In Dr. Racket, how would I display the 2 elements in a list in the opposite order?

Question

I want to write a function which displays 2 elements in a list in the opposite order. This is my code so far:

(define (reverse-order list)
  (cons (rest list) (cons (first list) empty)))

For some reason, this code is not working: For example: If I type in

 (reverse-order (cons 1 (cons 2 empty))) 

My desired output is

(cons 2 (cons 1 empty))

However, instead, I get

(cons (cons 2 empty) (cons 1 empty))

Any hints on how to go about solving this code will be very helpful.

Answer 1

That's because you're taking the rest of the list, which is '(2) , not the second element 2 ; and you should not call a parameter list , that clashes with a built-in procedure of the same name. Try this instead:

(define (reverse-order lst)
  (cons (second lst)
        (cons (first lst)
              empty)))

Or even simpler, and now you see why it was a bad idea to call the parameter list :

(define (reverse-order lst)
  (list (second lst) (first lst)))

Or just use the built-in reverse procedure :)

(define reverse-order reverse)

Anyway, it works as expected:

(reverse-order '(1 2))
=> '(2 1)