I have some problems with the output of the php quick sorting function.

{Code ...} output Array ([0] & amp; gt; [1] & amp; gt; [2] & amp; gt; 21 [3] & amp; gt; 333 [4] & amp; gt; 411 [5] & amp; gt; 666) Why are two more null values in front?

function qsort($arr){    if(!isset($arr[1])){        return $arr;    }    for ($i=0; $i < count($arr); $i++) {         if($arr[$i+1]<$arr[0]){            $larr[] = $arr[$i+1];        }        if($arr[$i+1]>=$arr[0]){            $rarr[] = $arr[$i+1];        }    }    $larr = qsort($larr);    $larr[] = $arr[0];    $rarr = qsort($rarr);    return array_merge($larr,$rarr);}print_r(qsort(array(411,21,333,666)));

Output Array ([0] => [1] => [2] => 21 [3] => 333 [4] => 411 [5] => 666)

Why is there two more null values in front?

Reply content:

function qsort($arr){    if(!isset($arr[1])){        return $arr;    }    for ($i=0; $i < count($arr); $i++) {         if($arr[$i+1]<$arr[0]){            $larr[] = $arr[$i+1];        }        if($arr[$i+1]>=$arr[0]){            $rarr[] = $arr[$i+1];        }    }    $larr = qsort($larr);    $larr[] = $arr[0];    $rarr = qsort($rarr);    return array_merge($larr,$rarr);}print_r(qsort(array(411,21,333,666)));

Output Array ([0] => [1] => [2] => 21 [3] => 333 [4] => 411 [5] => 666)

Why is there two more null values in front?

The end condition of the for loop is $ I.

Then you must turn off and report an error. Therefore, PHP does not report an error. Instead, it considers that the result is a null value and continues to be executed. Therefore, the sorting result contains a null value.

Correct:

  = $ Arr [0]) {$ rarr [] = $ arr [$ I] ;}}$ larr = qsort ($ larr ); $ larr [] = $ arr [0]; $ rarr = qsort ($ rarr); return array_merge ($ larr, $ rarr);} print_r (qsort (array (411,21, 333,666 )));

Https://3v4l.org/CIKED

    function querySort($arr,$order='asc') {        if (count($arr)<1) {            return $arr;        }        $arr_left = $arr_right = array();        $val = $arr[0]; unset($arr[0]);        foreach ($arr as $key => $value) {            if ($order=='desc') {                if ($value < $val) {                    $arr_left[] = $value;                } else {                    $arr_right[] = $value;                }            } else {                if ($value > $val) {                    $arr_left[] = $value;                } else {                    $arr_right[] = $value;                }            }        }        $arr_left = querySort($arr_left,$order);        $arr_right = querySort($arr_right,$order);        return array_merge($arr_left,[$val],$arr_right);    }    $arr = querySort(array(411,21,333,666));    echo "
";    var_dump($arr);
                                        
                                    
It should be the reason for array_merge
                                        
                                    
Why write and sort it like this? The built-in array sorting function can be used!