Scheme for getting the last item in the list

Question

I am trying to write a simple schema function that returns the last element of a list. My function looks as if it should work, but I managed to fail something:

(define (last_element l)( (cond (null? (cdr l)) (car l)) (last_element (cdr l)) )) (last_element '(1 2 3)) should return 3 

DrRacket continues to give me errors:

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

Since (null? '()) true, I don't understand why this is not working.

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

How to fix this feature?

Answer 1

Your syntax is completely wrong. You have an extra set of parentheses around the function body, not enough around cond clauses, and your recursive case is not even within cond , so 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))))) 

Answer 2

Just add: in professional-level Racket, the last function is part of the racket / list .

Answer 3

You can also do it like this. First find the length of the list with cdring it down. Then use list-ref x, which gives the x element from the list. For example, list-ref yourlistsname 0 gives the first element (basically a list car.) And (list-ref yourlistsname (- length 1)) gives the last element of the list.