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Python Program for Cocktail Sort

Last Updated : 23 Jul, 2025

Cocktail Sort is a variation of Bubble sort. The Bubble sort algorithm always traverses elements from left and moves the largest element to its correct position in first iteration and second largest in second iteration and so on. Cocktail Sort traverses through a given array in both directions alternatively. Algorithm: Each iteration of the algorithm is broken up into 2 stages:

1. The first stage loops through the array from left to right, just like the Bubble Sort. During the loop, adjacent items are compared and if value on the left is greater than the value on the right, then values are swapped. At the end of first iteration, largest number will reside at the end of the array.
2. The second stage loops through the array in opposite direction- starting from the item just before the most recently sorted item, and moving back to the start of the array. Here also, adjacent items are compared and are swapped if required.

# Python program for implementation of Cocktail Sort

def cocktailSort(a):
    n = len(a)
    swapped = True
    start = 0
    end = n-1
    while (swapped==True):

        # reset the swapped flag on entering the loop,
        # because it might be true from a previous
        # iteration.
        swapped = False

        # loop from left to right same as the bubble
        # sort
        for i in range (start, end):
            if (a[i] > a[i+1]) :
                a[i], a[i+1]= a[i+1], a[i]
                swapped=True

        # if nothing moved, then array is sorted.
        if (swapped==False):
            break

        # otherwise, reset the swapped flag so that it
        # can be used in the next stage
        swapped = False

        # move the end point back by one, because
        # item at the end is in its rightful spot
        end = end-1

        # from right to left, doing the same
        # comparison as in the previous stage
        for i in range(end-1, start-1,-1):
            if (a[i] > a[i+1]):
                a[i], a[i+1] = a[i+1], a[i]
                swapped = True

        # increase the starting point, because
        # the last stage would have moved the next
        # smallest number to its rightful spot.
        start = start+1

# Driver code to test above
a = [5, 1, 4, 2, 8, 0, 2]
cocktailSort(a)
print("Sorted array is:")
for i in range(len(a)):
    print ("%d" %a[i]),

Sorted array is:
0 1 2 2 4 5 8

Time Complexity : O(n²) for worst and average case and O(n) for best case.As

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