Pick random element
How does one pick a random element from a list?
C
<lang C>#include <stdio.h>
- include <stdlib.h>
- include <time.h>
int main(){
char array[] = { 'a', 'b', 'c' };
srand( time( 0 ) );
printf( "%c\n", array[ rand() % 3 ] );
return 0;
}</lang>
D
<lang D>import std.stdio, std.random;
void main() {
auto items = ["foo", "bar", "baz"]; auto r = items[uniform(0, items.length)]; writeln(r);
}</lang>
Haskell
Creating a custom function:
<lang haskell>import Random (randomRIO)
pick :: [a] -> IO a pick xs = randomRIO (0, length xs - 1) >>= return . (xs !!)
x <- pick [1 2 3]</lang>
Using the random-extras library:
<lang haskell>import Data.Random import Data.Random.Source.DevRandom import Data.Random.Extras
x <- runRVar (choice [1 2 3]) DevRandom</lang>
Icon and Unicon
The unary operator '?' selects a random element from its argument which may be a string, list, table, or set.
<lang Icon>procedure main()
L := [1,2,3] # a list x := ?L # random element
end</lang>
Java
<lang java>import java.util.Random; ... int[] array = {1,2,3}; return array[new Random().nextInt(array.length)]; // if done multiple times, the Random object should be re-used</lang>
For a List object rather than an array, substitute list.get(...) for array[...]. If preserving the order of the List isn't important, you could call Collections.shuffle(list); and then list.get(0);. You would need to shuffle each time unless you removed the item from the list.
JavaScript
<lang javascript>var array = [1,2,3]; return array[Math.floor(Math.random() * array.length)];</lang>
Liberty BASIC
The natural way to hold an array of text is in a space- or comma-delimited string, although an array could be used. <lang lb> list$ ="John Paul George Ringo Peter Paul Mary Obama Putin" wantedTerm =int( 10 *rnd( 1)) print "Selecting term "; wantedTerm; " in the list, which was "; word$( list$, wantedTerm, " ") </lang>
Selecting term 5 in the list, which was Peter
OCaml
<lang ocaml>let list_rand lst =
let len = List.length lst in List.nth lst (Random.int len)</lang>
# list_rand [1;2;3;4;5] ;; - : int = 3
PARI/GP
<lang parigp>pick(v)=v[random(#v)+1]</lang>
PHP
<lang php>$arr = array('foo', 'bar', 'baz'); $x = $arr[array_rand($arr)];</lang>
Perl
<lang perl>my @array = ('a', 'b', 'c'); print $array[ rand @array ];</lang>
Perl 6
Perl 6 has two functions to return random elements depending on whether you are doing selection with or without replacement.
Selection with replacement: (roll of a die) <lang perl6>(1..6).roll; # return 1 random value in the range 1 through 6 (1..6).roll(3); # return a list of 3 random values in the range 1 through 6 (1..6).roll(*); # return a lazy infinite list of random values in the range 1 through 6</lang>
Selection without replacement: (pick a card from a deck) <lang perl6>( 2..9, <J Q K A> X~ <♠ ♣ ♥ ♦> ).pick; # Pick a card ( 2..9, <J Q K A> X~ <♠ ♣ ♥ ♦> ).pick(5); # Draw 5 ( 2..9, <J Q K A> X~ <♠ ♣ ♥ ♦> ).pick(*); # Get a shuffled deck</lang>
PicoLisp
<lang PicoLisp>(get Lst (rand 1 (length Lst)))</lang>
Python
<lang python>>>> import random >>> random.choice(['foo', 'bar', 'baz']) 'baz'</lang>
Ruby
<lang ruby>irb(main):001:0> %w(north east south west).sample => "west" irb(main):002:0> (1..100).to_a.sample(2) => [17, 79]</lang>
<lang ruby>irb(main):001:0> %w(north east south west).choice => "south"</lang>
Smalltalk
<lang smalltalk>x := #(1 2 3) atRandom.</lang>
Tcl
Random selection from a list is implemented by composing lindex (for selection of an item from a list) and the pattern for generating an integral random number from the range . It's simpler to use when wrapped up as a helper procedure:
<lang tcl>proc randelem {list} {
lindex $list [expr {int(rand()*[llength $list])}]
} set x [randelem {1 2 3 4 5}]</lang>