P53_ShellSort.py

# Author: OMKAR PATHAK
# This program illustrates the shell sort implementation in Python

# 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."

#  Best Case O(n logn); Average Case O(depends on gap sequence); Worst Case O(n)

def shellSort(myList):
    gap = len(myList) // 2
    while gap > 0:
        for i in range(gap, len(myList)):
            currentItem = myList[i]
            j = i
            while j >= gap and myList[j - gap] > currentItem:
                myList[j] = myList[j - gap]
                j -= gap
            myList[j] = currentItem
        gap //= 2

    return myList

if __name__ == '__main__':
    myList = [12, 23, 4, 5, 3, 2, 12, 81, 56, 95]
    print(shellSort(myList))