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I'm trying to sort a multi-dimensional array by another array, but have so far come up short.
array_multisort seems be working only for real sorting.

Suppose I have these 2 arrays:

$order = array(2,3,1);

$data = array(
    array('id' => 1, 'title' => 'whatever'),
    array('id' => 2, 'title' => 'whatever'),
    array('id' => 3, 'title' => 'whatever')
);

Now I would like to sort my $data array according to the order in my $order array.
This is what I would like the result to be:

$data = array(
    array('id' => 2, 'title' => 'whatever'),
    array('id' => 3, 'title' => 'whatever')
    array('id' => 1, 'title' => 'whatever'),
);

I can accomplish this easily by running a nested loop, but that would not scale well (my array is pretty big, and the arrays have many more fields).

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3 Answers

up vote 2 down vote accepted

There is no built-in function for this in PHP and i am unable to think of any custom function, which would do this using usort. But array_map is simple enough, imo, so why not use it instead?

$sorted = array_map(function($v) use ($data) {
    return $data[$v - 1];
}, $order);
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im not able to use anonymous functions with my current version of php - how would this work without using them? – Gaz_Edge Nov 27 '12 at 17:17
have opened a question here stackoverflow.com/questions/13589707/… – Gaz_Edge Nov 27 '12 at 17:40
up vote 0 down vote

In your example the ids in the $data array are are numbered consecutively and starting at 1. The code I give below assumes this is always the case. If this is not the case, the code does not work.

$result = array();
$index = 0;
foreach ($order as $position) {
    $result[$index] = $data[$position - 1];
    $index++;
}

At http://codepad.org/YC8w0yHh you can see that it works for your example data.

EDIT

If the assumption mentioned above does not hold, the following code will achieve the same result:

<?php

$data = array(
    array('id' => 1, 'title' => 'whatever'),
    array('id' => 2, 'title' => 'whatever'),
    array('id' => 3, 'title' => 'whatever')
);

$order = array(2,3,1);
$order = array_flip($order);

function cmp($a, $b)
{
    global $order;

    $posA = $order[$a['id']];
    $posB = $order[$b['id']];

    if ($posA == $posB) {
        return 0;
    }
    return ($posA < $posB) ? -1 : 1;
}

usort($data, 'cmp');

var_dump($data);

See http://codepad.org/Q7EcTSfs for proof.

By calling array_flip() on the $order array it can be used for position lookup. This is like a hashtable lookup, which is linear in time, or O(n). You cannot do better.

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Your last solution is basically running a sugared nested loop. I don't think that would scale well. – MegaHit Oct 19 '11 at 0:01
@MegaHit see updated code in my answer, you cannot do better than that. – Jan-Henk Oct 19 '11 at 10:09
up vote 0 down vote

You could try using a custom sort with usort(). This way you can use the first array to determine the order of the second array.

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I had looked at usort, but a don't know if/how it works with multi-dimensional arrays. – MegaHit Oct 18 '11 at 22:25
That's what makes it so useful. You define a function which has access to the entire item whether it's just a string or a multidimensional array. So in the comparison function you could compare each values index in the order array. So something like this: return (array_search($a['id'], $order) > array_search($b['id'], $order)) ? -1 : 1; – Justin Lucas Oct 18 '11 at 22:31

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