Pick random element

Demonstrate how to pick a random element from a list.

ACL2

<lang Lisp>:set-state-ok t

(defun pick-random-element (xs state)

  (mv-let (idx state)
          (random$ (len xs) state)
     (mv (nth idx xs) state)))</lang>

Ada

The following program generates three 20-letter words. Each vowel and each consonant is picked randomly from a list of vowels resp. a list of consonants.

<lang Ada>with Ada.Text_IO, Ada.Numerics.Float_Random;

procedure Pick_Random_Element is

  package Rnd renames Ada.Numerics.Float_Random;
  Gen: Rnd.Generator; -- used globally

  type Char_Arr is array (Natural range <>) of Character;

  function Pick_Random(A: Char_Arr) return Character is
     -- Chooses one of the characters of A (uniformly distributed)
  begin
     return A(A'First + Natural(Rnd.Random(Gen) * Float(A'Last)));
  end Pick_Random;

  Vowels    : Char_Arr := ('a', 'e', 'i', 'o', 'u');
  Consonants: Char_Arr := ('t', 'n', 's', 'h', 'r', 'd', 'l');
  Specials  : Char_Arr := (',', '.', '?', '!');

begin

  Rnd.Reset(Gen);
  for J in 1 .. 3 loop
     for I in 1 .. 10 loop
        Ada.Text_IO.Put(Pick_Random(Consonants));
        Ada.Text_IO.Put(Pick_Random(Vowels));
     end loop;
     Ada.Text_IO.Put(Pick_Random(Specials) & " ");
  end loop;
  Ada.Text_IO.New_Line;

end Pick_Random_Element;</lang>

Sample Output:

horanohesuhodinahiru. desehonirosedisinelo, losihehederidonolahe?

Aime

<lang aime>list l;

l_append(l, 'a'); l_append(l, 'b'); l_append(l, 'c'); l_append(l, 'd'); l_append(l, 'e'); l_append(l, 'f');

o_byte(l_query(l, drand(5))); o_byte('\n');</lang>

AutoHotkey

True Arrays

Works with: AutoHotkey_L

<lang AHK>list := ["abc", "def", "gh", "ijklmnop", "hello", "world"] Random, randint, 1, % list.MaxIndex() MsgBox % List[randint]</lang>

Pseudo-Arrays

Works with: AutoHotkey_Basic

<lang AutoHotkey>list := "abc,def,gh,ijklmnop,hello,world" StringSplit list, list, `, Random, randint, 1, %list0% MsgBox % List%randint%</lang>

Bash

<lang Bash># borrowed from github.com/search?q=bashnative

rand() { printf $(( $1 * RANDOM / 32767 )) } rand_element () {

   local -a th=("$@")
   unset th[0]
   printf $'%s\n' "${th[$(($(rand "${#th[*]}")+1))]}"

}

echo "You feel like a $(rand_element pig donkey unicorn eagle) today"</lang>

BASIC

Works with: QBasic

Works with: PowerBASIC

Note the use of LBOUND and UBOUND. This is only necessary for arrays where the lower and upper limits aren't known. In this example, we know they are 0 and 10 respectively, and could have hard-coded those numbers. (For that matter, the "random selection" line could've just been entered as x = INT(RND * 11).)

<lang qbasic>'setup DIM foo(10) AS LONG DIM n AS LONG, x AS LONG FOR n = LBOUND(foo) TO UBOUND(foo)

   foo(n) = INT(RND*99999)

NEXT RANDOMIZE TIMER

'random selection x = INT(RND * ((UBOUND(foo) - LBOUND(foo)) + 1))

'output PRINT x, foo(x)</lang>

See also: Liberty BASIC, PureBasic, Run BASIC

BBC BASIC

<lang bbcbasic> DIM list$(5)

     list$() = "The", "five", "boxing", "wizards", "jump", "quickly"
     chosen% = RND(6)
     PRINT "Item " ; chosen% " was chosen which is '" list$(chosen%-1) "'"</lang>

Output:

Item 4 was chosen which is 'wizards'

Burlesque

<lang burlesque> blsq ) "ABCDEFG"123456 0 6rn-]!! 'G </lang>

123456 is the random seed. In order to pick another element you have to change the random seed.

