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Selection Sort Algorithm

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  • Difficulty Level : Easy
  • Last Updated : 01 Sep, 2022
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The selection sort algorithm sorts an array by repeatedly finding the minimum element (considering ascending order) from the unsorted part and putting it at the beginning. 

The algorithm maintains two subarrays in a given array.

  • The subarray which already sorted. 
  • The remaining subarray was unsorted.

In every iteration of the selection sort, the minimum element (considering ascending order) from the unsorted subarray is picked and moved to the sorted subarray. 

Flowchart of the Selection Sort: 
 

Selection-Sort-Flowhchart
How selection sort works?

Lets consider the following array as an example: arr[] = {64, 25, 12, 22, 11}

First pass:

  • For the first position in the sorted array, the whole array is traversed from index 0 to 4 sequentially. The first position where 64 is stored presently, after traversing whole array it is clear that 11 is the lowest value.
   64      25      12      22      11   
  • Thus, replace 64 with 11. After one iteration 11, which happens to be the least value in the array, tends to appear in the first position of the sorted list.
   11      25      12      22      64   

Second Pass:

  • For the second position, where 25 is present, again traverse the rest of the array in a sequential manner.
   11      25      12      22      64   
  • After traversing, we found that 12 is the second lowest value in the array and it should appear at the second place in the array, thus swap these values.
   11      12      25      22      64   

Third Pass:

  • Now, for third place, where 25 is present again traverse the rest of the array and find the third least value present in the array.
   11      12      25      22      64   
  • While traversing, 22 came out to be the third least value and it should appear at the third place in the array, thus swap 22 with element present at third position.
   11      12      22      25      64   

Fourth pass:

  • Similarly, for fourth position traverse the rest of the array and find the fourth least element in the array 
  • As 25 is the 4th lowest value hence, it will place at the fourth position.
   11      12      22      25      64   

Fifth Pass:

  • At last the largest value present in the array automatically get placed at the last position in the array
  • The resulted array is the sorted array.
   11      12      22      25      64   
Recommended Practice

Follow the below steps to solve the problem:

  • Initialize minimum value(min_idx) to location 0.
  • Traverse the array to find the minimum element in the array.
  • While traversing if any element smaller than min_idx is found then swap both the values.
  • Then, increment min_idx to point to the next element.
  • Repeat until the array is sorted.

Complete Interview Preparation - GFG

Below is the implementation of the above approach:

C++




// C++ program for implementation of
// selection sort
#include <bits/stdc++.h>
using namespace std;
 
//Swap function
void swap(int *xp, int *yp)
{
    int temp = *xp;
    *xp = *yp;
    *yp = temp;
}
 
void selectionSort(int arr[], int n)
{
    int i, j, min_idx;
 
    // One by one move boundary of
    // unsorted subarray
    for (i = 0; i < n-1; i++)
    {
       
        // Find the minimum element in
        // unsorted array
        min_idx = i;
        for (j = i+1; j < n; j++)
        if (arr[j] < arr[min_idx])
            min_idx = j;
 
        // Swap the found minimum element
        // with the first element
        if(min_idx!=i)
            swap(&arr[min_idx], &arr[i]);
    }
}
 
//Function to print an array
void printArray(int arr[], int size)
{
    int i;
    for (i=0; i < size; i++)
        cout << arr[i] << " ";
    cout << endl;
}
 
// Driver program to test above functions
int main()
{
    int arr[] = {64, 25, 12, 22, 11};
    int n = sizeof(arr)/sizeof(arr[0]);
    selectionSort(arr, n);
    cout << "Sorted array: \n";
    printArray(arr, n);
    return 0;
}
// This is code is contributed by rathbhupendra

C




// C program for implementation of selection sort
#include <stdio.h>
 
void swap(int *xp, int *yp)
{
    int temp = *xp;
    *xp = *yp;
    *yp = temp;
}
 
void selectionSort(int arr[], int n)
{
    int i, j, min_idx;
 
    // One by one move boundary of unsorted subarray
    for (i = 0; i < n-1; i++)
    {
        // Find the minimum element in unsorted array
        min_idx = i;
        for (j = i+1; j < n; j++)
          if (arr[j] < arr[min_idx])
            min_idx = j;
 
        // Swap the found minimum element with the first element
           if(min_idx != i)
            swap(&arr[min_idx], &arr[i]);
    }
}
 
/* Function to print an array */
void printArray(int arr[], int size)
{
    int i;
    for (i=0; i < size; i++)
        printf("%d ", arr[i]);
    printf("\n");
}
 
// Driver program to test above functions
int main()
{
    int arr[] = {64, 25, 12, 22, 11};
    int n = sizeof(arr)/sizeof(arr[0]);
    selectionSort(arr, n);
    printf("Sorted array: \n");
    printArray(arr, n);
    return 0;
}

Python3




# Python program for implementation of Selection
# Sort
import sys
A = [64, 25, 12, 22, 11]
 
# Traverse through all array elements
for i in range(len(A)):
     
    # Find the minimum element in remaining
    # unsorted array
    min_idx = i
    for j in range(i+1, len(A)):
        if A[min_idx] > A[j]:
            min_idx = j
             
