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Python Program for ShellSort

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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. 

Python Program for ShellSort

The provided Python code demonstrates Shell Sort, an optimization of the Insertion Sort algorithm. Shell Sort sorts elements in larger intervals initially, gradually reducing the interval size. The code defines the shellSort function that takes an array arr as input. It initializes a gap size gap as half the length of the array. The algorithm performs a modified insertion sort within each interval, gradually reducing the gap. Elements are compared and shifted to their correct positions. The driver code initializes an array, applies the shellSort function, and prints both the original and sorted arrays. This algorithm’s time complexity depends on the chosen gap sequence, offering improved performance over Insertion Sort.


# 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[] 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 ("\nArray after sorting:")
for i in range(n):


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

Time Complexity: O(n2)

Auxiliary Space: O(1)

Please refer complete article on ShellSort for more details!

Last Updated : 28 Aug, 2023
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