2010-02-15: array_slice is not confused about negative length

Posted at 2010-02-15 13:32:18 by SHD

I enjoy reading the amusingly named www.phpwtf.org on the rare occasions that they add a new post. A recent post complains about negative lengths when using array_slice (actually confusing it with array_splice at one point). Unfortunately, a technical issue means the site doesn't currently accept comments, so I'll simply make a blog post out of my comment there. The info may still be useful to other PHP programmers.

When using array_slice, a negative length does work, but it works differently to what one may expect. It does not start counting backwards from the start point (which would be intuitive, given a negative value for something called "length", nor does it have any result if the calculated end point ends up before the start point. The docs do actually specify this correctly: "If length is given and is negative then the sequence will stop that many elements from the end of the array." So:


<?php

$a = array ('a', 'b', 'c', 'd', 'e');

print_r (array_slice ($a, 2, -2));

print_r (array_slice ($a, 2, -1));

?>

outputs:

Array
(
    [0] => c
)
Array
(
    [0] => c
    [1] => d
)

That basically boils down to the following:


<?php

/*

 * Example function that operates as array_slice does *only*

 * with zero-based numerically indexed arrays and when

 * $offset is positive and $length is negative

 */

function slice ($array, $offset, $length)

{

    $result = array ();

    for ($i = $offset; $i < count ($array) + $length; ++$i)

    {

        $result [] = $array [$i];

    }

    return $result;

}

?>

In the example given on phpwtf.org, array_slice ($array, 0, -10) on an array with 1 element, that results in the for loop looping "for ($i = 0; $i < -9; ++$i)", which does nothing. Hence the empty array being returned.

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