PHP - Manual: array_rand

2024-11-15

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array_rand

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

array_rand — 从数组中随机取出一个或多个随机键

说明

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

从数组中取出一个或多个随机的单元，并返回随机条目对应的键（一个或多个）。它使用了伪随机数产生算法，所以不适合密码学场景。

参数

array: 输入的数组。
num: 指定要取出的单元数量。

返回值

如果只取出一个，array_rand() 返回随机单元的键名。否则就返回包含随机键名的数组。完成后，就可以根据随机的键获取数组的随机值。如果返回的是包含随机键名的数组，数组单元的顺序按照键名在原数组中的顺序排列。取出数量如果超过 array 的长度，就会导致 E_WARNING 错误，并返回 NULL。

更新日志

版本	说明
7.1.0	内部随机化算法已从 libc rand 函数更改为使用 » 梅森旋转算法（Mersenne Twister）随机数生成器。

范例

示例 #1 array_rand() 例子


<?php
$input = array("Neo", "Morpheus", "Trinity", "Cypher", "Tank");
$rand_keys = array_rand($input, 2);
echo $input[$rand_keys[0]] . "\n";
echo $input[$rand_keys[1]] . "\n";
?>

参见

shuffle() - 打乱数组

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User Contributed Notes 7 notes

down

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); ?>

down

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.

down

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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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 ) ) ) ) );

down

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)

down

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.

down

-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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官方地址：https://www.php.net/manual/en/function.array-rand.php

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array_rand

说明

参数

返回值

更新日志

范例

参见

User Contributed Notes 7 notes