Shell Sort Algorithm

In this tutorial, you will learn how shell sort works. Also, you will find working examples of shell sort in C, C++, Java and Python.

Shell sort is an algorithm that first sorts the elements far apart from each other and successively reduces the interval between the elements to be sorted. It is a generalized version of insertion sort.

In shell sort, elements at a specific interval are sorted. The interval between the elements is gradually decreased based on the sequence used. the performance of the shell sort depends on the type of sequence used for a given input array.

Some of the optimal sequences used are:

  • Shell’s original sequence: N/2 , N/4 , …, 1
  • Knuth’s increments: 1, 4, 13, …, (3k – 1) / 2
  • Sedgewick’s increments: 1, 8, 23, 77, 281, 1073, 4193, 16577...4j+1+ 3·2j+ 1.
  • Hibbard’s increments: 1, 3, 7, 15, 31, 63, 127, 255, 511…
  • Papernov & Stasevich increment: 1, 3, 5, 9, 17, 33, 65,...
  • Pratt: 1, 2, 3, 4, 6, 9, 8, 12, 18, 27, 16, 24, 36, 54, 81....

How Shell Sort Works?

    1. Suppose, we need to sort the following array.
      Shell sort step
    2. We are using the shell’s original sequence (N/2, N/4, ...1) as intervals in our algorithm.

      In the first loop, if the array size is N = 8 then, the elements lying at the interval of N/2 = 4 are compared and swapped if they are not in order.
      1. The 0th element is compared with the 4th element.
      2. If the 0th element is greater than the 4th one then, the 4th element is first stored in temp variable and the 0th element (ie. greater element) is stored in the 4th position and the element stored in temp is stored in the 0th position.
      Shell Sort step
      This process goes on for all the remaining elements.
      Shell Sort steps
    3. In the second loop, an interval of N/4 = 8/4 = 2 is taken and again the elements lying at these intervals are sorted.
      Shell Sort step
      You might get confused at this point.Shell Sort step
      The elements at 4th and 2nd position are compared. The elements at 2nd and 0th position are also compared. All the elements in the array lying at the current interval are compared.
       
    4. The same process goes on for remaining elements.
      Shell Sort step
    5. Finally, when the interval is N/8 = 8/8 =1 then the array elements lying at the interval of 1 are sorted. The array is now completely sorted.
      Shell Sort step

      Shell Sort Algorithm

      shellSort(array, size)
        for interval i <- size/2n down to 1
          for each interval "i" in array
              sort all the elements at interval "i"
      end shellSort

      Python, Java and C/C++ Examples

      # Python3 program for implementation of Shell Sort
      
      
      def shellSort(array, n):
          gap = n // 2
          while gap > 0:
              for i in range(gap, n):
                  temp = array[i]
                  j = i
                  while j >= gap and array[j - gap] > temp:
                      array[j] = array[j - gap]
                      j -= gap
      
                  array[j] = temp
              gap //= 2
      
      
      data = [9, 8, 3, 7, 5, 6, 4, 1]
      size = len(data)
      shellSort(data, size)
      print('Sorted Array in Ascending Order:')
      print(data)
      // Shell sort in Java programming
      
      import java.util.Arrays;
      
      class ShellSort{
        void shellSort(int array[], int n){
          for (int gap = n/2; gap > 0; gap /= 2){
            for (int i = gap; i < n; i += 1) {
              int temp = array[i];
              int j;
              for (j = i; j >= gap && array[j - gap] > temp; j -= gap){
                array[j] = array[j - gap];
              }
              array[j] = temp;
            }
          }
        }
      
        public static void main(String args[]){
          int[] data={9, 8, 3, 7, 5, 6, 4, 1};
          int size=data.length;
          ShellSort ss = new ShellSort();
          ss.shellSort(data, size);
          System.out.println("Sorted Array in Ascending Order: ");
          System.out.println(Arrays.toString(data));
        }
      }
      // Shell Sort in C programming
      
      #include <stdio.h>
      
      void shellSort(int array[], int n){
        for (int gap = n/2; gap > 0; gap /= 2){
          for (int i = gap; i < n; i += 1) {
            int temp = array[i];
            int j;
            for (j = i; j >= gap && array[j - gap] > temp; j -= gap){
              array[j] = array[j - gap];
            }
            array[j] = temp;
          }
        }
      }
      void printArray(int array[], int size){
        for(int i=0; i<size; ++i){
           printf("%d  ", array[i]);
        }
        printf("\n");
      }
      int main(){
        int data[]={9, 8, 3, 7, 5, 6, 4, 1}; 
        int size=sizeof(data) / sizeof(data[0]); 
        shellSort(data, size); 
        printf("Sorted array: \n"); 
        printArray(data, size);
      }
      // Shell Sort in C++ programming
      
      #include <iostream>
      using namespace std;
      
      void shellSort(int array[], int n)
      {
        for (int gap = n / 2; gap > 0; gap /= 2)
        {
          for (int i = gap; i < n; i += 1)
          {
            int temp = array[i];
            int j;
            for (j = i; j >= gap && array[j - gap] > temp; j -= gap)
            {
              array[j] = array[j - gap];
            }
            array[j] = temp;
          }
        }
      }
      void printArray(int array[], int size)
      {
        int i;
        for (i = 0; i < size; i++)
          cout << array[i] << " ";
        cout << endl;
      }
      int main()
      {
        int data[] = {9, 8, 3, 7, 5, 6, 4, 1};
        int size = sizeof(data) / sizeof(data[0]);
        shellSort(data, size);
        cout << "Sorted array: \n";
        printArray(data, size);
      }

      Complexity

      Shell sort is unstable sorting algorithm because this algorithm does not examine the elements lying in between the intervals.

      Time Complexity

      • Worst Case Complexity: less than or equal to O(n2)

        Worst case complexity for shell sort is always less than or equal to O(n2).

        According to Poonen Theorem, worst case complexity for shell sort is Θ(NlogN)2/(log logN)2) or Θ(NlogN)2/log logN) or Θ(N(logN)2) or something in between.
         
      • Best Case Complexity: O(n*log n)

        When the array is already sorted, the total number of comparison for each interval (or increment) is equal to the size of the array.
         
      • Average Case Complexity: O(n*log n)

        It is around O(n1.25).

      The complexity depends on the interval chosen. The above complexities differ for different increment sequence chosen. Best increment sequence is unknown.

      Space Complexity:

      The space complexity for shell sort is O(1).


      Shell Sort Applications

      Shell sort is used when:

      • calling a stack is overhead. uClibc library uses this sort.
      • recursion exceeds a limit. bzip2 compressor uses it.
      • Insertion sort does not perform well when the close elements are far apart. Shell sort helps in reducing the distance between the close elements. Thus, there will be less number of swappings to be performed.