Data Structure - Selection Sort

Overview

Selection sort is a simple sorting algorithm. This sorting algorithm is a in-place comparison based algorithm in which the list is divided into two parts, sorted part at left end and unsorted part at right end. Initially sorted part is empty and unsorted part is entire list.

Smallest element is seleced from the unsorted array and swapped with the leftmost element and that element becomes part of sorted array. This process continues moving unsorted array boundary by one element to the right.

This algorithm is not suitable for large data sets as its average and worst case complexity are of O(n²) where n are no. of items.

Pseudocode

Selection Sort ( A: array of item)
   procedure selectionSort( A : array of items )
   int indexMin
   for i = 1 to length(A) - 1 inclusive do:
      /* set current element as minimum*/
      indexMin = i    
      /* check the element to be minimum */
      for j = i+1 to length(A) - 1 inclusive do:
         if(intArray[j] < intArray[indexMin]){
             indexMin = j;
         }
      end for
      /* swap the minimum element with the current element*/
      if(indexMin != i) then
         swap(A[indexMin],A[i])
      end if
   end for
end procedure

Demo Program

#include <stdio.h>
#include <stdbool.h>
#define MAX 7

int intArray[MAX] = {4,6,3,2,1,9,7};

void printline(int count){
   int i;
   for(i=0;i <count-1;i++){
      printf("=");
   }
   printf("=\n");
}

void display(){
   int i;
   printf("[");
   // navigate through all items 
   for(i=0;i<MAX;i++){
      printf("%d ",intArray[i]);
   }
   printf("]\n");
}

void selectionSort(){

  int indexMin,i,j;       
  // loop through all numbers 
  for(i=0; i < MAX-1; i++){ 
	 // set current element as minimum 
	 indexMin = i;
	 // check the element to be minimum 
	 for(j=i+1;j<MAX;j++){
		if(intArray[j] < intArray[indexMin]){
		   indexMin = j;
		}
	 }

	 if(indexMin != i){
		printf("     Items swapped: [ %d, %d ]\n" ,intArray[i],intArray[indexMin]);            
		// swap the numbers 
		int temp=intArray[indexMin];
		intArray[indexMin] = intArray[i];
		intArray[i] = temp;
	 }          

	  printf("Iteration %d#:",(i+1));
	  display();
  }
}  

main(){
   printf("Input Array: ");
   display();
   printline(50);
   selectionSort();
   printf("Output Array: ");
   display();
   printline(50);
}

If we compile and run the above program then it would produce following result:

Input Array: [4, 6, 3, 2, 1, 9, 7]
==================================================
     Items swapped: [ 4, 1 ]
iteration 1#: [1, 6, 3, 2, 4, 9, 7]
     Items swapped: [ 6, 2 ]
iteration 2#: [1, 2, 3, 6, 4, 9, 7]
iteration 3#: [1, 2, 3, 6, 4, 9, 7]
     Items swapped: [ 6, 4 ]
iteration 4#: [1, 2, 3, 4, 6, 9, 7]
iteration 5#: [1, 2, 3, 4, 6, 9, 7]
     Items swapped: [ 9, 7 ]
iteration 6#: [1, 2, 3, 4, 6, 7, 9]
Output Array: [1, 2, 3, 4, 6, 7, 9]
==================================================