C++ Program for Cocktail Sort

Last Updated : 13 Jun, 2022

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:

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.
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.

CPP

   // C++ implementation of Cocktail Sort
#include <bits/stdc++.h>
using namespace std;
 
// Sorts array a[0..n-1] using Cocktail sort
void CocktailSort(int a[], int n)
{
 bool swapped = true;
 int start = 0;
 int end = n - 1;
 
 while (swapped) {
  // 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 (int i = start; i < end; ++i) {
   if (a[i] > a[i + 1]) {
    swap(a[i], a[i + 1]);
    swapped = true;
   }
  }
 
  // if nothing moved, then array is sorted.
  if (!swapped)
   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;
 
  // from right to left, doing the
  // same comparison as in the previous stage
  for (int i = end - 1; i >= start; --i) {
   if (a[i] > a[i + 1]) {
    swap(a[i], a[i + 1]);
    swapped = true;
   }
  }
 
  // increase the starting point, because
  // the last stage would have moved the next
  // smallest number to its rightful spot.
  ++start;
 }
}
 
/* Prints the array */
void printArray(int a[], int n)
{
 for (int i = 0; i < n; i++)
  printf("%d ", a[i]);
 printf("\n");
}
 
// Driver code
int main()
{
 int arr[] = { 5, 1, 4, 2, 8, 0, 2 };
 int n = sizeof(arr) / sizeof(arr[0]);
 CocktailSort(arr, n);
 printf("Sorted array :\n");
 printArray(arr, n);
 return 0;
}
 
  

Output:

Sorted array : 0 1 2 2 4 5 8

Worst and Average Case Time Complexity: O(n²).
Best Case Time Complexity: O(n). The best-case occurs when the array is already sorted.
Auxiliary Space: O(1)

Please refer complete article on Cocktail Sort for more details!