getting the largest number in a list in scheme

Question

I do not understand why my function to get the largest number does not want to work. If I am thinking about this correctly, if the first atom is smaller than the second atom the

Answer 1

There are 2 errors in your code:

1) in the else clause, you should recursively call yourself, dropping the second element:

(else (getlargest (cons (car a_list) (cddr a_list))))))

2) you are missing the case of a list of only one element, where cadr will fail

((null? (cdr a_list)) (car a_list))

And I personally prefer to yield #f if the list is empty. Thus the code would look like

(define (getlargest a_list)
  (cond
    ((null? a_list)       
     #f)
    ((null? (cdr a_list))
     (car a_list))
    ((< (car a_list) (cadr a_list))
     (getlargest (cdr a_list)))
    (else 
     (getlargest (cons (car a_list) (cddr a_list))))))

Of course, a solution using foldl is preferable:

(define (getlargest lst)
  (foldl (lambda (e r) (if (or (not r) (> e r)) e r))
         #f
         lst))

or, probably slightly more efficient:

(define (getlargest lst)
  (if (null? lst)
      #f
      (foldl (lambda (e r) (if (> e r) e r))
             (car lst)
             (cdr lst))))