Writing merge sort in PHP - php

Question

I have tried to write a basic merge sort in PHP involving a small array, yet the problem is it takes about a minute or so to execute, and returns:
Fatal error: Allowed memory size of 536870912 bytes exhausted (tried
to allocate 35 bytes) in /Users/web/www/merge.php on line 39
Does anyone have an idea where the code might be going wrong (if at all)? I've been staring at this for a good hour now.
<?php
$array = array(8,1,2,5,6,7);
print_array($array);
merge_sort($array);
print_array($array);
function merge_sort(&$list){
if( count($list) <= 1 ){
return $list;
}
$left = array();
$right = array();
$middle = (int) ( count($list)/2 );
// Make left
for( $i=0; $i < $middle; $i++ ){
$left[] = $list[$i];
}
// Make right
for( $i = $middle; $i < count($list); $i++ ){
$right[] = $list[$i];
}
// Merge sort left & right
merge_sort($left);
merge_sort($right);
// Merge left & right
return merge($left, $right);
}
function merge(&$left, &$right){
$result = array();
while(count($left) > 0 || count(right) > 0){
if(count($left) > 0 && count(right) > 0){
if($left[0] <= $right[0]){
$result[] = array_shift($left);
} else {
$result[] = array_shift($right);
}
} elseif (count($left) > 0){
$result[] = array_shift($left);
} elseif (count($right) > 0){
$result[] = array_shift($right);
}
}
print_array($result);exit;
return $result;
}
function print_array($array){
echo "<pre>";
print_r($array);
echo "<br/>";
echo "</pre>";
}
?>

Answer 1

In your merge function, you call count on right instead of $right. PHP assumes this is a string constant (at least in 5.3.9) and when casted into an array that always has one element. So count(right) is always one, and you never exit the first merge.

Answer 2

Try this approach. Instead of shifting it, slice.
Also, for in while loop for the merge function, you need to do an and && comparison instead
of ||
function mergeSort($array)
{
if(count($array) == 1 )
{
return $array;
}
$mid = count($array) / 2;
$left = array_slice($array, 0, $mid);
$right = array_slice($array, $mid);
$left = mergeSort($left);
$right = mergeSort($right);
return merge($left, $right);
}
function merge($left, $right)
{
$res = array();
while (count($left) > 0 && count($right) > 0)
{
if($left[0] > $right[0])
{
$res[] = $right[0];
$right = array_slice($right , 1);
}
else
{
$res[] = $left[0];
$left = array_slice($left, 1);
}
}
while (count($left) > 0)
{
$res[] = $left[0];
$left = array_slice($left, 1);
}
while (count($right) > 0)
{
$res[] = $right[0];
$right = array_slice($right, 1);
}
return $res;
}

Answer 3

Have a look at this, the algorithm is already implemented, using array_push and array splice instead of just array_shift.
http://www.codecodex.com/wiki/Merge_sort#PHP

Answer 4

I implement merge sort this way
function mergeSort($Array)
{
$len = count($Array);
if($len==1){
return $Array;
}
$mid = (int)$len / 2;
$left = mergeSort(array_slice($Array, 0, $mid));
$right = mergeSort(array_slice($Array, $mid));
return merge($left, $right);
}
function merge($left, $right)
{
$combined = [];
$totalLeft = count($left);
$totalRight = count($right);
$rightIndex = $leftIndex=0;
while ($leftIndex < $totalLeft && $rightIndex < $totalRight) {
if ($left[$leftIndex] > $right[$rightIndex]) {
$combined[]=$right[$rightIndex];
$rightIndex++;
}else {
$combined[] =$left[$leftIndex];
$leftIndex++;
}
}
while($leftIndex<$totalLeft){
$combined[]=$left[$leftIndex];
$leftIndex++;
}
while ($rightIndex<$totalRight){
$combined[] =$right[$rightIndex];
$rightIndex++;
}
return $combined;
}

