array_rand
(PHP 4, PHP 5)
array_rand — Pick one or more random entries out of an array
Description
Picks one or more random entries out of an array, and returns the key (or keys) of the random entries.
Parameters
-
array -
The input array.
-
num -
Specifies how many entries should be picked.
Return Values
When picking only one entry, array_rand() returns
the key for a random entry. Otherwise, an array of keys for the random
entries is returned. This is done so that random keys can be picked
from the array as well as random values. Trying to pick more elements
than there are in the array will result in an
E_WARNING level error, and NULL will be returned.
Changelog
| Version | Description |
|---|---|
| 5.2.10 | The resulting array of keys is no longer shuffled. |
| 4.2.0 | The random number generator is seeded automatically. |
Examples
Example #1 array_rand() example
<?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";
?>
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Коментарии
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); ?>
It doesn't explicitly say it in the documentation, but PHP won't pick the same key twice in one call.<?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, $n ) ) ) ) );
<?php
/**
* Wraps array_rand call with additional checks
*
* TLDR; not so radom as you'd wish.
*
* NOTICE: the closer you get to the input arrays length, for the n parameter, the output gets less random.
* e.g.: array_random($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)]);
}
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.
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
)
*/
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)