Cocktail shaker sort,
also known as bidirectional bubble sort, cocktail sort, shaker sort (which can also refer to a variant of
selection sort
In computer science, selection sort is an in-place comparison sorting algorithm. It has an O(''n''2) time complexity, which makes it inefficient on large lists, and generally performs worse than the similar insertion sort. Selection sort is no ...
), ripple sort, shuffle sort,
or shuttle sort, is an extension of
bubble sort. The algorithm extends bubble sort by operating in two directions. While it improves on bubble sort by more
quickly moving items to the beginning of the list, it provides only marginal performance improvements.
Like most variants of bubble sort, cocktail shaker sort is used primarily as an educational tool. More performant algorithms such as
quicksort
Quicksort is an efficient, general-purpose sorting algorithm. Quicksort was developed by British computer scientist Tony Hoare in 1959 and published in 1961, it is still a commonly used algorithm for sorting. Overall, it is slightly faster than ...
,
merge sort
In computer science, merge sort (also commonly spelled as mergesort) is an efficient, general-purpose, and comparison-based sorting algorithm. Most implementations produce a stable sort, which means that the order of equal elements is the same ...
, or
timsort
Timsort is a hybrid, stable sorting algorithm, derived from merge sort and insertion sort, designed to perform well on many kinds of real-world data. It was implemented by Tim Peters in 2002 for use in the Python programming language. The algor ...
are used by the sorting libraries built into popular programming languages such as Python and Java.
Pseudocode
The simplest form goes through the whole list each time:
procedure cocktailShakerSort(A : list of sortable items) is
do
swapped := false
for each i in 0 to length(A) − 2 do:
if A
> A
+ 1then
// test whether the two elements are in the wrong order
swap(A
A
+ 1 // let the two elements change places
swapped := true
end if
end for
if not swapped then
// we can exit the outer loop here if no swaps occurred.
break do-while loop
end if
swapped := false
for each i in length(A) − 2 to 0 do:
if A
> A
+ 1then
swap(A
A
+ 1
swapped := true
end if
end for
while swapped
// if no elements have been swapped, then the list is sorted
end procedure
The first rightward pass will shift the largest element to its correct place at the end, and the following leftward pass will shift the smallest element to its correct place at the beginning. The second complete pass will shift the second largest and second smallest elements to their correct places, and so on. After ''i'' passes, the first ''i'' and the last ''i'' elements in the list are in their correct positions, and do not need to be checked. By shortening the part of the list that is sorted each time, the number of operations can be halved (see
bubble sort).
This is an example of the algorithm in MATLAB/OCTAVE with the optimization of remembering the last swap index and updating the bounds.
function A = cocktailShakerSort(A)
% `beginIdx` and `endIdx` marks the first and last index to check
beginIdx = 1;
endIdx = length(A) - 1;
while beginIdx <= endIdx
newBeginIdx = endIdx;
newEndIdx = beginIdx;
for ii = beginIdx:endIdx
if A(ii) > A(ii + 1)
(ii+1), A(ii)= deal(A(ii), A(ii+1));
newEndIdx = ii;
end
end
% decreases `endIdx` because the elements after `newEndIdx` are in correct order
endIdx = newEndIdx - 1;
for ii = endIdx:-1:beginIdx
if A(ii) > A(ii + 1)
(ii+1), A(ii)= deal(A(ii), A(ii+1));
newBeginIdx = ii;
end
end
% increases `beginIdx` because the elements before `newBeginIdx` are in correct order
beginIdx = newBeginIdx + 1;
end
end
Differences from bubble sort
Cocktail shaker sort is a slight variation of
bubble sort.
It differs in that instead of repeatedly passing through the list from bottom to top, it passes alternately from bottom to top and then from top to bottom. It can achieve slightly better performance than a standard bubble sort. The reason for this is that
bubble sort only passes through the list in one direction and therefore can only move items backward one step each iteration.
An example of a list that proves this point is the list (2,3,4,5,1), which would only need to go through one pass of cocktail sort to become sorted, but if using an ascending
bubble sort would take four passes. However one cocktail sort pass should be counted as two bubble sort passes. Typically cocktail sort is less than two times faster than bubble sort.
Another optimization can be that the algorithm remembers where the last actual swap has been done. In the next iteration, there will be no swaps beyond this limit and the algorithm has shorter passes. As the cocktail shaker sort goes bidirectionally, the range of possible swaps, which is the range to be tested, will reduce per pass, thus reducing the overall running time slightly.
Complexity
The complexity of the cocktail shaker sort in
big O notation
Big ''O'' notation is a mathematical notation that describes the limiting behavior of a function when the argument tends towards a particular value or infinity. Big O is a member of a family of notations invented by Paul Bachmann, Edmund Lan ...
is
for both the worst case and the average case, but it becomes closer to
if the list is mostly ordered before applying the sorting algorithm. For example, if every element is at a position that differs by at most k (k ≥ 1) from the position it is going to end up in, the complexity of cocktail shaker sort becomes
The cocktail shaker sort is also briefly discussed in the book ''
The Art of Computer Programming
''The Art of Computer Programming'' (''TAOCP'') is a comprehensive monograph written by the computer scientist Donald Knuth presenting programming algorithms and their analysis. Volumes 1–5 are intended to represent the central core of com ...
'', along with similar refinements of bubble sort. In conclusion, Knuth states about bubble sort and its improvements:
References
Sources
*
External links
Interactive demo of cocktail sort*
{{DEFAULTSORT:Cocktail Sort
Sorting algorithms
Comparison sorts
Stable sorts
Articles with example pseudocode