PHP 8.2.0 Alpha 2 available for testing

array_rand

(PHP 4, PHP 5, PHP 7, PHP 8)

array_randEscolhe uma ou mais chaves aleatórias de um array

Descrição

array_rand(array $array, int $num = 1): int|string|array

Escolhe aleatóriamente um ou mais itens de um array, e retorna a chave (ou chaves) do itens selecionados. Utiliza um gerador de números pseudo randômicos que não é apropriado para uso criptográfico.

Parâmetros

array

O array de entrada.

num

Especifica quantos elementos deseja obter.

Valor Retornado

Se estiver selecionando apenas um elemento, array_rand() retorna a chave para este elemento aleatório. De outra forma, ele retorna um array com as chaves desses elementos selecionados. Assim é possível selecionar os elementos pelas chaves no array assim como valores aleatórios. Se múltiplas chaves forem retornadas, elas serão retornadas na ordem em que estavam presentes no array original. Tentar selecionar mais elementos que a quantidade presente no array resultará em um erro E_WARNING, e NULL será retornado.

Changelog

Versão Descrição
7.1.0 O algoritmo de embaralhamento interno foi alterado para usar o Gerador de Números Aleatórios » Mersenne Twister em vez da função rand da libc.

Exemplos

Exemplo #1 Exemplo de array_rand()

<?php
$input 
= array("Neo""Morpheus""Trinity""Cypher""Tank");
$rand_keys array_rand($input2);
echo 
$input[$rand_keys[0]] . "\n";
echo 
$input[$rand_keys[1]] . "\n";
?>

Veja Também

  • shuffle() - Mistura os elementos de um array

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User Contributed Notes 7 notes

up
66
Anonymous
12 years ago
If the array elements are unique, and are all integers or strings, here is a simple way to pick $n random *values* (not keys) from an array $array:

<?php array_rand(array_flip($array), $n); ?>
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31
Anonymous
9 years ago
It doesn't explicitly say it in the documentation, but PHP won't pick the same key twice in one call.
up
16
grzeniufication
4 years ago
<?php

/**
* Wraps array_rand call with additional checks
*
* TLDR; not so radom as you'd wish.
*
* NOTICE: the closer you get to the input arrays length, for the n parameter, the  output gets less random.
* e.g.: array_random($a, count($a)) == $a will yield true
* This, most certainly, has to do with the method used for making the array random (see other comments).
*
* @throws OutOfBoundsException – if n less than one or exceeds size of input array
*
* @param array $array – array to randomize
* @param int $n – how many elements to return
* @return array
*/
function array_random(array $array, int $n = 1): array
{
    if (
$n < 1 || $n > count($array)) {
        throw new
OutOfBoundsException();
    }

    return (
$n !== 1)
        ?
array_values(array_intersect_key($array, array_flip(array_rand($array, $n))))
        : array(
$array[array_rand($array)]);
}
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13
grzeniufication
4 years ago
<?php
// An example how to fetch multiple values from array_rand
$a = [ 'a', 'b', 'c', 'd', 'e', 'f', 'g' ];
$n = 3;

// If you want to fetch multiple values you can try this:
print_r( array_intersect_key( $a, array_flip( array_rand( $a, $n ) ) ) );

// If you want to re-index keys wrap the call in 'array_values':
print_r( array_values( array_intersect_key( $a, array_flip( array_rand( $a, $n ) ) ) ) );
up
1
divinity76+spam at gmail dot com
2 months ago
for a cryptographically secure version, try

<?php

/**
* fetch a random key from array, using a cryptograpically secure rng
* discussed+reviewed at https://codereview.stackexchange.com/questions/275832/cryptographically-secure-version-of-the-core-array-rand-function/
*
* @param array $array
* @throws ValueError if array is empty
* @return int|string key
*/
function array_rand_cryptographically_secure(array $array)/*: int|string*/ {
   
