Pick random element

Pick random element
You are encouraged to solve this task according to the task description, using any language you may know.

Demonstrate how to pick a random element from a list.

ACL2

:set-state-ok t
 
(defun pick-random-element (xs state)
   (mv-let (idx state)
           (random$ (len xs) state)
      (mv (nth idx xs) state)))

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.

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;

Sample Output:

horanohesuhodinahiru. desehonirosedisinelo, losihehederidonolahe?

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');

AutoHotkey

True Arrays

Works with: AutoHotkey_L

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

Pseudo-Arrays

Works with: AutoHotkey_Basic

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

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"

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).)

'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)

See also: Liberty BASIC, PureBasic, Run BASIC

BBC BASIC

      DIM list$(5)
      list$() = "The", "five", "boxing", "wizards", "jump", "quickly"
      chosen% = RND(6)
      PRINT "Item " ; chosen% " was chosen which is '" list$(chosen%-1) "'"

Output:

Item 4 was chosen which is 'wizards'

Burlesque

 
blsq ) "ABCDEFG"123456 0 6rn-]!!
'G
 

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

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;
}

C++

#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 ;
}

C#

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);
  }
}

Clojure

(rand-nth coll)

where coll is some sequential collection. Equivalent to:

(nth coll (rand-int (count coll)))

CoffeeScript

array = [1,2,3]
console.log array[Math.floor(Math.random() * array.length)]

CommonLisp

(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*))

Output:

FIVE 
THREE 
ONE

D

import std.stdio, std.random;
 
void main() {
    const items = ["foo", "bar", "baz"];
    items[uniform(0, $)].writeln;
}

Déjà Vu

print choose [ "one" "two" "chicken" ]

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).
 

Euphoria

constant s = {'a', 'b', 'c'}
puts(1,s[rand($)])

Factor

( scratchpad ) { "a" "b" "c" "d" "e" "f" } random .
"a"

F#

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

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))])
}

Groovy

Solution:

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]}"
}

Output:

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

Haskell

Creating a custom function:

import Random (randomRIO)
 
pick :: [a] -> IO a
pick xs = randomRIO (0, length xs - 1) >>= return . (xs !!)
 
x <- pick [1 2 3]

Using the random-extras library:

import Data.Random
import Data.Random.Source.DevRandom
import Data.Random.Extras
 
x <- runRVar (choice [1 2 3]) DevRandom

Icon and Unicon

The unary operator '?' selects a random element from its argument which may be a string, list, table, or set.

procedure main()
   L := [1,2,3]  # a list
   x := ?L       # random element
end

J

   ({~ ?@#) 'abcdef'
b

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

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

var array = [1,2,3];
return array[Math.floor(Math.random() * array.length)];

K

  1?"abcdefg"
,"e"

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.

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.

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, " ")

Selecting term 5 in the list, which was Peter

Logo

Works with: UCB Logo

pick [1 2 3]

Lua

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

Mathematica

RandomChoice[{a, b, c}]
->c

MATLAB / Octave

In case list is a cell array:

        list = {'a','b','c'}; 
	list{ceil(rand(1)*length(list))}

If list is a vector:

        list = 1:1000; 
	list(ceil(rand(1)*length(list)))

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) )

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
 

Objeck

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

OCaml

With a list:

let list_rand lst =
  let len = List.length lst in
  List.nth lst (Random.int len)

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

With an array:

let array_rand ary =
  let len = Array.length ary in
  ary.(Random.int len)

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

PARI/GP

pick(v)=v[random(#v)+1]

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.

Perl

my @array = qw(a b c);
print $array[ rand @array ];

Perl 6

In a nutshell, picking an element from a list is implemented with a method conveniently called "pick":

say (1, 2, 3).pick;

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)

(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

Selection without replacement: (pick a card from a deck)

# 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

Or you can always use the normal rand built-in to generate a subscript (which automatically truncates any fractional part):

@array[@array * rand]

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:

say Bool.pick;  # returns either True or False

PHP

$arr = array('foo', 'bar', 'baz');
$x = $arr[array_rand($arr)];

PicoLisp

(get Lst (rand 1 (length Lst)))

PL/I

   declare t(0:9) character (1) static initial
      ('a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j');
   put ( t(10*random()) );

output:

e

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()
Next 
 
If OpenConsole()
  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

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

>>> import random
>>> random.choice(['foo', 'bar', 'baz'])
'baz'

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"

Racket

 
#lang racket
(define (pick-item l)
  (list-ref l (random (length l))))
 

REXX

version 1

This REXX example takes the task very literally.

/*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.*/

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).

 
/* 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)))    
 

Ruby

Works with: Ruby version 1.9

irb(main):001:0> %w(north east south west).sample
=> "west"
irb(main):002:0> (1..100).to_a.sample(2)
=> [17, 79]

Works with: Ruby version 1.8, but not 1.9

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

Run BASIC

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

Seed7

$ include "seed7_05.s7i";
 
const proc: main is func
  begin
    writeln(rand([] ("foo", "bar", "baz")));
  end func;

Smalltalk

x := #(1 2 3) atRandom.

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:

proc randelem {list} {
    lindex $list [expr {int(rand()*[llength $list])}]
}
set x [randelem {1 2 3 4 5}]

TXR

Translation of: Tcl

@(do (defun randelem (vec)
       (vecref vec (random nil (length vec)))))
@(bind x @(randelem #("a" "b" "c" "d")))

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

Output:

Selecting term 3  in the list, which was George

XPL0

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