Sorting Algorithms- 1 | Algorithms - Computer Science Engineering (CSE) PDF Download

Selection Sort

The selection sort algorithm sorts an array by repeatedly finding the minimum element (considering ascending order) from unsorted part and putting it at the beginning. The algorithm maintains two subarrays in a given array.

1. The subarray which is already sorted.
2. Remaining subarray which is unsorted.

In every iteration of selection sort, the minimum element (considering ascending order) from the unsorted subarray is picked and moved to the sorted subarray.
Following example explains the above steps:
arr[] = 64 25 12 22 11
// Find the minimum element in arr[0...4]
// and place it at beginning
11 25 12 22 64
// Find the minimum element in arr[1...4]
// and place it at beginning of arr[1...4]
11 12 25 22 64
// Find the minimum element in arr[2...4]
// and place it at beginning of arr[2...4]
11 12 22 25 64
// Find the minimum element in arr[3...4]
// and place it at beginning of arr[3...4]
11 12 22 25 64

Flowchart of the Selection Sort:

• C++
  // C++ program for implementation of selection sort
  #include <bits/stdc++.h>
  using namespace std;
  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
          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
          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;
  }
• Python
  # 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]),
• 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.
  ?>

Output:
Sorted array:
11 12 22 25 64
Time Complexity: O(n²) as there are two nested loops.
Auxiliary Space: O(1)
The good thing about selection sort is it never makes more than O(n) swaps and can be useful when memory write is a costly operation.
Stability: The default implementation is not stable. However it can be made stable. Please see stable selection sort for details.
In Place: Yes, it does not require extra space.

Snapshots:

Quiz on Selection Sort

Other Sorting Algorithms on GeeksforGeeks/GeeksQuiz:

• Bubble Sort
• Insertion Sort
• Merge Sort
• Heap Sort
• QuickSort
• Radix Sort
• Counting Sort
• Bucket Sort
• ShellSort