Scheme getting last element in list

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?

Solution 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)))))

Solution 2

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

Solution 3

you can retrieve the last element of a list by calling

(define (lastElem list) (car (reverse list)))

or, recursively using if built-in

(define (last list) (if (zero? (length (cdr list))) (car list) (last (cdr list))))

Comments

[Greg Hendershott]

@calccrypto The glass-half-full version of this answer is that you had nearly all the individual elements correct. Just keep in mind that parentheses aren't the same thing as curly braces; you'll want to break that habit of an "opening (" on the first line, and putting )s on their own line. Also, let DrRacket guide you with the indentation. You'll get the hang of it soon.