Transpose a matrix in racket (list of lists

Question

I got a list of lists in racket and have to transpose them.

(: transpose ((list-of(list-of %a)) -> (list-of (list-of %a))))

(check-expect (transpose (lis         


        

Answer 1

for/list can be used sequentially to create a list of lists with transposed items:

(define (transpose_ lol)                    ; lol is list of lists
  (for/list ((i (length (list-ref lol 0)))) ; loop for length of first inner list
    (for/list ((il lol))                    ; for each inner list (il)
      (list-ref il i))))                    ; get its item

Testing:

(transpose_ (list (list 1 2 3)
                  (list 4 5 6)))

Output:

'((1 4) (2 5) (3 6))