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I'd like to sort an array of numbers (while maintaining the indexes) in a descending order.

The thing is that if there are duplicate values, I'd like them to be sorted by the matching values in a second array.

For instance, if the array is:

0 => 13, 
1 => 21, 
2 => 15, 
3 => 21

And the second array is:

0 => 3, 
1 => 2, 
2 => 7, 
3 => 4

Then I'd like 3 => 21 to come before 1 => 21, because the matching value of index 3 in the second array is bigger than index 1 (4 > 2). So the final sorted array would be:

3 => 21, 
1 => 21, 
2 => 15, 
0 => 13

The actual purpose of this is as follows:

Figure a poll. Every user has to rank the options from 1 to X (X = number of options).

If a user ranks the an answer as the first rank, it gets X points, the second one gets X-1, and so on.

So the first array is actually the score of each option (the index is the option's id).

The second array is the number of users who ranked the option first (the index is the option's id, matching to the indexes in the first array).

So when there are two or more options with the same score, the one that has more first rank votes is ranked higher.

Hope that makes sense.
Thanks!

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We/I would rather deal with the actual array and the expected outcome over "instances" – RobertPitt Feb 15 '11 at 18:30
1  
This is the actual array. – Lior Feb 15 '11 at 18:32
I smell homework! Explanation or it's suspect. ;) – Stephen Feb 15 '11 at 18:35
Actually, no, it's not an exercise. – Lior Feb 15 '11 at 18:36
I'll add the actual purpose of this to the first post in a minute. – Lior Feb 15 '11 at 18:37

2 Answers

up vote 0 down vote accepted

Convert it to:

$a = array(
    0 => array(13, 3), 
    1 => array(21, 2),
    2 => array(15, 7),
    3 => array(21, 4)
);

And then use arsort($a) More info: http://php.net/manual/en/function.arsort.php

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How exactly do I swap them? And what if there are more than 2 duplicate values? Thanks. – Lior Feb 15 '11 at 19:04
check the updated answer – Denis Feb 15 '11 at 19:05
Awesome, it's not mentioned in the function documentation. Thanks! – Lior Feb 15 '11 at 19:15
up vote 0 down vote

If I understand correctly, all you need is:

array_multisort($array1, $array2);

The bad part is that if you are using indexed arrays, they become reindexed. Your example becomes: array(13, 15, 21, 21) and array(3, 2, 4, 7).

You can get around the reindex by prepending some character (e.g., '0', or 'x') to each key.

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