超越PHP - Exceed PHP: shuffle

说明

void shuffle ( array array)

本函数打乱（随机排列单元的顺序）一个数组。必须用 srand() 播下本函数的随机数发生器种子。
例子 1. shuffle() 例子

<?php $numbers = range (1,20); srand ((float)microtime()*1000000); shuffle ($numbers); while (list (, $number) = each ($numbers)) { echo "$number "; } ?>

注: 在 PHP 4.2.0 中，无需用函数 srand() 或 mt_srand()来搜寻随机数生成器，它将被自动完成。

参见 arsort()，asort()，ksort()，rsort()，sort() 和 usort()。

add a note User Contributed Notes
shuffle

Justin Knoll
14-Nov-2003 04:57

Here's an implementation of a canonical shuffling algorithm. 



Its
O(n), not O(n^2). The implementation of Fisher-Yates below uses array
splicing inside a for loop over the array, making it O(n^2).



It
creates a uniform distribution of permutations - the definition of a
shuffle. I'm not sure that all of the ad hoc algorithms below do this -
they might create shuffles that look random on casual inspection, but
aren't.



Many implementations of Fisher-Yates check to make sure
that $i != $j before swapping. I figure that for large lists, that check
incurs more cost than just swapping every time.




<?php

/**

 * Shuffles an array using the Fisher-Yates
shuffle.

 *

 * D. E. Knuth: The Art of Computer Programming, Volume
2,

 * Third edition. Section 3.4.2, Algorithm P, pp 145. Reading:


* Addison-Wesley, 1997. ISBN: 0-201-89684-2.

 *

 * R. A. Fisher and
F. Yates: Statistical Tables. London, 1938.

 * Example 12.

 *


* @author Justin Knoll

 * @param array The array to be sorted - passed
by reference.

 * @return The shuffled array.

 */

function
fisherYatesShuffle(&$array)

{

	for ($i = count($array); --$i;
$i > 0)

	{

 		$j = @mt_rand(0, $i+1);

		$temp =
$array[$i];

 		$array[$i] = $array[$j];

 		$array[$j] =
$temp;

	}

}

?>

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