CODEWITHSUNDEEP
haskell
Andriy Drozdyuk
Andriy Drozdyuk

Reputation: 61041

Reorganizing a list of lists

How to solve this problem?

The problem is to reorder the list-of-lists of doubles:

[ [a, b, c],  [aa, bb, cc] ]

into this:

[ [a, aa],   [b, bb],  [c, cc] ]

After poking about I came up with the following (a function that increasingly diggs deeper and deeper into sublists, taking their head and joining them together):

organize xs = organize' xs head
--recursive function (type stolen from ghci)
organize':: [[a]] -> ([a] -> b) -> [b]
organize' [] f = []
organize' xs f = (map f xs)++(organize' xs (f . tail)

This doesn't work too good (which I thought it did) - in my joy of success I completely missed the error:

 Exception: Prelude.head: empty list

Upvotes: 1

Views: 127

Answers (1)

Jeremy Kerr
Jeremy Kerr

Reputation: 1935

Your mention of "doubles" implies that you want a list of 2-tuples (ie, "doubles"), rather than a list of 2-element lists. (Or perhaps this wording was particular to my Function Programming 101 lecturer!)

In which case, zip does exactly this:

zip [1, 2, 3] [4, 5, 6] = [(1,4),(2,5),(3,6)]

If you do need a list of 2-element lists (instead of tuples), you can use zipWith:

organize [xs,ys] = zipWith (\x y -> [x,y]) xs ys

Or are you looking for something that will work with any number of lists? In that case (as others have commented) transpose from Data.List is what you're after:

transpose [[1,2,3],[4,5,6]] = [[1,4],[2,5],[3,6]]

Upvotes: 2

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