Question

Im trying count the number of positive elements in a list. Here is what I have so far:

 (define howMany
   (lambda (list)
      (cond
         [(not (list? list)) 0]
         [(null? list) 0]
         [(> list 0) (+ 1 (howMany (cdr list)))])))

It keeps giving me an error, “expects type real number”, how would you fix this?

Oh im calling this like so:

(howMany '(6 7 8))

Answer 1

There are a couple of bugs in your code.

(> list 0) should be (> (car list) 0) as you want to check if the first element of the list is greater than 0. You cannot apply the default implementation of > to a list either.

(+ 1 (howMany (cdr list))) will also fail as howMany does not always evaluate to a number. You have to maintain a counter by passing that as an argument to the recursively called procedure. One way to do this is:

(define (howmany lst)
  (let loop ((n 0) (lst lst))
    (if (null? lst) n
      (loop (if (> (car lst) 0) (add1 n) n) (cdr lst)))))

Test:

> (howmany '(1 2 3 4 5))
5
> (howmany '(1 2 3 -4 5))
4
> (howmany '(1 -2 3 -4 5))
3
> (howmany '(-1 -2 3 -4 5))
2