Stooge sort is a recursive sorting algorithm. It is notable for its exceptionally bad time complexity of = The running time of the algorithm is thus slower compared to reasonable sorting algorithms, and is slower than bubble sort, a canonical example of a fairly inefficient sort. It is however more efficient than Slowsort. The name comes from The Three Stooges.
The algorithm is defined as follows: If the value at the start is larger than the value at the end, swap them. If there are 3 or more elements in the list, then: Stooge sort the initial 2/3 of the list Stooge sort the final 2/3 of the list Stooge sort the initial 2/3 of the list again
It is important to get the integer sort size used in the recursive calls by rounding the 2/3 upwards, e.g. rounding 2/3 of 5 should give 4 rather than 3, as otherwise the sort can fail on certain data.
Implementation
Pseudocode
function stoogesort(array L, i = 0, j = length(L)-1){
if L[i] > L[j] then // If the leftmost element is larger than the rightmost element swap(L[i],L[j]) // Then swap them if (j - i + 1) > 2 then // If there are at least 3 elements in the array t = floor((j - i + 1) / 3) stoogesort(L, i, j-t) // Sort the first 2/3 of the array stoogesort(L, i+t, j) // Sort the last 2/3 of the array stoogesort(L, i, j-t) // Sort the first 2/3 of the array again return L }
Haskell
-- Not the best but equal to above
stoogesort :: (Ord a) => [a] -> [a]
stoogesort [] = []
stoogesort src = innerStoogesort src 0 ((length src) - 1)
innerStoogesort :: (Ord a) => [a] -> Int -> Int -> [a]
innerStoogesort src i j
| (j - i + 1) > 2 = src''''
| otherwise = src'
where
src' = swap src i j -- need every call
t = floor (fromIntegral (j - i + 1) / 3.0) src'' = innerStoogesort src' i (j - t) src''' = innerStoogesort src'' (i + t) j src'''' = innerStoogesort src''' i (j - t)
swap :: (Ord a) => [a] -> Int -> Int -> [a]
swap src i j
| a > b = replaceAt (replaceAt src j a) i b
| otherwise = src
where
a = src !! i
b = src !! j
replaceAt :: [a] -> Int -> a -> [a]
replaceAt (x:xs) index value
| index == 0 = value : xs
| otherwise = x : replaceAt xs (index - 1) value
References
Sources
External links
Sorting Algorithms (including Stooge sort) Stooge sort – implementation and comparison