PHP - Sort multi-dimensional array by another array

Question

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

Answer 1

This would be how I would do. I would use a custom usort function (arr_sort) in conjunction with the $data array.

<?php
$order = array(2,3,1);
$data = array(
    array('id' => 1, 'title' => 'whatever'),
    array('id' => 2, 'title' => 'whatever'),
    array('id' => 3, 'title' => 'whatever')
);
function arr_sort($a,$b){
  global $order;
  foreach ($order as $key => $value) {
    if ($value==$a['id']) {
      return 0;
      break;
    }
    if ($value==$b['id']) {
      return 1;
      break;
    }
  }
}
usort($data,'arr_sort');
echo "<pre>";
print_r($data);
echo "<pre>";

Answer 2

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.

Answer 3

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.

Answer 4

For those of you who want to sort data based on an array with actual IDs, rather than based on an array with indexes like in the accepted answer - you can use the following simple comparison function for the usort:

usort($data, function($a, $b) use ($order) {
    $posA = array_search($a['id'], $order);
    $posB = array_search($b['id'], $order);
    return $posA - $posB;
});

---

So the following example will work fine and you won't get the Undefined offset notices and an array with null values:

$order = [20, 30, 10];

$data = [
    ['id' => 10, 'title' => 'Title 1'],
    ['id' => 20, 'title' => 'Title 2'],
    ['id' => 30, 'title' => 'Title 3']
];

usort($data, function($a, $b) use ($order) {
    $posA = array_search($a['id'], $order);
    $posB = array_search($b['id'], $order);
    return $posA - $posB;
});

echo '<pre>', var_dump($data), '</pre>';

Output:

array(3) {
  [0]=>
  array(2) {
    ["id"]=>
    int(20)
    ["title"]=>
    string(7) "Title 2"
  }
  [1]=>
  array(2) {
    ["id"]=>
    int(30)
    ["title"]=>
    string(7) "Title 3"
  }
  [2]=>
  array(2) {
    ["id"]=>
    int(10)
    ["title"]=>
    string(7) "Title 1"
  }
}

Answer 5

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);