x
5 0w -0w
q)dz[x;y]
5 0 -0f

573. Exclusive or

q)x:0 0 1 1
q)y:0 1 0 1
   ~x=y
0 1 1 0
q)x:0 0 1 1
q)y:0 1 0 1
q)not x=y
0110b

574. Y where x is 0

q)x:0 7 8 0 2
q)y:10 4 6 7 3
   x+y*x=0
10 7 8 7 2
q)x:0 7 8 0 2
q)y:10 4 6 7 3
q)x+y*x=0
10 7 8 7 2

575. Kronecker delta of x and y

q)x:0 0 1 1
q)y:0 1 0 1
   x=y
1 0 0 1
q)x:0 0 1 1
q)y:0 1 0 1
q)x=y
1001b

576. Append y items g before each item of x

q)x:1 3 5
q)y:2
q)g:10
   y#g
10 10
   (y#g),/:x
(10 10 1
10 10 3
10 10 5)
   ,/(y#g),/:x
10 10 1 10 10 3 10 10 5
q)x:1 3 5
q)y:2
q)g:10
q)y#g
10 10
q)(y#g),/:x
10 10 1
10 10 3
10 10 5
q)raze (y#g),/:x
10 10 1 10 10 3 10 10 5

577. Append y items g after each item of x

q)x:1 3 5
q)y:2
q)g:10
   ,/x,\:y#g
1 10 10 3 10 10 5 10 10
q)raze x,\:y#g
1 10 10 3 10 10 5 10 10

578. Merge items from x and y alternately

q)x:1 3 5 7
q)y:2 4 6 8
   ,/x,'y
1 2 3 4 5 6 7 8
q)raze x,'y
1 2 3 4 5 6 7 8

579. Variable length lines

q)x:"by and by"
q)y:"God caught his eye"
   ,(x;y)
,("by and by"
"God caught his eye")
q)(x;y)
"by and by"
"God caught his eye"

580. See 490

581. Insert y after each item of x

q)x:"abc"
q)y:"d"
   ,/x,'y
"adbdcd"
q)x:"abc"
q)y:"d"
q)raze x,'y
"adbdcd"

582. See 197

583. Array and its negative

q)x:1 -3 5
   x,'-x
(1 -1
-3 3
5 -5)
q)x:1 -3 5
q)x,'neg x
1  -1
-3 3
5  -5

584. Omitted

585. Omitted

586. Omitted

587. First column as a matrix

q)x:(0 1 2 3
4 5 6 7
8 9 10 11)
   x[;,0]
(,0
,4
,8)
q)x[;enlist 0]
0
4
8

588. 2-row matrix from two vectors

q)x:"abcd"
q)y:"efgh"
   (,x),(,y)
("abcd"
"efgh")
q)x:"abcd"
q)y:"efgh"
q)(enlist x),(enlist y)
"abcd"
"efgh"

589. 2-column matrix from two vectors

q)x:"abcd"
q)y:"efgh"
   x,'y
("ae"
"bf"
"cg"
"dh")
q)x,'y
"ae"
"bf"
"cg"
"dh"

590. Increasing rank of y to rank of x

   x:("abcd"
> "efgh")
   x
("abcd"
"efgh")
   y:"ijkl"
   #^x
2
   #^y
1
   #^,y
2
   ((#^x)-#^y),:/y
,"ijkl"

591. Reshape vector x into 2-column matrix

q)x:"abcdefgh"
   ((_ 0.5*#x),2)#x
("ab"
"cd"
"ef"
"gh")
q)x:"abcdefghi"
q)((floor 0.5*count x),2)#x
"ab"
"cd"
"ef"
"gh"
q)((floor 0.5*count x),2)#x:"ab"
"ab"
q)((floor 0.5*count x),2)#x:"abcd"
"ab"
"cd"
q)((floor 0.5*count x),2)#x:"abcde"
"ab"
"cd"
q)((floor 0.5*count x),2)#x:"abcdef"
"ab"
"cd"
"ef"
q)((floor 0.5*count x),2)#x:"abcdefgh"
"ab"
"cd"
"ef"
"gh"

592. Vector from array

   x:2 1 2 1 2 1# !8
   x
2 1 2 1 2 1
   ,//x
0 1 2 3 4 5 6 7
   ,//x
,8

593. Matrix of y rows, each x

q)x:"abcd"
q)y:3
   y#,x
("abcd"
"abcd"
"abcd")
q)x:"abcd"
q)y:3
q)y#enlist x
"abcd"
"abcd"
"abcd"

