? Roll, Deal, Permute
Random lists, with or without duplicates
Roll and Deal
Select items randomly, generate random values
x?y ?[x;y] / Roll
neg[x]?y ?[neg[x];y] / Deal
Select
Where
xis an int atomyis a list
returns abs[x] randomly selected items of y.
Where x is
- positive items are selected independently (Roll)
- negative and
x>=neg count y, items are selected from different indexes ofy(Deal)
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
Duplicate items in y
If y contains duplicate items, so may the result of Deal.
q)-2?`bye`bye`blackbird
`bye`bye
Generate
Where
xis an int atomyis an atom > 0
returns a list of abs[x] items of the same type as y, generated as follows
right domain (y) range operator
----------------------------------------------------------------
integer >0 til y Roll, Deal
0Ng GUIDs Roll, Deal
float, temporal ≥0 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) Roll
from abcdefghijklmnop
Where x is negative (Deal), y must have integer or GUID type
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
Roll and Deal return list results
For an atom result, instead of first 1?x, use rand.
Deal of GUID atom
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 / Deal one GUID ten times
5
The range of GUIDs is large enough that Roll and Deal often return the same result.
q)count distinct 1000000000?0Ng / Roll a billion GUIDs
1000000000
For a set of distinct GUIDs, use Deal to generate them in one operation
Permute
0N?x
Where x is
- a non-negative int atom, returns the items of
til xin random order - a list, returns the items of
xin 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) |