Question

I’m having a bit of trouble implementing this program in Scheme, although I think I’m 90% of the way there. Unfortunately I need to be a little vague about it since this is a homework assignment.
I want to (A B C D ) to return ( B D) . but i am getting an error that says The object (), passed as an argument to safe-car, is not a pair
| ”
This is my code:

(DEFINE (other_el lis)
  (COND
   (( NULL? lis ) '())
   ((LIST? lis)
    (append (CADR lis) (other_el (CDR lis))))
   (ELSE (show " USAGE: (other_el [LIST])"))))

Answer 1

There are a number of minor issues with this code that should be mentioned before I demonstrate the proper code.

1. Do not capitalize procedures’ names such as cdr and define in Scheme.
2. Do not display an error message manually. Use exceptions.
3. You should always indent your code. (edit: it looks like someone has edited the question’s code to include indentation)

Anyway, here is the function you are looking for:

(define (evens lst)
  (if (or (null? lst)             ; if the list is empty 
          (null? (cdr lst)))      ; or the list has a single element
      '()                         ; then return the empty list
      (cons (cadr lst)            ; otherwise `cons` the second element
            (evens (cddr lst))))) ; and recursively advance two elements

I tested the function in DrRacket 5.3 and (evens ‘(A B C D)) returns ‘(B D), as you specified. If you have any trouble, let me know. Good luck with your homework!