Question

I have a problem to unfold a matrix. Here is how the output of the programm should look like. I am little bit stuck.

unfoldMatrix :: [ [a] ] -> [a]
Main> unfoldMatrix [[1, 2, 3],
                [4, 5, 6],
                [7, 8, 9],
                [10, 11, 12]]
[1,4,7,10,11,12,9,6,3,2,5,8]

My code works but it’s output is in this format

[[1,4,7],[8,9],[6,3],[2],[5],[]]

Any ideat how to change the code to work as wanted?

transpose2:: [[a]]->[[a]]
transpose2 ([]:_) = []
transpose2 x = (map head x) : transpose2 (map tail x)


unfoldMatrix:: [[a]]->[[a]]
unfoldMatrix ([]:_) = []
unfoldMatrix x =(map head x):unfoldMatrix(tail2(x))

rotate90 :: [ [ a ] ] -> [ [ a ] ]
rotate90 = (map reverse).transpose2

tail2:: [[a]]->[[a]]
tail2 = (tail).rotate90

Answer 1

You don’t need transpose2, if all sublists are equally long, that’s the same as transpose from Data.List. So your unfold would simply be

Prelude Data.List> let unfold xxs@((_:_):_) = map head xxs ++ unfold (map reverse . transpose $ map tail xxs); unfold _ = []
Prelude Data.List> unfold [[1,2,3],[4,5,6],[7,8,9]]
[1,4,7,8,9,6,3,2,5]
Prelude Data.List> unfold [[1,2,3],[4,5,6],[7,8,9],[10,11,12]]
[1,4,7,10,11,12,9,6,3,2,5,8]