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>

C++

<lang cpp>#include <iostream>

include <random>
include <vector>

int main( ) {

  std::vector<int> numbers { 11 , 88 , -5 , 13 , 4 , 121 , 77 , 2 } ;
  std::random_device seed ;
  // generator 
  std::mt19937 engine( seed( ) ) ;
  // number distribution
  std::uniform_int_distribution<int> choose( 0 , numbers.size( ) - 1 ) ;
  std::cout << "random element picked : " << numbers[ choose( engine ) ] 
     << " !\n" ;
  return 0 ;

}</lang>

C#

<lang csharp>using System; using System.Collections.Generic;

class RandomElementPicker {

 static void Main() {
   var list = new List<int>(new[]{0, 1, 2, 3, 4, 5, 6, 7, 8, 9});
   var rng = new Random();
   var randomElement = list[rng.Next(list.Count)];
   Console.WriteLine("I picked element {0}", randomElement);
 }

}</lang>

Clojure

where coll is some sequential collection. Equivalent to:

<lang Clojure>(nth coll (rand-int (count coll)))</lang>

CoffeeScript

<lang coffeescript>array = [1,2,3] console.log array[Math.floor(Math.random() * array.length)]</lang>

CommonLisp

<lang lisp>(defvar *list* '(one two three four five))

(print (nth (random (length *list*)) *list*)) (print (nth (random (length *list*)) *list*)) (print (nth (random (length *list*)) *list*))</lang>

Output:

FIVE 
THREE 
ONE

D

<lang d>import std.stdio, std.random;

void main() {

   const items = ["foo", "bar", "baz"];
   items[uniform(0, $)].writeln;

}</lang>

Déjà Vu

<lang dejavu>print choose [ "one" "two" "chicken" ]</lang>

Erlang

<lang erlang>% Implemented by Arjun Sunel -module(pick_random). -export([main/0]).

main() -> List =[1,2,3,4,5], index = random:uniform(length(List)), lists:nth(index,List). </lang>

Euphoria

<lang euphoria>constant s = {'a', 'b', 'c'} puts(1,s[rand($)])</lang>

Factor

<lang factor>( scratchpad ) { "a" "b" "c" "d" "e" "f" } random . "a"</lang>

F#

<lang fsharp>let list = ["a"; "b"; "c"; "d"; "e"] let rand = new System.Random() printfn "%s" x.[rand.Next(list.Length)]</lang>

GAP

<lang gap>a := [2, 9, 4, 7, 5, 3]; Random(a);</lang>

Go

<lang go>package main

import (

   "fmt"
   "math/rand"
   "time"

)

var list = []string{"bleen", "fuligin", "garrow", "grue", "hooloovoo"}

func main() {

   rand.Seed(time.Now().UnixNano())
   fmt.Println(list[rand.Intn(len(list))])

}</lang>

Groovy

Solution: <lang groovy>def list = [25, 30, 1, 450, 3, 78] def random = new Random();

(0..3).each {

   def i = random.nextInt(list.size())
   println "list[${i}] == ${list[i]}"

}</lang>

Output:

list[3] == 450
list[2] == 1
list[5] == 78
list[3] == 450

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>

J

<lang j> ({~ ?@#) 'abcdef' b</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>

K

<lang K> 1?"abcdefg" ,"e"</lang>

LabVIEW

This image is a VI Snippet, an executable image of LabVIEW code. The LabVIEW version is shown on the top-right hand corner. You can download it, then drag-and-drop it onto the LabVIEW block diagram from a file browser, and it will appear as runnable, editable code.

