cocktail shaker sort

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) − 1 do:

if A[i] > A[i + 1] then // test whether the two elements are in the wrong order

swap(A[i], A[i + 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) − 1 to 0 do:

if A[i] > A[i + 1] then

swap(A[i], A[i + 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)

[A(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)

[A(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

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.

The complexity of the cocktail shaker sort in big O notation is $O(n^2)$ for both the worst case and the average case, but it becomes closer to $O(n)$ 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 $O(kn).$

The cocktail shaker sort is also briefly discussed in the book The Art of Computer Programming, along with similar refinements of bubble sort. In conclusion, Knuth states about bubble sort and its improvements:

{{quote|But none of these refinements leads to an algorithm better than straight insertion [that is, insertion sort]; and we already know that straight insertion isn't suitable for large N. [...] In short, the bubble sort seems to have nothing to recommend it, except a catchy name and the fact that it leads to some interesting theoretical problems.|D. E. Knuth

}}

• Dual Cocktail Shaker sort is a variant of Cocktail Shaker Sort that performs a forward and backward pass per iteration simultaneously, improving performance compared to the original.

{{Reflist}}

• {{cite journal|first=R.|last=Hartenstein|journal=The Grand Challenge to Reinvent Computing|title=A new World Model of Computing|publisher=CSBC|date=July 2010|place=Belo Horizonte, Brazil|url=http://www.inf.pucminas.br/sbc2010/anais/pdf/semish/st03_02.pdf|access-date=2011-01-14|archive-url=https://web.archive.org/web/20130807043613/http://www.inf.pucminas.br/sbc2010/anais/pdf/semish/st03_02.pdf|archive-date=2013-08-07|url-status=dead}}

{{Wikibooks|Algorithm implementation|Sorting/Cocktail sort|Cocktail sort}}

• [http://www.hermann-gruber.com/lehre/sorting/Shaker/Shaker-en.html Interactive demo of cocktail sort]
• [https://web.archive.org/web/20061008105719/http://www.cs.ubc.ca/~harrison/Java/sorting-demo.html Java source code and an animated demo of cocktail sort (called bi-directional bubble sort) and several other algorithms]
• {{cite web |url=http://www.sharpdeveloper.net/content/archive/2007/08/14/dot-net-data-structures-and-algorithms.aspx |archive-url=https://web.archive.org/web/20120212173240/http://www.sharpdeveloper.net/content/archive/2007/08/14/dot-net-data-structures-and-algorithms.aspx |title=.NET Implementation of cocktail sort and several other algorithms |archive-date=2012-02-12}}

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