Python: Shell sort

Last update on April 23 2025 12:50:17 (UTC/GMT +8 hours)

7. Shell Sort

Write a Python program to sort a list of elements using shell sort algorithm.
Note : According to Wikipedia "Shell sort or Shell's method, is an in-place comparison sort. It can be seen as either a generalization of sorting by exchange (bubble sort) or sorting by insertion (insertion sort). The method starts by sorting pairs of elements far apart from each other, then progressively reducing the gap between elements to be compared. Starting with far apart elements can move some out-of-place elements into position faster than a simple nearest neighbor exchange."

Sample Solution:

Python Code:

def shellSort(alist):
    sublistcount = len(alist)//2
    while sublistcount > 0:
      for start_position in range(sublistcount):
        gap_InsertionSort(alist, start_position, sublistcount)

      print("After increments of size",sublistcount, "The list is",nlist)

      sublistcount = sublistcount // 2

def gap_InsertionSort(nlist,start,gap):
    for i in range(start+gap,len(nlist),gap):

        current_value = nlist[i]
        position = i

        while position>=gap and nlist[position-gap]>current_value:
            nlist[position]=nlist[position-gap]
            position = position-gap

        nlist[position]=current_value


nlist = [14,46,43,27,57,41,45,21,70]
shellSort(nlist)
print(nlist)

Sample Output:

After increments of size 4 The list is [14, 41, 43, 21, 57, 46, 45, 27, 70]                                   
After increments of size 2 The list is [14, 21, 43, 27, 45, 41, 57, 46, 70]                                   
After increments of size 1 The list is [14, 21, 27, 41, 43, 45, 46, 57, 70]                                   
[14, 21, 27, 41, 43, 45, 46, 57, 70]

Flowchart:

For more Practice: Solve these Related Problems:

Write a Python program to implement shell sort and experiment with different gap sequences to compare performance.
Write a Python script to sort a list using shell sort and print the intermediate list after each gap reduction.
Write a Python program to modify shell sort to sort a list of tuples based on a specified key.
Write a Python function to implement shell sort and analyze the number of comparisons for various input sizes.

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