594. See 203

595. One-row matrix from vector

   x:2 3 5 7 11
   x
2 3 5 7 11
   ^x
,5
   ,x
,2 3 5 7 11
   ^,x
1 5

596. See 366

597. Omitted

598. Omitted

599. Number of columns in array x

q)x:1 1 1 1 1 678#0
   ^x
1 1 1 1 1 678
   *|^x
678

600. Number of columns in matrix x

   x:3 19#0
   ^x
3 19
   (^x)[1]
19

601. Number of rows in matrix x

   x:17 2#0
   #x
17
q)count x
17

602. Choosing according to sign

   sg:{(x>0)-(x<0)}
   sg -4.5 0 6.78
-1 0 1
   y:"-0+"
   x:-54
   y[1+sg[x]]
"-"
   x:0
   y[1+sg[x]]
"0"
   x:1234.5
   y[1+sg[x]]
"+"
q)signum -4.5 0 6.8
q)y[1+signum[x]]
"-"
q)x:0
q)y[1+signum[x]]
"0"
q)x:1234.5
q)y[1+signum[x]]
"+"

603. Conditional change of sign

   y:1+!6
   y
1 2 3 4 5 6
   x:0 1 0 1 1 0
   y*1 -1[x]
1 -2 3 -4 -5 6
q)y:1+til 6
q)y
1 2 3 4 5 6
q)x:0 1 0 1 1 0
q)y*1 -1[x]
1 -2 3 -4 -5 6

604. Omitted

605. Indexing plotting characters with boolean index

   x:~3 6 5 7 2<\:1+!7
   x
(1 1 1 0 0 0 0
1 1 1 1 1 1 0
1 1 1 1 1 0 0
1 1 1 1 1 1 1
1 1 0 0 0 0 0)
   " *"[x]
("*** "
"****** "
"***** "
"*******"
"** ")
q)x:not 3 6 5 7 2<\:1+til 7
q)x
1110000b
1111110b
1111100b
1111111b
1100000b
q)" *"[x]
"***    "
"****** "
"*****  "
"*******"
"**     "

606. Omitted

607. Vector from column of matrix

   x:3 4# !12
   x
(0 1 2 3
4 5 6 7
8 9 10 11)
   x[;0]
0 4 8
q)x:3 4#til 12
q)x
0 1 2  3
4 5 6  7
8 9 10 11
q)x[;0]
0 4 8

608. Zeroing an array

q)x:1 2 3 4
q)x
1 2 3 4
   x[]:0
q)x
0 0 0 0
q)x:2 3#!6
q)x
(0 1 2
3 4 5)
   x[;]:0
q)x
(0 0 0
0 0 0)
q)x:9
q)x
9
q)x:0
q)x
0
q)x:3 4#til 12
q)x
0 1 2  3
4 5 6  7
8 9 10 11
q)x[;0]
0 4 8
q)x:1 2 3 4
q)x
1 2 3 4
q)x[]:0
q)x
0 0 0 0
q)x:2 3#til 6
q)x
0 1 2
3 4 5
q)x[;]:0
q)x
0 0 0
0 0 0
q)x:9
q)x
9
q)x:0

609. Omitted

610. Y cyclic repetitions of vector x

q)x:"abcd"
q)y:3
   (y*#x)#x
"abcdabcdabcd"
q)(y*count x)#x
"abcdabcdabcd"

611. Multiply each row of x by vector y

q)x:3 4#1+!12
q)x
(1 2 3 4
5 6 7 8
9 10 11 12)
q)y:1 10 100 10000
   x*\:y
(1 20 300 40000
5 60 700 80000
9 100 1100 120000)
q)x*\:y
1 20  300  40000
5 60  700  80000
9 100 1100 120000

612. Rank of array y (number of dimensions)

q)x:2 1 2 1 3 1 4#0
   #^x
7
   #^9
0
   #^7 8 9
1
   #^(1 2 3;4 5 6)
2
q)rank(1 2 3;4 5 6)
0 1
q)count rank(1 2 3;4 5 6)
2

613. See 441

614. Array with shape of y and x as its rows

q)y:3 4# !12
q)y
(0 1 2 3
4 5 6 7
8 9 10 11)
q)x:"abcd"
   (^y)#x
("abcd"
"abcd"
"abcd")
/not quite the same
q)y:3 4#til 12
q)x:"abcd"
q)3 4#x
"abcd"
"abcd"
"abcd"

615. First atom in x

q)x:2 2 2 2# !8
q)x
(((0 1
2 3)
(4 5
6 7))
((0 1
2 3)
(4 5
6 7)))
   *//x
0
/not quite the same
q)x:4 2#(4 2#til 8)
q)x
0 1 2 3
4 5 6 7
0 1 2 3
4 5 6 7
q)x 0
0 1
2 3
q)x 1
4 5
6 7
q)x 2
0 1
2 3
q)x 3
4 5
6 7
q)prd/[x]
0

