transpose of a list of lists

Question

I\'m trying to make a recursive function to get the transpose of a list of lists, n x p to p x n. But i\'m unable to do so. I\'ve been able to make

Answer 1

I know this is an old question, but I recently had to solve this as part of an exercise I was doing, and I came across @sepp2k's solution, but I couldn't understand how it worked, so I tried to arrive at it by myself.

This is essentially the same algorithm, but a little bit more terse, as it does not destructure the list of lists. I thought I would post it here in case anyone else is searching, and might find this way of expressing it useful:

let rec transpose = function
   | [] 
   | [] :: _ -> []
   | rows    -> 
       List.map List.hd rows :: transpose (List.map List.tl rows)

Answer 2

let rec transpose list = match list with
| []             -> []
| []   :: xss    -> transpose xss
| (x::xs) :: xss ->
    (x :: List.map List.hd xss) :: transpose (xs :: List.map List.tl xss)