Cocktail shaker sort, also known as bidirectional bubble sort, cocktail sort, shaker sort (which can also refer to a variant of selection sort), 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 efficient algorithms such as quicksort, merge sort, or timsort 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[i] > A[i + 1] then <span style="color:green">// test whether the two elements are in the wrong order</span>
swap(A[i], A[i + 1]) <span style="color:green">// let the two elements change places</span>
swapped := true
end if
end for
if not swapped then
<span style="color:green">// we can exit the outer loop here if no swaps occurred.</span>
break do-while loop
end if
swapped := false
for each i in length(A) − 2 to 0 do:
if A[i] > A[i + 1] then
swap(A[i], A[i + 1])
swapped := true
end if
end for
while swapped <span style="color:green">// if no elements have been swapped, then the list is sorted</span>
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.
<syntaxhighlight lang="matlab">
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
</syntaxhighlight>
Differences from bubble sort
Cocktail shaker sort is a slight variation of bubble sort.