PHP Sorting Array Based on a Value

Question

Yes I know there are tons of questions like this. I want to re-order my array based on the 'magnitude' key in this array in descending order. I've been hovering all around the right method to use (array_multisort(), arsort(), etc.) but haven't figured out how to get exactly what I want.

Array ( [alCrn] => 27113 [epcid] => IMC601 [last_access] => 2015-03-14 15:26:40 [magnitude] => 1 ) 
Array ( [alCrn] => 27114 [epcid] => IMC602 [last_access] => 2015-03-16 20:45:17 [magnitude] => 8 ) 
Array ( [alCrn] => 27115 [epcid] => IMC603 [last_access] => 2015-03-15 17:57:58 [magnitude] => 2 ) 
Array ( [alCrn] => 27120 [epcid] => IMC608 [last_access] => 2015-03-15 23:56:16 [magnitude] => 12 ) 
Array ( [alCrn] => 27122 [epcid] => IMC612 [last_access] => 2015-03-15 19:55:02 [magnitude] => 2 ) 
Array ( [alCrn] => 27123 [epcid] => IMC615 [last_access] => 2015-03-17 14:00:34 [magnitude] => 6 )

The problem was...

It had to do with the array not being properly formatted. I had to take the results of a MySQL query and put them all into an array like so:

$count = 0;
while($prefix_rows = $prefix_results->fetch_assoc()){
    //the results are not yet ordered according to the number of clicks...save each row in an array and sort using php in next step
    $prefix_rows_arr[$count]['alCrn'] = $prefix_rows['alCrn'];
    $prefix_rows_arr[$count]['epcid'] = $prefix_rows['epcid'];
    $prefix_rows_arr[$count]['last_access'] = $prefix_rows['last_access'];
    $prefix_rows_arr[$count]['magnitude'] = $prefix_rows['magnitude'];

    $count++;               
}

After this, I needed a foreach loop to organize the $prefix_rows_arr:

foreach($prefix_rows_arr as $row){
    $arr[] = $row;
}

Then finally the usort() worked as expected

function compareMagnitudes($a, $b){
    if ($a['magnitude'] == $b['magnitude']) {
        return 0;
    }
    return ($a['magnitude'] > $b['magnitude']) ? -1 : 1;    
}
usort($arr, 'compareMagnitudes');

`usort` should be enough, any basic example from the doc will guide you. — Clément Malet, Mar 19 '15 at 17:19
You acknowledge that there are lots of existing questions, but you don't explain what is different that you need that those existing questions don't answer, or show us any of the code you've tried. That makes it feel like you just want us to write the code for you, which isn't the purpose of this site. — IMSoP, Mar 19 '15 at 17:26
Sometimes it's hard to articulate a problem one is not completely familiar with. I didn't think showing random bits of sort code was going to help in this situation. It's a Catch 22 on this site sometimes. — Ravioli87, Mar 19 '15 at 18:00

score 1 · Answer 1 · answered Mar 19 '15 at 17:20

You can define a comparaison function, and pass it as the second argument to the usort function which sorts an array in descending order.

function cmp($a, $b) {
    if ($a['key'] == $b['key']) {
        return 0;
    }
    return ($a['key'] < $b['key']) ? -1 : 1;
}

usort($your_array, "cmp");

Replace the key in the comparaison function by the proper value, magnitude in your case.

PHP Sorting Array Based on a Value

1 Answers1