How do I break a Scheme list into args to be passed to a procedure?

Question

I want to use the predefined (max) function (R5RS) with a list of numbers, which varies in length. Unfortunately, (max) accepts input like this:

(max 2 43 5 6)
=> 43

I'm attempting to use it like so:

(define lst '(3 5 53 4 53 54 32))
(max lst)

This produces the following error.

max: expects argument of type <real number>; given (3 5 53 4 53 54 32)

How can I break this list into the individual arguments I need, as I'm passing them to (max)?

Answer 1

You might consider using apply (though be warned, that this may impose limits with respect to the number of elements/arguments are acceptable, depending on your implementation of Scheme):

(apply max '(3 4 2 1 78 2 1))

In general, if the number of elements is not known to be small, it might be safer to do it manually:

(define max* (list)
    (if (null? list) (negative-infinity)
        (let loop ((list (cdr list)) (best (car list)))
            (if (null? list) best (loop (cdr list) (max best (car list)))))))

Answer 2

Using apply basically translates into running (max 3 4 2 1 78 2 1) but the number of arguments a procedure can be passed is not infinite on some systems. For max, you can take advantage of its commutativity by using fold:

(fold-left max -inf.0 '(3 4 2 1 78 2 1))

Answer 3

You can write a procedure that compares the first item to the second,

(define (largestele lst)
        (if (null? (cdr lst))
            (car lst)
            (if (> (car lst) (cadr lst))
                (largestele (cons (car lst) (cddr lst)))
                (largestele (cdr lst)))))