Selection

In computer science, a selection sort is a sorting algorithm, specifically an in-place comparison sort. It has O(n2) time complexity, making it inefficient on large lists, and generally performs worse than the similar insertion sort. Selection sort is noted for its simplicity, and it has performance advantages over more complicated algorithms in certain situations, particularly where auxiliary memory is limited.

The below image shows how the selection sort works-

enter image description here

Below pseudo code helps in creating a program(in any language) or understanding selection sort.

procedure selection sort 
list  : array of items
n     : size of list

for i = 1 to n - 1
/* set current element as minimum*/
  min = i    

  /* check the element to be minimum */

  for j = i+1 to n 
     if list[j] < list[min] then
        min = j;
     end if
  end for

  /* swap the minimum element with the current element*/
  if indexMin != i  then
     swap list[min] and list[i]
  end if

end for

end procedure

Advantages :

  • it’s too simple to understand
  • it performs well on a small list
  • no additional temporary storage is required beyond what is needed to hold the original list

Image Reference: RMIT University

Selection Sort (Python)

Animation to show how selection sort works

enter image description here

The below example shows selection sort in Python

def sort_selection(my_list):

for pos_upper in xrange( len(my_list)-1, 0, -1):
    max_pos = 0
    for i in xrange(1, pos_upper + 1):
        if(my_list[i] > my_list[max_pos]):
            max_pos = i
            print "resetting max_pos = " + str(max_pos)

    my_list[pos_upper], my_list[max_pos] = my_list[max_pos], my_list[pos_upper]
    print "pos_upper: " + str(pos_upper) + " max_pos: " + str(max_pos) + " my_list: " + str(my_list)

return my_list


if __name__ == "__main__":

    my_list = [54,26,93,17,77,31,44,55,20]
    print "my_list: " + str(my_list)
    print sort_selection(my_list)

Output of the program:

my_list: [54, 26, 93, 17, 77, 31, 44, 55, 20]
resetting max_pos = 2
pos_upper: 8 max_pos: 2 my_list: [54, 26, 20, 17, 77, 31, 44, 55, 93]
resetting max_pos = 4
pos_upper: 7 max_pos: 4 my_list: [54, 26, 20, 17, 55, 31, 44, 77, 93]
resetting max_pos = 4
pos_upper: 6 max_pos: 4 my_list: [54, 26, 20, 17, 44, 31, 55, 77, 93]
pos_upper: 5 max_pos: 0 my_list: [31, 26, 20, 17, 44, 54, 55, 77, 93]
resetting max_pos = 4
pos_upper: 4 max_pos: 4 my_list: [31, 26, 20, 17, 44, 54, 55, 77, 93]
pos_upper: 3 max_pos: 0 my_list: [17, 26, 20, 31, 44, 54, 55, 77, 93]
resetting max_pos = 1
pos_upper: 2 max_pos: 1 my_list: [17, 20, 26, 31, 44, 54, 55, 77, 93]
resetting max_pos = 1
pos_upper: 1 max_pos: 1 my_list: [17, 20, 26, 31, 44, 54, 55, 77, 93]
[17, 20, 26, 31, 44, 54, 55, 77, 93]

Image Reference: Pirate Learner

Selection Sort (Java)

Animation to show how selection sort works

enter image description here

Below example shows selection sort in ascending order:

public class MySelectionSort {
 
    public static int[] doSelectionSort(int[] arr){
         
        for (int i = 0; i < arr.length - 1; i++)
        {
            int index = i;
            for (int j = i + 1; j < arr.length; j++)
                if (arr[j] < arr[index]) 
                    index = j;
      
            int smallerNumber = arr[index];  
            arr[index] = arr[i];
            arr[i] = smallerNumber;
        }
        return arr;
    }

I’ve written a sample main() method to show the output of the selection sort:

    public static void main(String a[]){
         
        int[] arr1 = {10,34,2,56,7,67,88,42};
        int[] arr2 = doSelectionSort(arr1);
        for(int i:arr2){
            System.out.print(i);
            System.out.print(", ");
        }
    }
}

Output of the program :

2, 7, 10, 34, 42, 56, 67, 88

Below example shows selection sort in descending order:

public static void doDescendingSelectionSort ( int [ ] num )
{
     int i, j, first, temp;  
     for ( i = num.length - 1; i > 0; i - - )  
     {
          first = 0;   //initialize to subscript of first element
          for(j = 1; j <= i; j ++)   //locate smallest element between positions 1 and i.
          {
               if( num[ j ] < num[ first ] )         
                 first = j;
          }
          temp = num[ first ];   //swap smallest found with element in position i.
          num[ first ] = num[ i ];
          num[ i ] = temp; 
      }           
}

Image Reference : Wikipedia