PHP - how to sort multidimensional array by comparing two different sub array values

Question

Im so tired of finding the way to sort this multidimensional array. I have tried usort() but it doesnt sort the way I want to. Or just maybe I could not figure out the correct logic to use. usort() seems hard to understand for me.

I wanted to sort my data(example below), by finding first which has the higher value (between keys a and b) for each of the sub arrays. And then the sub array which has the highest value either from its keys a or b will be on top.

For example this array:

Array
(
    [0] => Array
        (

         [a]=>5
         [b]=>
         [c]=>apple

        )

    [1] => Array
        (

         [a]=>5
         [b]=>7
         [c]=>guava

        )

    [2] => Array
        (

         [a]=>6
         [b]=>
         [c]=>banana

        )
    [3] => Array
        (

         [a]=>5
         [b]=>
         [c]=>avocado

        )

)

should be sorted like this:

Array
(
    [0] => Array
        (

         [a]=>5
         [b]=>7
         [c]=>guava

        )

    [1] => Array
        (

         [a]=>6
         [b]=>
         [c]=>banana

        )

    [2] => Array
        (

         [a]=>5
         [b]=>
         [c]=>apple

        )
    [3] => Array
        (

         [a]=>5
         [b]=>
         [c]=>avocado

        ) 

So how exactly I am able to do this? Im so confused how to use usort. What is the best PHP function for sorting this?

Answer 1

The way usort() works is that it requires a way to compare the elements of the array to each other to decide what comes first. You provide it a function, and it calls that function for each element pair in the input array. If the function returns a negative value, it means the first argument is less than the second argument and should come before it in the sorted array. The opposite is true if it returns a positive value.

Let's say you're comparing two sub-arrays s1 and s2. Using the logic you provided, s1 is "less than" s2 -- that is, s1 should come before s2 -- if max(s1.a,s1.b) > max(s2.a,s2.b). Thus, the sort function should return a negative value if this is true. If you subtract s1's max from s2's max, this should yield the proper result.

Try this:

function sort_function(array $arr1, array $arr2) {
    $arr1_val = max($arr1['a'],$arr1['b']);
    $arr2_val = max($arr2['a'],$arr2['b']);
    return $arr2_val - $arr1_val;
}

usort($arr,sort_function);

Assuming $arr is your array.

Answer 2

Your comparison function:

function cmp($a,$b){
    // clean up the keys so they don't cause weird cross-type comparisons
    (empty($a['a']) ? $a['a'] = 0 : NULL;
    (empty($a['b']) ? $a['b'] = 0 : NULL;
    (empty($b['a']) ? $b['a'] = 0 : NULL;
    (empty($b['b']) ? $b['b'] = 0 : NULL;

    /// figure out the pertinent value to compare
    $value_from_a_to_compare = max($a['a'],$a['b']);
    $value_from_b_to_compare = max($b['a'],$b['b']);

    if($value_from_a_to_compare ==  $value_from_b_to_compare) {
        return 0;
    }

    if($value_from_a_to_compare >  $value_from_b_to_compare) {
        return 1;
    } else {
       return -1;
    }


}

Call this from usort

usort($your_array,'cmp');
var_dump($your_array);

Answer 3

There are several methods, including writing the function written by dethtron5000 that would enable you to use usort.

I will describe a different suggestion: create a secondary array, in which you put the highest numbers from the first array and then use array_multisort to sort both, using the values from the secondary array.

$helper = array();
foreach ($data as $value)
{
    $helper[] = max($value['a'],$value['b']);
}
array_multisort($helper,$data);