    # Swap the found minimum element with
    # the first element       
    A[i], A[min_idx] = A[min_idx], A[i]
 
# Driver code to test above
print ("Sorted array")
for i in range(len(A)):
    print("%d" %A[i],end=" ")

Java




// Java program for implementation of Selection Sort
class SelectionSort
{
    void sort(int arr[])
    {
        int n = arr.length;
 
        // One by one move boundary of unsorted subarray
        for (int i = 0; i < n-1; i++)
        {
            // Find the minimum element in unsorted array
            int min_idx = i;
            for (int j = i+1; j < n; j++)
                if (arr[j] < arr[min_idx])
                    min_idx = j;
 
            // Swap the found minimum element with the first
            // element
            int temp = arr[min_idx];
            arr[min_idx] = arr[i];
            arr[i] = temp;
        }
    }
 
    // Prints the array
    void printArray(int arr[])
    {
        int n = arr.length;
        for (int i=0; i<n; ++i)
            System.out.print(arr[i]+" ");
        System.out.println();
    }
 
    // Driver code to test above
    public static void main(String args[])
    {
        SelectionSort ob = new SelectionSort();
        int arr[] = {64,25,12,22,11};
        ob.sort(arr);
        System.out.println("Sorted array");
        ob.printArray(arr);
    }
}
/* This code is contributed by Rajat Mishra*/

C#




// C# program for implementation
// of Selection Sort
using System;
 
class GFG
{
    static void sort(int []arr)
    {
        int n = arr.Length;
 
        // One by one move boundary of unsorted subarray
        for (int i = 0; i < n - 1; i++)
        {
            // Find the minimum element in unsorted array
            int min_idx = i;
            for (int j = i + 1; j < n; j++)
                if (arr[j] < arr[min_idx])
                    min_idx = j;
 
            // Swap the found minimum element with the first
            // element
            int temp = arr[min_idx];
            arr[min_idx] = arr[i];
            arr[i] = temp;
        }
    }
 
    // Prints the array
    static void printArray(int []arr)
    {
        int n = arr.Length;
        for (int i=0; i<n; ++i)
            Console.Write(arr[i]+" ");
        Console.WriteLine();
    }
 
    // Driver code
    public static void Main()
    {
        int []arr = {64,25,12,22,11};
        sort(arr);
        Console.WriteLine("Sorted array");
        printArray(arr);
    }
 
}
// This code is contributed by Sam007

PHP




<?php
// PHP program for implementation
// of selection sort
function selection_sort(&$arr, $n)
{
    for($i = 0; $i < $n ; $i++)
    {
        $low = $i;
        for($j = $i + 1; $j < $n ; $j++)
        {
            if ($arr[$j] < $arr[$low])
            {
                $low = $j;
            }
        }
         
        // swap the minimum value to $ith node
        if ($arr[$i] > $arr[$low])
        {
            $tmp = $arr[$i];
            $arr[$i] = $arr[$low];
            $arr[$low] = $tmp;
        }
    }
}
 
// Driver Code
$arr = array(64, 25, 12, 22, 11);
$len = count($arr);
selection_sort($arr, $len);
echo "Sorted array : \n";
 
for ($i = 0; $i < $len; $i++)
    echo $arr[$i] . " ";
 
// This code is contributed
// by Deepika Gupta.
?>

Javascript




<script>
// Javascript program for implementation of selection sort
function swap(arr,xp, yp)
{
    var temp = arr[xp];
    arr[xp] = arr[yp];
    arr[yp] = temp;
}
 
function selectionSort(arr,  n)
{
    var i, j, min_idx;
 
    // One by one move boundary of unsorted subarray
    for (i = 0; i < n-1; i++)
    {
        // Find the minimum element in unsorted array
        min_idx = i;
        for (j = i + 1; j < n; j++)
        if (arr[j] < arr[min_idx])
            min_idx = j;
 
        // Swap the found minimum element with the first element
        swap(arr,min_idx, i);
    }
}
 
function printArray( arr,  size)
{
    var i;
    for (i = 0; i < size; i++)
        document.write(arr[i] + " ");
    document.write(" <br>");
}
 
var arr = [64, 25, 12, 22, 11];
    var n = 5;
    selectionSort(arr, n);
    document.write("Sorted array: <br>");
    printArray(arr, n);
 
// This code is contributed by akshitsaxenaa09.
</script>

Output

Sorted array: 
11 12 22 25 64 

Complexity Analysis of Selection Sort:

Time Complexity: The time complexity of Selection Sort is O(N2) as there are two nested loops:

  • One loop to select an element of Array one by one = O(N)
  • Another loop to compare that element with every other Array element = O(N)

Therefore overall complexity = O(N) * O(N) = O(N*N) = O(N2)

Auxiliary Space: O(1) as the only extra memory used is for temporary variables while swapping two values in Array. The selection sort never makes more than O(N) swaps and can be useful when memory write is a costly operation. 

Sort an array of strings using Selection Sort

Is Selection Sort Algorithm stable?

Stability: The default implementation is not stable. However, it can be made stable. Please see stable selection sort for details.

Is Selection Sort Algorithm in place?

Yes, it does not require extra space.

Snapshots: Quiz on Selection Sort
Other Sorting Algorithms on GeeksforGeeks/GeeksQuiz: Coding practice for sorting

Please write comments if you find anything incorrect, or if you want to share more information about the topic discussed above 


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