Wim Godden's professional blog                                                                                                    

Important : the following technique doesn’t work for any arrays where the values are :

• boolean (true/false)
• null
• objects
• resources

Suppose you get data from some source (an XML file, a CSV file, …) and you put it into an array. Now suppose this data is full of duplicates. For example, you have :

array(
    0 => 'horse',
    1 => 'pig',
    2 => 'pig',
    3 => 'cow',
    4 => 'horse',
...

How can you get the unique values from this array ?

The standard way would be to do :

$a = array_uniques($a);

Works fine, except that it’s extremely slow for large arrays.

A better way would be :

$a = array_keys(array_flip($a));

But marginally faster is :

$a = array_flip(array_flip($a));

So how big is the speed difference ? For large arrays, a double array_flip can easily be 20 times faster.

For reference, here’s a small benchmark :

$a = array();
for ($x=0; $x < 1000000; $x++) {
 $a[] = rand(0,1000);
}
 
$starttime = microtime(true);
$b = array_unique($a);
echo (microtime(true) - $starttime) . "\n";
 
$starttime = microtime(true);
$b = array_keys(array_flip($a));
echo (microtime(true) - $starttime) . "\n";
 
$starttime = microtime(true);
$b = array_flip(array_flip($a));
echo (microtime(true) - $starttime) . "\n";

The result :

2.06489086151
0.101167201996
0.0999970436096

This entry was posted on Friday, May 22nd, 2009 at 15:33 and is filed under Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.