$max = count ( $array ) - 1;
    if (
$max < 0) {
        throw new
ValueError ( 'Argument #1 ($array) cannot be empty' );
    }
    return
key ( array_slice ( $array, random_int ( 0, $max ), 1, true ) );
}

$tests = [
        [
5, 6, 7],
        [
'a' => 1, 'b' => 2, 'c' => 3],
        [
'zero', 4 => 'four', 9 => 'nine'],
        [
"PEAN"=>0],
        []
];
foreach (
$tests as $test) {
    echo
array_rand_cryptographically_secure($test) . "\n";
}

?>
(this is an improved version, which unlike the first version, avoids copying *all* the keys)
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0
fulachan2 at gmail dot com
8 months ago
array_rand () takes a random value without ever being able to go back in its choice of random value.
A simple example:
I decide to mix an array of 10 entries to retrieve 3 values. This choice will give increasing and random values.

    $myarray = range(1,10);
   
    $pm = array_rand($myarray,3);     
    // $pm return array(0->0,1->6,2->8)

But if I decide to shuffle an array of 10 entries to get 10 entries, array_rand () will choose to assign a value to each return value and therefore the return array will not be random.

    $gm = array_rand($myarray,count($myarray));
    // $gm not random array(0->0,1->1,2->2,3->3,4->4,5->5,6->6,7->7,8->8,9->9)

The easiest way to have a truly random value:
either use array_rand () in a loop of 1 value at a time

$deg = range(-60,60);
$size = range(16,64);
$color = ["blue","red","green","pink","orange","purple","grey","darkgreen","darkkhaki"];
$i = 0;
$longueur = 10;
do{
    ++$i;
    printf("<span style='transform: rotateZ(%ddeg); display: inline-block;font-size: %spx; color:%s'>%s</span>",
        $deg[array_rand($deg)],
        $size[array_rand($size)],
        $color[array_rand($color)],
        $alnum[array_rand($alnum)]);

}while($i < $longueur);

------------------
or simply use shuffle () to shuffle the array really randomly.
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-2
Vitaly
6 months ago
Generate random index in weights array.
Implementation of Vose's Alias Method.

<?php
class AliasMethod
{
    protected
$count;
    protected
$prob;
    protected
$alias;

    public function
__construct($weight)
    {
       
$count = count($weight);
       
$sum = array_sum($weight);
       
$d = $count / $sum;
       
$small = [];
       
$large = [];

        foreach (
$weight as $i => & $w) {
            if ( (
$w *= $d) < 1)
               
$small[] = $i;
            else
               
$large[] = $i;
        }
        unset(
$w);

       
$prob = [];
       
$alias = [];

        while (!empty(
$small) AND !empty($large)) {
           
$l = array_pop($small);
           
$g = array_pop($large);
           
$prob[$l] = $weight[$l];
           
$alias[$l] = $g;

            if ( (
$weight[$g] += $weight[$l] - 1) < 1)
               
$small[] = $g;
            else
               
$large[] = $g;
        }

        foreach (
$large as $i)
           
$prob[$i] = 1;

        foreach (
$small as $i)
           
$prob[$i] = 1;

       
$this->prob = $prob;
       
$this->alias = $alias;
       
$this->count = $count;

    }

    public function
next(): int
   
{
       
$i = mt_rand(0, $this->count - 1);
        if (
mt_rand() / mt_getrandmax() <= $this->prob[$i])
            return
$i;
        else
            return
$this->alias[$i];
    }
}

// Usage:

$weight = [1, 2, 1, 1, 100, 1, 1, 5, 1, 1];
$rnd = new AliasMethod($weight);

$results = array_fill(0, count($weight), 0);

for(
$i = 0; $i < 100000; ++$i) {
   
$results[$rnd->next()]++;
}
print_r($results);

/*
Array
(
    [0] => 901
    [1] => 1785
    [2] => 899
    [3] => 907
    [4] => 87655
    [5] => 867
    [6] => 836
    [7] => 4371
    [8] => 910
    [9] => 869
)
*/
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