Scheme combining two lists in a certain way

Question

This is my second question in Scheme. Say I have 2 lists

'("a" "b" "c")

'("e" "f" "g")

I want to combine them in this way:

'(("a" "e") ("b" "f") ("c" "g"))

Is this possible?

Answer 1

You use SRFI-1 List library. the function name is zip. When you apply zip to a zip result you're back to where you started so unzip is usually the same procedure except it takes a list of lists instead of many list arguments.

(zip '(1 2 3) '(a b c))         ; ==> ((1 a) (2 b) (3 c))
(unzip '((1 2 3) (a b c)))      ; ==> ((1 a) (2 b) (3 c))
(unzip '((1 a) (2 b) (3 c)))    ; ==> ((1 2 3) (a b c))

If you follow the link you'll find a reference implementation. However you usually don't need it as most implementations do include them. E.g. In Racket R6RS you (import (srfi :1)) or in the racket language you (require srfi/1). In Chicken you (declare (uses srfi-1)) so the syntax between the implementations varies greatly.

You tagged lisp so for completeness a Common Lisp implementation would look like this:

(defun zip (&rest lsts)
  (apply #'mapcar #'list lsts)) 

(defun unzip (lsts)
  (apply #'mapcar #'list lsts))