usort problem

[paulsim]

I have a multidimensionalarray - $students.

They are normally in order of surname.

Another field contains exam marks.

I sort them in terms of exam marks, highest at the top using usort, and this works fine --- in a way.

However if several students have the same exam marks, the set containing the same exam mark in now no longer in order of surname:-

Start with:-
surname mark

Allen 77
Evans 77
Jones 78
Simons 78
Smith 77
Williams 77

sort using usort() - field to compare is "mark"

Sorts to

1 Jones 78
2 Evans 78
3 Simons 78
4 Williams 77
5 Smith 77
6 Allen 77

So students are ranked by mark, highest at the top, but within a group of the same mark the names are no longer alphabetical. somethng to do with the way usort does its sorting I suppose.

any ideas how to get the names in order within the same mark set????

Cheers

paulsim

[Weedpacket]

Have the usort callback function look at both mark and surname elements. If the two marks are different, that's enough to put the elements in order. If they're the same, use the surname as a tiebreaker.

[swr]

If you want a "stable" sort that doesn't re-order elements that compare equal, use merge sort (below). However, Weedpacket's advice is better, and will result in faster sorting.

function mergesort(&$array, $cmp_function = 'strcmp') {
    // Arrays of size < 2 require no action.
    if (count($array) < 2) return;
    // Split the array in half
    $halfway = count($array) / 2;
    $array1 = array_slice($array, 0, $halfway);
    $array2 = array_slice($array, $halfway);
    // Recurse to sort the two halves
    mergesort($array1, $cmp_function);
    mergesort($array2, $cmp_function);
    // If all of $array1 is <= all of $array2, just append them.
    if (call_user_func($cmp_function, end($array1), $array2[0]) < 1) {
        $array = array_merge($array1, $array2);
        return;
    }
    // Merge the two sorted arrays into a single sorted array
    $array = array();
    $ptr1 = $ptr2 = 0;
    while ($ptr1 < count($array1) && $ptr2 < count($array2)) {
        if (call_user_func($cmp_function, $array1[$ptr1], $array2[$ptr2]) < 1) {
            $array[] = $array1[$ptr1++];
        }
        else {
            $array[] = $array2[$ptr2++];
        }
    }
    // Merge the remainder
    while ($ptr1 < count($array1)) $array[] = $array1[$ptr1++];
    while ($ptr2 < count($array2)) $array[] = $array2[$ptr2++];
    return;
}