Lasso

<lang Lasso>local( my array = array('one', 'two', 3) )

myarray -> get(integer_random(#myarray -> size, 1))</lang>

-> two

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

Logo

Works with: UCB Logo

Lua

<lang lua>math.randomseed(os.time()) local a = {1,2,3} print(a[math.random(1,#a)])</lang>

Mathematica

<lang Mathematica>RandomChoice[{a, b, c}] ->c</lang>

MATLAB / Octave

In case list is a cell array: <lang Matlab> list = {'a','b','c'}; list{ceil(rand(1)*length(list))}</lang>

If list is a vector: <lang Matlab> list = 1:1000; list(ceil(rand(1)*length(list)))</lang>

NetLogo

<lang NetLogo>;; from list containnig only literals and literal constants user-message one-of [ 1 3 "rooster" blue ]

from list containing variables and reporters

user-message one-of (list (red + 2) turtles (patch 0 0) )</lang>

NetRexx

<lang netrexx>/* NetRexx */ options replace format comments java crossref savelog symbols nobinary

iArray = [ 1, 2, 3, 4, 5 ] -- a traditional array iList = Arrays.asList(iArray) -- a Java Collection "List" object iWords = '1 2 3 4 5' -- a list as a string of space delimited words

v1 = iArray[Random().nextInt(iArray.length)] v2 = iList.get(Random().nextInt(iList.size())) v3 = iWords.word(Random().nextInt(iWords.words()) + 1) -- the index for word() starts at one

say v1 v2 v3 </lang>

Objeck

<lang objeck>values := [1, 2, 3]; value := values[(Float->Random() * 100.0)->As(Int) % values->Size()];</lang>

OCaml

With a list: <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

With an array: <lang ocaml>let array_rand ary =

 let len = Array.length ary in
 ary.(Random.int len)</lang>

# array_rand [|1;2;3;4;5|] ;;
- : int = 3

PARI/GP

<lang parigp>pick(v)=v[random(#v)+1]</lang>

Pascal

<lang pascal>Program PickRandomElement (output);

const

 s: array [1..5] of string = ('1234', 'ABCDE', 'Charlie', 'XB56ds', 'lala');

begin

 randomize;
 writeln(s[low(s) + random(length(s))]);

end.</lang>

Perl

<lang perl>my @array = qw(a b c); print $array[ rand @array ];</lang>

Perl 6

In a nutshell, picking an element from a list is implemented with a method conveniently called "pick": <lang Perl 6>say (1, 2, 3).pick;</lang>

There are various ways of doing something similar, though. Perl 6 has actually two methods (with associated functional forms) 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># define the deck constant deck = 2..9, <J Q K A> X~ <♠ ♣ ♥ ♦>; deck.pick; # Pick a card deck.pick(5); # Draw 5 deck.pick(*); # Get a shuffled deck</lang> Or you can always use the normal rand built-in to generate a subscript (which automatically truncates any fractional part): <lang perl6>@array[@array * rand]</lang> However, the pick and roll methods (not to be confused with the pick-and-roll method in basketball) are more general insofar as they may be used on any enumerable type: <lang perl6>say Bool.pick; # returns either True or False</lang>

PHP

<lang php>$arr = array('foo', 'bar', 'baz'); $x = $arr[array_rand($arr)];</lang>

PicoLisp

<lang PicoLisp>(get Lst (rand 1 (length Lst)))</lang>

PL/I

<lang PL/I> declare t(0:9) character (1) static initial

     ('a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j');
  put ( t(10*random()) );</lang>

output:

PureBasic

<lang PureBasic>Procedure.s pickRandomElement(List source.s())

 Protected x = ListSize(source())
 
 If x > 0
   SelectElement(source(), Random(x - 1)) ;element numbering is zero - based
   ProcedureReturn source()
 EndIf

EndProcedure

initialize list elements

DataSection

 elements:
 Data.s "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten"

EndDataSection

elementCount = 10

NewList item.s()

Restore elements Define i For i = 1 To #elementCount

 AddElement(item())
 Read.s item()

 Print("Source list:  ")
 ForEach item()
   Print(item() + " ")
 Next
 PrintN(#CRLF$)
  
 Print("Random picks from list:  ")
 For i = 1 To 10
   Print(pickRandomElement(item()) + " ")
 Next
  
 Print(#CRLF$ + #CRLF$ + "Press ENTER to exit"): Input()
 CloseConsole()

EndIf</lang> Sample output:

Source list:  One Two Three Four Five Six Seven Eight Nine Ten

Random picks from list:  Seven Nine Two Six Four Four Nine Three Six Two

Python

<lang python>>>> import random >>> random.choice(['foo', 'bar', 'baz']) 'baz'</lang>

