How to Generate All Permutations of a PHP Array?

Generate All Permutations of a PHP Array

Problem:

Given an array of strings, generate all possible permutations of its elements. For example, for the array ['peter', 'paul', 'mary'], we should obtain:

• peter-paul-mary
• peter-mary-paul
• paul-peter-mary
• paul-mary-peter
• mary-peter-paul
• mary-paul-peter

Solution 1: pc_permute Function

This function uses recursion to swap and reorder elements within the array, generating permutations.

function pc_permute($items, $perms = array()) {
    if (empty($items)) { 
        echo join(' ', $perms) . "<br />";
    } else {
        for ($i = count($items) - 1; $i >= 0; --$i) {
             $newitems = $items;
             $newperms = $perms;
             list($foo) = array_splice($newitems, $i, 1);
             array_unshift($newperms, $foo);
             pc_permute($newitems, $newperms);
         }
    }
}

Solution 2: pc_next_permutation Function

An alternative approach is to utilize the next permutation algorithm.

function pc_next_permutation($p, $size) {
    // Find the largest index i where p[i] < p[i+1]
    for ($i = $size - 1; $p[$i] >= $p[$i+1]; --$i) { }

    // If i is -1, no next permutation exists
    if ($i == -1) { return false; }

    // Find the largest index j where p[j] > p[i]
    for ($j = $size; $p[$j] <= $p[$i]; --$j) { }

    // Swap p[i] and p[j]
    $tmp = $p[$i]; $p[$i] = $p[$j]; $p[$j] = $tmp;

    // Reverse the order of the elements from i+1 to size
    for (++$i, $j = $size; $i < $j; ++$i, --$j) {
         $tmp = $p[$i]; $p[$i] = $p[$j]; $p[$j] = $tmp;
    }

    return $p;
}

Usage:

$arr = array('peter', 'paul', 'mary');

pc_permute($arr);

or

$set = split(' ', 'she sells seashells');
$size = count($set) - 1;
$perm = range(0, $size);
$j = 0;

do { 
     foreach ($perm as $i) { $perms[$j][] = $set[$i]; }
} while ($perm = pc_next_permutation($perm, $size) and ++$j);

foreach ($perms as $p) {
    print join(' ', $p) . "\n";
}

Reference:

• http://docstore.mik.ua/orelly/webprog/pcook/ch04_26.htm

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