? Roll, Deal, Permute

Random selections, with or without duplicates

Roll and Deal

A list of random selections.

Syntax: x?y, ?[x;y] (Roll)
Syntax: neg[x]?y, ?[neg[x];y] (Deal)

Where x is an int atom, returns a list of x randomly selected items, without duplication if x is negative.

Where y is

• a list, the result items are items of y

  
      q)5?`Arthur`Steve`Dennis
      `Arthur`Arthur`Steve`Dennis`Arthur
      q)2?("a";0101b;`abc;`the`quick;2012.06m)
      `abc
      2012.06m
      q)-3?`the`quick`brown`fox
      `brown`quick`fox
      

• an atom, the result items have the same type as y and are generated as follows

y	range	operator
integer	til y	Roll, Deal
0Ng	GUIDs	Roll, Deal
float, temporal	0 to y	Roll
0i	ints	Roll
0	longs	Roll
0b	01b	Roll
" "	.Q.a	Roll
0x0	bytes	Roll
numeric symbol `n	symbols, each of n chars (n≤8) from abcdefghijklmnop	Roll

q)10?5                                        / roll 10 (5-sided dice)
4 2 1 1 3 2 0 0 2 2   
q)-5?20                                       / deal 5 
13 11 8 12 19   
q)-10?10                                      / first 10 ints in random order
9 3 5 7 2 0 6 1 4 8
q)(asc -10?10)~asc -10?10
1b

q)-1?0Ng                                      / deal 1 GUID
,fd2db048-decb-0008-0176-01714e5eeced
q)count distinct -1000?0Ng                    / deal 1000 GUIDs
1000

q)5?4.5                                       / roll floats
3.13239 1.699364 2.898484 1.334554 3.085937 

q)4?2012.09m                                  / roll months    
2006.02 2007.07 2007.07 2008.06m

q)30?" "
"tusrgoufcetphltnkegcflrunpornt"

q)16?0x0                                      / roll 16 bytes
0x8c6b8b64681560840a3e178401251b68

q)20?0b                                       / roll booleans
00000110010101000100b

q)10?`3                                       / roll short symbols
`bon`dec`nei`jem`pgm`kei`lpn`bjh`flj`npo
q)rand `6
`nemoad

Successive calls of Deal

Deal of GUID uses a mix of process ID, current time and IP address to generate the GUID, and successive calls may not allow enough time for the current time reading to change.

q)count distinct {-1?0ng}each til 10
5

Permute

Syntax: 0N?x

Where x is

• a non-negative int atom, returns the items of til x in random order
• a list, returns the items of x in random order

(Since V3.3.)

q)0N?10                         / permute til 10
8 2 4 1 6 0 5 3 7 9
q)0N?5 4 2                      / permute items
4 5 2
q)0N?"abc"                      / permute items
"bac"
q)0N?("the";1 2 4;`ibm`goog)    / permute items
`ibm`goog
1 2 4
"the"

Seed

Deal, Roll, Permute and rand use a constant seed on kdb+ startup: scripts using them can be repeated with the same results. You can see and set the value of the seed with system command \S.)

To use GUIDs as identifiers, use Deal, not Roll.

$ q
..
q)1?0Ng                                    / roll 1 GUID
,8c6b8b64-6815-6084-0a3e-178401251b68
q)\\
$ q
..
q)1?0Ng                                    / roll 1 GUID
,8c6b8b64-6815-6084-0a3e-178401251b68
q)\\
$ q
..
q)-1?0Ng                                   / deal 1 GUID
,2afe0040-2a1b-bfce-ef3e-7160260cf992
q)\\
$ q
..
q)-1?0Ng                                   / deal 1 GUID
,753a8739-aa6b-3cb4-2e31-0fcdf20fd2f0

Roll uses the current seed (\S 0N). Deal uses a seed based on process properties and the current time. This means -10?0Ng is different from {first -1?0Ng}each til 10.

Errors

error	cause
length	neg x exceeds count y
type	x is negative (Roll only)

rand