PHP - shuffle only part of an array

Question

I have an array as so:

$animals = array (
    'giraffe',
    'lion',
    'hippo',
    'dog',
    'cat',
    'rabbit',
    'fly',
    'hamster',
    'gerbil'
    'goldfish'
);

That is the order i want the array to be in apart from these 2 - hamster and gerbil

I would like to randomise between which one comes first. I know i can use:

shuffle($animals);

To randomise them all but i want to do it for just these 2. So if i was to do a print_r($animals) i may get hamster come before gerbil but another time get gerbil before hamster

Answer 1

You can splice the array for those to elements, randomize their order and put them back in the original array:

$sub = array_splice($animals, 7, 2);
shuffle($sub);
array_splice($animals, 7, 0, $sub);
var_dump($animals);

Answer 2

Adding 2 variations of Fisher-Yates-Knuth unbiased shuffling algorithm, with only included indices or excluded indices (php-like pseudo-code)

function shuffle_include( $a, $inc ) 
{
    // $a is array to shuffle
    // $inc is array of indices to be included only in the shuffle
    // all other elements/indices will remain unaltered

    // fisher-yates-knuth shuffle variation O(n)
    $N = count($inc);
    while ( $N-- )
    { 
        $perm = rnd( 0, $N ); 
        $swap = $a[ $inc[$N] ]; 
        $a[ $inc[$N] ] = a[ $inc[$perm] ]; 
        $a[ $inc[$perm] ] = $swap; 
    }
    // in-place
    return $a;
}

function shuffle_exclude( $a, $exc ) 
{
    // $a is array to shuffle
    // $exc is array of indices to be excluded from the shuffle
    // all other elements/indices will be shuffled
    // assumed excluded indices are given in ascending order
    $inc = array();
    $i=0; $j=0; $l = count($a); $le = count($exc)
    while ($i < $l)
    {
        if ($j >= $le || $i<$exc[$j]) $inc[] = $i;
        else $j++;
        $i++;
    }
    // rest is same as shuffle_include function above

    // fisher-yates-knuth shuffle variation O(n)
    $N = count($inc);
    while ( $N-- )
    { 
        $perm = rnd( 0, $N ); 
        $swap = $a[ $inc[$N] ]; 
        $a[ $inc[$N] ] = $a[ $inc[$perm] ]; 
        $a[ $inc[$perm] ] = $swap; 
    }
    // in-place
    return $a;
}

Example:

$a = array(1,2,3,4,5,6);

print_r( shuffle_include( $a, array(0,1,2) ) );
// sample output: [2,1,3,4,5,6] , only 0,1,2 indices are shuffled

print_r( shuffle_exclude( $a, array(0,1,2) ) );
// sample output: [1,2,3,6,5,4], all other indices are shuffled except 0,1,2

NOTE That PHP's shuffle function itself uses a variation of Fisher-Yates-Knuth shuffling algorithm

NOTE2 All shuffle algorithms given (and PHP's original shuffle function) have a (average) time complexity of $O(n)$ (n=size of array to shuffle)

for other variations of shuffle see:

1. Efficiently pick n random elements from PHP array (without shuffle)