Answer 5

Here is the class in PHP to implement the Merge Sort -
<?php
class mergeSort{
public $arr;
public function __construct($arr){
$this->arr = $arr;
}
public function mSort($l,$r){
if($l===null || $r===null){
return false;
}
if ($l < $r)
{
// Same as ($l+$r)/2, but avoids overflow for large $l and $r
$m = $l+floor(($r-$l)/2);
// Sort first and second halves
$this->mSort($l, $m);
$this->mSort($m+1, $r);
$this->merge($l, $m, $r);
}
}
// Merges two subarrays of $this->arr[]. First subarray is $this->arr[$l..$m]. Second subarray is $this->arr[$m+1..$r]
public function merge($l, $m, $r)
{
if($l===null || $m===null || $r===null){
return false;
}
$n1 = $m - $l + 1;
$n2 = $r - $m;
/* create temp arrays */
$L=array();
$R=array();
/* Copy data to temp arrays $L[] and $R[] */
for ($i = 0; $i < $n1; $i++)
$L[$i] = $this->arr[$l + $i];
for ($j = 0; $j < $n2; $j++)
$R[$j] = $this->arr[$m + 1+ $j];
/* Merge the temp arrays back into $this->arr[$l..$r]*/
$i = 0; // Initial index of first subarray
$j = 0; // Initial index of second subarray
$k = $l; // Initial index of merged subarray
while ($i < $n1 && $j < $n2)
{
if($L[$i] <= $R[$j])
{
$this->arr[$k] = $L[$i];
$i++;
}
else
{
$this->arr[$k] = $R[$j];
$j++;
}
$k++;
}
/* Copy the remaining elements of $L[], if there are any */
while($i < $n1)
{
$this->arr[$k] = $L[$i];
$i++;
$k++;
}
/* Copy the remaining elements of $R[], if there are any */
while($j < $n2)
{
$this->arr[$k] = $R[$j];
$j++;
$k++;
}
}
}
$arr = array(38, 27, 43, 5, 9, 91, 12);
$obj = new mergeSort($arr);
$obj->mSort(0,6);
print_r($obj->arr);
?>

Answer 6

I was looking for a optimized Mergesort algorithm in PHP. There are 5 algorithms in the answers, so I tested those, and mine too. Using PHP 7.2.7, these are the times:
Sorting 1000 random numbers:
Avanche 1 0.0396 seconds
Avanche 2 0.0347 seconds
Kartik 0.0291 seconds
Kripa 0.0282 seconds
Samuel 0.0247 seconds
Mine 0.0144 seconds
Sorting 10 random numbers:
Avanche 1 0.000222 seconds
Kartik 0.000216 seconds
Kripa 0.000159 seconds
Avanche 2 0.000144 seconds
Samuel 0.000128 seconds
Mine 0.000098 seconds
So, although I encourage to whom read it to make it faster (that was I was looking for, and I believe it can be done), I let you my implementation too, cause seems to be faster than the other answers:
//This function needs start and end limits
function mergeSortRec(&$a,$start,$end){
if($start<$end){
$center=($start+$end)>>1; //Binary right shift is like divide by 2
mergeSortRec($a, $start, $center);
mergeSortRec($a, $center+1, $end);
//Mixing the 2 halfs
$aux=array();
$left=$start; $right=$center;
//Main loop
while($left<$center && $right<=$end){
if($a[$left]<$a[$right]){
$aux[]=$a[$left++];
}else{
$aux[]=$a[$right++];
}
}
//Copy the rest of the first half
while($left<$center) $aux[]=$a[$left++];
//Copy the rest of the second half
while($right<=$end) $aux[]=$a[$right++];
//Copy the aux array to the main array
foreach($aux as $v) $a[$start++]=$v;
}
}
//This is the function easier to call
function mergeSort(&$a) {
mergeSortRec($a,0,count($a)-1);
}
If you post a new answer, let me a comment to test it and add it.
Edit: I did some new optimizations, for those looking for a better implementation.

Answer 7

Your merge sort accepts a list by reference
function merge_sort(&$list)
So you need to assign it the new merged and sorted list. So instead of
return merge($left, $right);
do
$list = $this->merge($left, $right);
That should do it, just remove the exit and fix the count variable

Answer 8

MergeSort in PHP
<?php
class Solution
{
function mergeSort(&$arr)
{
if(count($arr) > 1) {
$mid = floor(count($arr)/2);
$left = array_slice($arr, 0, $mid);
$right = array_slice($arr, $mid);
$this->mergeSort($left);
$this->mergeSort($right);
// Merge the results.
$i = $j = $k = 0;
while(($i < count($left)) && ($j < count($right))) {
if($left[$i] < $right[$j]) {
$arr[$k] = $left[$i];
$i++;
} else {
$arr[$k] = $right[$j];
$j++;
}
$k++;
}
while($i < count($left)) {
$arr[$k] = $left[$i];
$i++;
$k++;
}
while($j < count($right)) {
$arr[$k] = $right[$j];
$j++;
$k++;
}
}
}
}
$s = new Solution();
$tmp = [12, 7, 11, 13, 5, 6, 7];
$s->mergeSort($tmp);
print_r($tmp);