How does array_diff work?

Question

How does array_diff() work? It obviously couldn't work as follows:

function array_diff($arraya, $arrayb)
{
    $diffs = array();
    foreach ($arraya as $keya => $valuea)
    {
        $equaltag = 0;
        foreach ($arrayb as $valueb)     
        {
            if ($valuea == $valueb)
            {
                $equaltag =1;
                break;
            }
        }
        if ($equaltag == o)
        {
              $diffs[$keya]=$valuea;
        }

    }
    return $diffs;                          
}                                  //couldn't be worse than this

Does anyone know a better solution?

EDIT @animuson:

function array_diff($arraya, $arrayb)
{
    foreach ($arraya as $keya => $valuea)
    {
        if (in_array($valuea, $arrayb))
        {
            unset($arraya[$keya]);
        }
    }
    return $arraya;
}

Answer 1

user187291's suggestion to do it in PHP via hash tables is simply great! In a rush of adrenaline taken from this phantastic idea, I even found a way to speed it up a little more (PHP 5.3.1):

function leo_array_diff($a, $b) {
    $map = array();
    foreach($a as $val) $map[$val] = 1;
    foreach($b as $val) unset($map[$val]);
    return array_keys($map);
}

With the benchmark taken from user187291's posting:

LEO=0.0322  leo_array_diff()
ME =0.1308  my_array_diff()
YOU=4.5051  your_array_diff()
PHP=45.7114 array_diff()

The array_diff() performance lag is evident even at 100 entries per array.

Note: This solution implies that the elements in the first array are unique (or they will become unique). This is typical for a hash solution.

Note: The solution does not preserve indices. Assign the original index to $map and finally use array_flip() to preserve keys.

function array_diff_pk($a, $b) {
    $map = array_flip($a);
    foreach($b as $val) unset($map[$val]);
    return array_flip($map);
}

PS: I found this while looking for some array_diff() paradoxon: array_diff() took three times longer for practically the same task if used twice in the script.

Answer 2

UPDATE

• see below for faster/better code.

• array_diff behaviour is much better in php 5.3.4, but still ~10 times slower than Leo's function.

• also it's worth noting that these functions are not strictly equivalent to array_diff since they don't maintain array keys, i.e. my_array_diff(x,y) == array_values(array_diff(x,y)).

/UPDATE

A better solution is to use hash maps

function my_array_diff($a, $b) {
    $map = $out = array();
    foreach($a as $val) $map[$val] = 1;
    foreach($b as $val) if(isset($map[$val])) $map[$val] = 0;
    foreach($map as $val => $ok) if($ok) $out[] = $val;
    return $out;
}

$a = array('A', 'B', 'C', 'D');
$b = array('X', 'C', 'A', 'Y');

print_r(my_array_diff($a, $b)); // B, D

benchmark

function your_array_diff($arraya, $arrayb)
{
    foreach ($arraya as $keya => $valuea)
    {
        if (in_array($valuea, $arrayb))
        {
            unset($arraya[$keya]);
        }
    }
    return $arraya;
}

$a = range(1, 10000);
$b = range(5000, 15000);

shuffle($a);
shuffle($b);

$ts = microtime(true);
my_array_diff($a, $b);
printf("ME =%.4f\n", microtime(true) - $ts);

$ts = microtime(true);
your_array_diff($a, $b);
printf("YOU=%.4f\n", microtime(true) - $ts);

result

ME =0.0137
YOU=3.6282

any questions? ;)

and, just for fun,

$ts = microtime(true);
array_diff($a, $b);
printf("PHP=%.4f\n", microtime(true) - $ts);

result

ME =0.0140
YOU=3.6706
PHP=19.5980

that's incredible!

Answer 3

The best solution to know how it works it to take a look at its source-code ;-)
(Well, that's one of the powers of open source -- and if you see some possible optimization, you can submit a patch ;-) )

For array_diff, it should be in ext/standard -- which means, for PHP 5.3, it should be there : branches/PHP_5_3/ext/standard

And, then, the array.c file looks like a plausible target ; the php_array_diff function, line 3381, seems to correspond to array_diff.

(Good luck going through the code : it's quite long...)