616. Scalar from one-item vector

q)x:,8
q)x
,8
   ""#x
8
   x[0]
8
*x
8
q)x 0
8
q)first x
8

617. Omitted

618. Deferred

619. Deferred

620. Omitted

621. See 583

622. Retain value of items marked by y, zero others

q)x:3 7 15 1 292
q)y:1 0 1 1 0
   x*y
3 0 15 1 0
q)x:3 7 15 1 292
q)y:1 0 1 1 0
q)x*y
3 0 15 1 0

623. Conditional change of sign

q)x:-9
q)y:0
   x*-1^y
-9.0
q)y:1
   x*-1^y
9.0
/^ is very different between old k2 k3 and k4/q?
q)x:-9
q)y:0
q)x*-1^y
0
q)y:1
q)x*-1^y
-9

624. Zero numerical array

q)x:2 3#99
q)x
(99 99 99
99 99 99)
   x*0
(0 0 0
0 0 0)
q)x*0
0 0 0
0 0 0

625. Omitted

626. Omitted

627. Omitted

628. Omitted

629. Error to stop execution

&`

630. Omitted

631. Omitted

1001. Payback

cumulative accumulation factors (see Zark APL Tutor News 1998 Quarter 2)

   caf:{[W;R]*\(#W)#1+R}
   pay:{[B;T;R;W]C*B-+\W%(#W)#T _ 1,C:caf[W;R]}
The version pay2 replaces T by END, and permits END to be any value between 0 and 1.
   pay2:{[B;END;R;W]CPA:*\A:1+R
   CPA*B-+\W%CPA%A*1-END}
B initial balance
T time of withdrawal: 0 start of period, 1 end
R interest rate per period
W withdrawal amount
   B:1000
   T:0
   R:0.05
   W:200 300 400 200
   pay[B;T;.R;W]
840 567 175.35 -25.8825
   T:1
   pay[B;T;.R;W]
850 592.5 222.125 33.23125
   R:0.05 0.04 0.06 0.05
   pay[B;T;R;W]
840 561.6 171.296 -30.1392
   T:1
   pay[B;T;R;W]
850 584 219.04 29.992
q)caf:{[W;R]prds(count W)#1+R}
q)pay:{[B;T;R;W]C*B-sums W%(count W)#T _ 1,C:caf[W;R]}
q)pay2:{[B;END;R;W]CPA:prds A:1+R;CPA*B-sums W%CPA%A*1-END}
q)B:1000
q)T:0
q)R:0.05
q)W:200 300 400 200
q)pay[B;T;R;W]
840 567 175.35 -25.8825
q)T:1
q)pay[B;T;R;W]
850 592.5 222.125 33.23125
q)R:0.05 0.04 0.06 0.05
q)T:0
q)pay[B;T;R;W]
840 561.6 171.296 -30.1392
q)T:1
q)pay[B;T;R;W]
850 584 219.04 29.992

1002. Round summands

ensure sum of rounded summands matches round of sum

   f:{y%:x
     i:_y
     x*@[i;{(_.5+/x)#>x}[y-i];+;1]}
   y:42.35 38.45 19.20
   _.5++/y
100
   _.5+y
42 38 19
   +/_.5+y
99
   x:1
   z:f[x;y]
   z
42 39 19
   +/z
100
   y:42.65 37.60 19.75
   _.5++/y
100
   _.5+y
43 38 20
   +/_.5+y
101
   z:f[x;y]
   z
43 37 20
   +/z
100

1003. Maximum sum of infixes

q)f:{|/0(0|+)\x}
q)x:-100 2 3 4 -100 6 7 8 9 -100
f x
30
/need more study
q)f:{max 0 (0|+)\x}
q)x:-100 2 3 4 -100 6 7 8 9 -100
q)f x
30

1004. Range union

   i:(1 3;8 10;11 12;2 4)
   / given ordered (lefts;rights)
   / interval 0 and where left is greater than 1+ max previous right
   f:{(x i;1!y i:0,&x>1+y:-1!|\y)}
   +f .+{x@<x}i /flip f apply flip sort i
(1 4;8 12)
q)f:{(x i;1 rotate y i:0,where x>1+y:-1 rotate(|\)y)}
q)flip f . flip asc i
1 4
8 12

1005. Pointer chasing

For r a primitive root of prime p, the additive list formed by (r*!p)!p has an interesting property, first discussed by August Crelle in the early 19th century. For example, if we take such a list for the primitive root 3 of 7:

a:(3*!7)!7
/ list of successive sums of 3, starting from 0, mod 7:
a
0 3 6 2 5 1 4
then if we treat the items of this list as pointers, and write
a\1
1 3 2 6 4 5
we find that the new list is the successive powers of 3, mod 7.
q)a:(3*til 7)mod 7
q)a
0 3 6 2 5 1 4
q)a\[1]
1 3 2 6 4 5

