Racket: Recursive List Creation from 2 Lists

Question

I am supposed to create a function that takes two lists as the arguments and returns a list of pairs from combining the input lists where the pairs are a combination of the elements at the given index.

Example Input:

(zipper '(1 2 3) '(4 9 5 7))

Example Output:

'((1 4) (2 9) (3 5))

If one input list is longer than the other, extra elements of the longer list are ignored. The implementation has to be recursive.

The code I created:

(define (zipper list1 list2)
(if (or (empty? list1) (empty? list2))
  '()
(list (list (zipper (rest list1) (rest list2)) ))))

The output:

'((((((((()))))))))
'((((((()))))))
'()

Can someone please help? I've also tried using 'cons' to put the elements together, so am not sure how the elements are getting lost. I'm brand new to Racket. Thanks.

Where is this a b c coming from in the test case? Provide more test cases. I am sure I can do it but not with the given information (What is a single pair of lists? that is not what your output illustrates. You mean a list of pairs?) — Tyler Nichols
You are absolutely right. I had copied the wrong test case into my post. Edited. — Brian Jay

Tyler Nichols Tyler Nichols · Accepted Answer · 2017-09-28T18:48:47

Pretty simple, all we have to do is make a function that will return empty as a base case if either list is empty, otherwise we will cons the (first list1, first list2) to the recursive call.

(define (zipper list1 list2)
  (cond [(or (empty? list1)
             (empty? list2)) empty]
        [else (cons 
               (cons (first list1)
                     (first list2)) ;; List of first elements
               (zipper (rest list1) (rest list2)))]))

Racket: Recursive List Creation from 2 Lists

1 Answers