R

<lang r># a vector (letters are builtin) letters

[1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s"
[20] "t" "u" "v" "w" "x" "y" "z"

picking one element

sample(letters, 1)

[1] "n"

picking some elements with repetition, and concatenating to get a word

paste(sample(letters, 10, rep=T), collapse="")

[1] "episxgcgmt"</lang>

Racket

lang racket

(define (pick-item l)

 (list-ref l (random (length l))))

</lang>

REXX

version 1

This REXX example takes the task very literally. <lang rexx>/*REXX program to pick a random element from a list (tongue in cheek). */

_= 'hydrogen helium lithium beryllium boron carbon nitrogen oxygen' _=_ 'fluorine neon sodium magnesium aluminum silicon phosphorous sulfur' _=_ 'chlorine argon potassium calcium scandium titanium vanadium chromium' _=_ 'manganese iron cobalt nickel copper zinc gallium germanium arsenic' _=_ 'selenium bromine krypton rubidium strontium yttrium zirconium' _=_ 'niobium molybdenum technetium ruthenium rhodium palladium silver' _=_ 'cadmium indium tin antimony tellurium iodine xenon cesium barium' _=_ 'lanthanum cerium praseodymium neodymium promethium samarium europium' _=_ 'gadolinium terbium dysprosium holmium erbium thulium ytterbium' _=_ 'lutetium hafnium tantalum tungsten rhenium osmium irdium platinum' _=_ 'gold mercury thallium lead bismuth polonium astatine radon francium' _=_ 'radium actinium thorium protactinium uranium neptunium plutonium' _=_ 'americium curium berkelium californium einsteinum fermium mendelevium' _=_ 'nobelium lawrencium rutherfordium dubnium seaborgium bohrium hassium' _=_ 'meitnerium darmstadtium roentgenium copernicium Ununtrium'

w=words(_) say 'random element =' word(_,random(1,w))

                                      /*stick a fork in it, we're done.*/</lang>

output

random element = ytterbium

version 2

Slightly simplified:

Note that this version doesn't work (receives a syntax error 12) with REXXes that have a
smaller limit of the total length of a clause, in particular PC/REXX and Personal REXX
which have a limit of 1,000 characters). <lang rexx> /* REXX ***************************************************************

18.10.2012 Walter Pachl Not only the list of elements shortened:-)
- - - - /

wl='hydrogen helium lithium beryllium boron carbon nitrogen oxygen',

  'fluorine neon sodium magnesium aluminum silicon phosphorous sulfur',   
  '...',                                                                  
  'meitnerium darmstadtium roentgenium copernicium Ununtrium'

Say word(wl,random(1,words(wl))) </lang>

Ruby

Works with: Ruby version 1.9

<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>

Works with: Ruby version 1.8, but not 1.9

<lang ruby>irb(main):001:0> %w(north east south west).choice => "south"</lang>

Run BASIC

<lang runbasic>list$ = "a,b,c,d,e,f,g,h,i,j" letter = rnd(1) * 10 print "Selected letter:"; word$(list$,letter,",")</lang>

Seed7

<lang seed7>$ include "seed7_05.s7i";

const proc: main is func

 begin
   writeln(rand([] ("foo", "bar", "baz")));
 end func;</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 $[0,n)$ . 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>

TXR

Translation of: Tcl

<lang txr>@(do (defun randelem (vec)

      (vecref vec (random nil (length vec)))))

@(bind x @(randelem #("a" "b" "c" "d")))</lang>

TUSCRIPT

<lang tuscript>$$ MODE TUSCRIPT list="John'Paul'George'Ringo'Peter'Paul'Mary'Obama'Putin" sizeList=SIZE(list) selectedNr=RANDOM_NUMBERS (1,sizeList,1) selectedItem=SELECT(list,#selectednr) PRINT "Selecting term ",selectedNr," in the list, which was ",selectedItem</lang> Output:

Selecting term 3  in the list, which was George

XPL0

<lang XPL0>code Ran=1, Text=12; int List; [List:= ["hydrogen", "helium", "lithium", "beryllium", "boron"]; \(Thanks REXX) Text(0, List(Ran(5))); ]</lang>