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Data Structure and Algorithm - Selection Sort

In this tutorial, we will learn a simple sorting algorithm - Selection Sort.

Problem to Solve

Given a list of numbers as shown below, please sort them in ascending order.

$numbers = [21,25,100,98,89,77];


  • You are required to use Selection Sort algorithm when sorting the numbers.
  • You are required to implement the algorithm in PHP language.


Selection Sort works by maintaining a sorted sub-list, finding the smallest item from the master list and swap it to the last element of the sub-list until all items are sorted.

The sorted sub-list's length is increased by one, whereas the master list's length is shrunk by one after each swap.


Pseudocode of Selection Sort algorithm can be written as follows:

FOR each element of the master list indexed by i
    Set current element of master list as the sub-list[i] element
    Find the smallest item from the master list (staring from i)
    Swap it with the last element of sub-list

PHP Implementation

We need an outer FOR loop to iterate through the master list and an inner FOR loop to find the smallest item from the master list.

$masterList = [21, 25, 100, 98, 89, 77];
$subList = [];
for ($i = 0; $i < count($masterList); $i++) {
    $subList[$i] = $masterList[$i];
    // Find the smallest item
    $smallestIndex = $i;
    for ($j = $i; $j < count($masterList); $j++) {
        if ($masterList[$j] < $masterList[$smallestIndex]) {
            $smallestIndex = $j;
    // Swap
    $tmp = $subList[count($subList) - 1];
    $subList[count($subList) - 1] = $masterList[$smallestIndex];
    $masterList[$smallestIndex] = $tmp;
// Output:
    [0] => 21
    [1] => 25
    [2] => 77
    [3] => 89
    [4] => 98
    [5] => 100

Take note that the inner loop starts from i.

The End

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