A scheme procedure that returns a list of every other element

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

Answer 1

This one is considerably simpler than the previous question you asked. Bear in mind, you don't have to calculate the length at each step (that could be very inefficient), or use append operations to solve it (use a cons instead); here's the structure of the answer, because it looks like homework I'l let you fill-in the blanks:

(define (every-other lst)
  (if (or <???>                    ; if the list is empty 
          <???>)                   ; or the list has a single element
      <???>                        ; then return the empty list
      (cons <???>                  ; otherwise `cons` the second element
            (every-other <???>)))) ; and recursively advance two elements

If you need to do some error checking first, use another function and call the above procedure after you're certain that the arguments are correct:

(define (other_el lst)
  (if (list? lst)
      (every-other lst)
      (error "USAGE: (other_el [LIST])")))

Use it like this:

(other_el '(A B C D E G))
=> '(B D G)

Answer 2

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!