Question

Im trying to write a simple scheme function that returns the last element of a list. My function looks like it should work, but I managed to fail on something:

(define (last_element l)(
  (cond (null? (cdr l)) (car l))
  (last_element (cdr l))
))

(last_element '(1 2 3)) should return 3

DrRacket keeps on giving me the errors:

mcdr: contract violation
  expected: mpair?
  given: ()

Since (null? '()) is true, I don’t get why this doesn’t work.

This is a function I think I will need for a homework assignment (writing the function last-element is not the assignment), and the instructions say that I cannot use the built-in function reverse, so I can’t just do (car (reverse l))

How do I fix this function?

Answer 1

Your syntax is totally wrong. You have an extra set of parentheses around the body of the function, not enough around the cond clauses, and your recursive case isn’t even within the cond, so it gets done whether the test succeeds or fails. The following procedure should work:

(define (last_element l)
  (cond ((null? (cdr l)) (car l))
        (else (last_element (cdr l)))))