Question

I have this simple quicksort function (I got it from uncle “G”)

function quicksort( &$list, $l , $r ) {

$i = $l;
$j = $r;

$tmp = $list[(int)( ($l+$r)/2 )];

do {
    while( $list[$i] < $tmp )
        $i++;

    while( $tmp < $list[$j] )
        $j--;

    if( $i <= $j ) {

        $w = $list[$i];
        $list[$i] = $list[$j];
        $list[$j] = $w;

        //_swp($list[$i],$list[$j]);

        $i++;
        $j--;
    }
}while( $i <= $j );


if( $l < $j )
    quicksort($list, $l, $j);


if( $i < $r )
    quicksort($list, $i, $r);

return $list;
}

And I have this little function to swap two variables.

function _swp(&$a,&$b){
    $a=$a+$b;
    $b=$a-$b;
    $a=$a-$b;
}

How come I can’t use _swp($a,$b) in quicksort function instead of this lines?

$w = $list[$i];
$list[$i] = $list[$j];
$list[$j] = $w;

If I comment out these 3 lines of code and enter call to _swp function I got bad results…
Please explain.
Best regards

Answer 1

the unexpected behavior is probably the “random” occurence of zeros in the sorted list. This happens because there is a special case while swapping:

if( $i <= $j ) {
    // swapping here using references!
    _swp($list[$i],$list[$j]);
    $i++;
    $j--;
}

The problem is found directly in the condition for swapping itself: if $i==$j then there are two references to the same variable. Thus calling _swp($list[$i],$list[$j]); will firstly add both variables $a = $a + $b. Considering $a and $b actually access the same variable content, $a and $b will then have the same value. In the next step $b = $a - $b will then be zero as $a is equal to $b. The third operation will leave the result to 0.

An easy solution for this is inserting another condition:

if( $i <= $j ) {
    // ensure $i to be truly smaller than $j
    if( $i < $j ) {
        _swp($list[$i],$list[$j]);
    }
    $i++;
    $j--;
}

I hope this will help you.
Cheers,
Fabian