ShellSort

3

ShellSort is mainly a variation of Insertion Sort. In insertion sort, we move elements only one position ahead. When an element has to be moved far ahead, many movements are involved. The idea of shellSort is to allow exchange of far items. In shellSort, we make the array h-sorted for a large value of h. We keep reducing the value of h until it becomes 1. An array is said to be h-sorted if all sublists of every h’th element is sorted.

Following is C++ implementation of ShellSort.

C++

// C++ implementation of Shell Sort
#include  <iostream>
using namespace std;

/* function to sort arr using shellSort */
int shellSort(int arr[], int n)
{
    // Start with a big gap, then reduce the gap
    for (int gap = n/2; gap > 0; gap /= 2)
    {
        // Do a gapped insertion sort for this gap size.
        // The first gap elements a[0..gap-1] are already in gapped order
        // keep adding one more element until the entire array is
        // gap sorted 
        for (int i = gap; i < n; i += 1)
        {
            // add a[i] to the elements that have been gap sorted
            // save a[i] in temp and make a hole at position i
            int temp = arr[i];

            // shift earlier gap-sorted elements up until the correct 
            // location for a[i] is found
            int j;            
            for (j = i; j >= gap && arr[j - gap] > temp; j -= gap)
                arr[j] = arr[j - gap];
            
            //  put temp (the original a[i]) in its correct location
            arr[j] = temp;
        }
    }
    return 0;
}

void printArray(int arr[], int n)
{
    for (int i=0; i<n; i++)
        cout << arr[i] << " ";
}

int main()
{
    int arr[] = {12, 34, 54, 2, 3}, i;
    int n = sizeof(arr)/sizeof(arr[0]);

    cout << "Array before sorting: \n";
    printArray(arr, n);

    shellSort(arr, n);

    cout << "\nArray after sorting: \n";
    printArray(arr, n);

    return 0;
}

Java

// Java implementation of ShellSort
class ShellSort
{
    /* An utility function to print array of size n*/
    static void printArray(int arr[])
    {
        int n = arr.length;
        for (int i=0; i<n; ++i)
            System.out.print(arr[i] + " ");
        System.out.println();
    }

    /* function to sort arr using shellSort */
    int sort(int arr[])
    {
        int n = arr.length;

        // Start with a big gap, then reduce the gap
        for (int gap = n/2; gap > 0; gap /= 2)
        {
            // Do a gapped insertion sort for this gap size.
            // The first gap elements a[0..gap-1] are already
            // in gapped order keep adding one more element
            // until the entire array is gap sorted
            for (int i = gap; i < n; i += 1)
            {
                // add a[i] to the elements that have been gap
                // sorted save a[i] in temp and make a hole at
                // position i
                int temp = arr[i];

                // shift earlier gap-sorted elements up until
                // the correct location for a[i] is found
                int j;
                for (j = i; j >= gap && arr[j - gap] > temp; j -= gap)
                    arr[j] = arr[j - gap];

                // put temp (the original a[i]) in its correct
                // location
                arr[j] = temp;
            }
        }
        return 0;
    }

    // Driver method
    public static void main(String args[])
    {
        int arr[] = {12, 34, 54, 2, 3};
        System.out.println("Array before sorting");
        printArray(arr);

        ShellSort ob = new ShellSort();
        ob.sort(arr);

        System.out.println("Array after sorting");
        printArray(arr);
    }
} 
/*This code is contributed by Rajat Mishra */

Python

# Python program for implementation of Shell Sort

def shellSort(arr):

    # Start with a big gap, then reduce the gap
    n = len(arr)
    gap = n/2

    # Do a gapped insertion sort for this gap size.
    # The first gap elements a[0..gap-1] are already in gapped 
    # order keep adding one more element until the entire array
    # is gap sorted
    while gap > 0:

        for i in range(gap,n):

            # add a[i] to the elements that have been gap sorted
            # save a[i] in temp and make a hole at position i
            temp = arr[i]

            # shift earlier gap-sorted elements up until the correct
            # location for a[i] is found
            j = i
            while  j >= gap and arr[j-gap] >temp:
                arr[j] = arr[j-gap]
                j -= gap

            # put temp (the original a[i]) in its correct location
            arr[j] = temp
        gap /= 2


# Driver code to test above
arr = [ 12, 34, 54, 2, 3]

n = len(arr)
print ("Array before sorting:")
for i in range(n):
    print(arr[i]),

shellSort(arr)

print ("\nArray after sorting:")
for i in range(n):
    print(arr[i]),

# This code is contributed by Mohit Kumra


Output:
Array before sorting:
12 34 54 2 3
Array after sorting:
2 3 12 34 54

Time Complexity: Time complexity of above implementation of shellsort is O(n2). In the above implementation gap is reduce by half in every iteration. There are many other ways to reduce gap which lead to better time complexity. See this for more details.

References:
https://www.youtube.com/watch?v=pGhazjsFW28
http://en.wikipedia.org/wiki/Shellsort

 

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Quiz on Shell Sort

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