The shell sort improves on the insertion sort by breaking the original list into a number of smaller sub-lists, each of which is sorted using an insertion sort.

The unique way that these sub-lists are chosen is the key to the shell sort. Instead of breaking the list into sub-lists of contiguous items, the shell sort uses an increment i, sometimes called the gap, to create a sub-list by choosing all items that are i items apart.

Resources for Review

Check out the resources below for a review of Shell sort:

• Wikipedia
• Visual Algo
• Sorting Algorithms Animcation with Pseudocode

def shell_sort(arr):
    sublistcount = len(arr)/2
    
    # While we still have sub lists
    while sublistcount > 0:
        for start in range(sublistcount):
            # Use a gap insertion
            gap_insertion_sort(arr,start,sublistcount)

      

        sublistcount = sublistcount / 2

def gap_insertion_sort(arr,start,gap):
    for i in range(start+gap,len(arr),gap):

        currentvalue = arr[i]
        position = i

        # Using the Gap
        while position>=gap and arr[position-gap]>currentvalue:
            arr[position]=arr[position-gap]
            position = position-gap
        
        # Set current value
        arr[position]=currentvalue

arr = [45,67,23,45,21,24,7,2,6,4,90]
shell_sort(arr)
print(arr)

[2, 4, 6, 7, 21, 23, 24, 45, 45, 67, 90]

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Hassan Raza