1006. Partitions of y with no part less than x

   part:{t:x _!1+_ y%2
     (,/t,''t _f'y-t),y}
   part[3;10]
(3 3 4
3 7
4 6
5 5
10)
   part[1;6]
(1 1 1 1 1 1
1 1 1 1 2
1 1 1 3
1 1 2 2
1 1 4
1 2 3
1 5
2 2 2
2 4
3 3
6)
   #:'part[1]'1+!10
1 2 3 5 7 11 15 22 30 42
q)part:{t:x _ til 1+ floor y%2;(raze t,''t part'y-t),y}
q)part[3;10]
3 3 4
3 7
4 6
5 5
10
q)part[1;6]
1 1 1 1 1 1
1 1 1 1 2
1 1 1 3
1 1 2 2
1 1 4
1 2 3
1 5
2 2 2
2 4
3 3
6
q)count each part[1]'[1+til 10]
1 2 3 5 7 11 15 22 30 42

1007. Pascal's triangle

   pt:{+':0,x,0}
   4 pt\1
(1
1 1
1 2 1
1 3 3 1
1 4 6 4 1)
   pt pt pt pt 1
1 4 6 4 1
   4 pt/1
1 4 6 4 1
/not quite the same
q)pt:{+':[0,x,0]}
q)pt pt pt pt 1
0 0 0 0 1 4 6 4 1
q)4 pt/1
0 0 0 0 1 4 6 4 1
q)4 pt\1
1
0 1 1
0 0 1 2 1
0 0 0 1 3 3 1
0 0 0 0 1 4 6 4 1
/ this version gives required output
q)pt:{0+':x,0}
q)4 pt\ 1
1
1 1
1 2 1
1 3 3 1
1 4 6 4 1

1008. Polygon area

   area: 0.5*+/{-/y*|x}':

The dyad {-/y*|x} yields the determinant of a 2-by-2 matrix. The dyad area yields the area of a polygon whose x-y coordinates are, in order, the rows of a 2-column matrix, where the last row is the same as the first row.

   x:(7 2;10 5;6 8;3 6;4 3;7 2)
   area x
24.5

1009. Great circle distance

   gcd:{_cos?(*/_sin x)+(*/_cos x)*_cos[-/y]}

The great circle distance in radians between two points on a sphere whose latitudes in radians are in x and longitudes in radians are in y.

1010. Nautical miles from radians

   nmr:{x*180*60%3.141592653589798238}
q)nmr:{x*180*60%3.141592653589798238}

1011. Degrees from degrees and minutes

   dfdm:{+/x%1 60}
q)dfdm:{sum x%1 60}
q)dfdm 60 0
60f
q)dfdm 60 3
60.05
q)dfdm 60 10
60.16667
q)dfdm 60 30
60.5

1012. Fibonacci numbers

   fn:{x{x,+/-2#x}/0 1}
   fn:{x(|+\)\1 1.0}
q)fn:{x{x,sum -2#x}/0 1}

1013. Tree from depth;value

   tdv:{[d;v](1#v),(c _ d-1)_f'(c:&1=d)_ v}
q)tdv:{[d;v](1#v),(c _ d-1)tdv'(c:where 1=d)_ v}

1014. Depth from tree

   dt:{0,/1+_f'1_ x}
q)dt:{0,/1+dt'[1_ x]}

1015. Value from tree

   vt:{(1#x),/_f'1_ x}
   vt:{(1#x),/vt'[1_ x]}
   d:0 1 2 2 1 1
   v:0 1 2 3 4 5
   t:tdv[d;v]
   t
(0
(1
,2
,3)
,4
,5)
   dt t
0 1 2 2 1 1
   vt t
0 1 2 3 4 5
tdv[d;v]
(0;(1;,2;,3);,4;,5)
q)dt t
0 1 2 2 1 1
q)vt t
0 1 2 3 4 5

These 3 recursions stop when they run out of data

1016. Streak of numbers with same sign

q)f:{1+(x;0)y}\[1;]differ signum@
q)n
2 3 4 -2 -7 1 4 2
q)f n
1 2 3 1 2 1 2 3

1017. Permutations

   perm:{(1 0#x){,/(.q.rotate 1)\'x,'y}/x}
   perm`a`b`c
(`a`b`c;`b`c`a;`c`a`b;`b`a`c;`a`c`b;`c`b`a)
q)perm:{(1 0#x){raze(1 rotate)scan'x,'y}/x}
q)perm`a`b`c
a b c
b c a
c a b
b a c
a c b
c b a
q)

translated from Kevin Lawler's k3 impl.