# Randomness

## rand¶

Syntax: rand x

Where x is

• a positive numeric atom, returns a numeric atom in the range [0,x).

q)rand 100
10
q)rand each 20#6  /roll twenty 6-sided dice
2 5 4 5 1 0 5 2 4 5 1 2 0 1 1 2 1 0 0 5
q)rand 3.14159
1.277572
q)rand 2012.09.12
2008.02.04


• a list, returns an item chosen randomly from x

q)rand 1 30 45 32
32


First roll

rand is exactly equivalent to {first 1?x}. If you need a vector result, consider using roll instead of rand. The following expressions all roll 100 six-sided dice.

rand each 100#6
{first 1?x} each 6
100?6


## ? (roll)¶

Syntax: x ? y

Roll returns a list of random selections. Where x is a positive int atom and

• y is a numeric atom, returns x values randomly chosen from the range til y.
q)20?5
4 3 3 4 1 2 2 0 1 3 1 4 0 2 2 1 4 4 2 4
q)5?4.5
3.13239 1.699364 2.898484 1.334554 3.085937
q)4?2012.09m
2006.02 2007.07 2007.07 2008.06m


Short symbols

There is a shorthand special case for generating short symbols (length between 1 and 8) using the first 16 lower-case letters of the alphabet.

q)10?3
bondecneijempgmkeilpnbjhfljnpo
q)rand 6


• y is a list, returns x items randomly chosen from y.
q)10?ArthurSteveDennis
ArthurArthurSteveDennisArthurArthurArthurDennisArthurDennis
q)2?("a";0101b;abc;thequick;2012.06m)
abc
2012.06m


## ? (deal)¶

Syntax: -x ? y

Deal returns random selections without repetition. Where -x is a negative int atom, and

• y is an int atom such that y>=x, returns x integers chosen randomly and without repetition from the range til y.
q)-20?100  /20 different integers from 0-99
2 40 66 52 86 45 94 84 38 26 33 23 78 49 51 59 44 37 48 53
q)-20?20.  /first 20 integers in random order
10 19 2 6 17 16 14 8 3 12 13 1 5 11 4 9 18 15 0 7
q)(asc -20?20)~asc -20?20
1b


GUIDs

To deal a unique GUID, use the null GUID for y.

q)-1?0Ng
,fd2db048-decb-0008-0176-01714e5eeced


• y is a list of unique values, and x>=count y, returns x items randomly chosen without repetition from y.
q)-3?ArthurSteveDennis
SteveArthurDennis
q)-4?ArthurSteveDennis
'length


## Sowing the seed¶

Deal, rand and roll use a constant seed on q invocation: scripts using them can be repeated with the same results. You can see and change the value of the seed by using system command "\S".)

Warning

To use GUIDs as identifiers, ensure x is negative. Otherwise, you will get duplicates, given the same seed:

$q q)1?0Ng ,8c6b8b64-6815-6084-0a3e-178401251b68 q)\\  $q

q)1?0Ng
,8c6b8b64-6815-6084-0a3e-178401251b68
`