Question

In a PHP project I have some data I want to sort using a linear time, simple counting sort:

$ar = array(7, 2, 0, 3, 8, 0, 12, 7, 6, 7);
$count = array();
foreach ($ar as $v)
    $count[$v]++;
$sorted = array();    
foreach ($count as $v => $c)
    for ($i = 0; $i < $c; $i++)
        $sorted[] = $v;

The problem is that the above obviously doesn’t work. The php array works more like a hashmap than an array. The code can be made to work by inserting ksort($count) before the final loop, but ksort runs in O(nlogn) which destroys the entire point.

Is there any way to do a linear time sort in php? Perhaps using some paramter to array(), or some entirely different structure?

Answer 1

You didn’t follow the algorithm correctly. This is O(n).

$ar = array(7, 2, 0, 3, 8, 0, 12, 7, 6, 7);
$count = array();
foreach ($ar as $v) {
    $count[$v] = isset($count[$v]) ? $count[$v] + 1 : 1;
}
$sorted = array();
$min = min($ar);
$max = max($ar);
for ($i=$min; $i<=$max; $i++) {
    if (isset($count[$i])) {
        for ($j=0; $j<$count[$i]; $j++) {
            $sorted[] = $i;
        }
    }
}

also, see array_count_values(), or alternatively compute the min and max inside the counting loop.