List of lengths from list of strings using map, filter, or fold-right

Question

You are given a list of strings.

Generate a procedure such that applying this procedure to such a list would result in a list of the lengths of each of the strings in the input.

Use map, filter, or fold-right.

(lengths (list "This" "is" "not" "fun")) => (4 2 3 3)

(define lengths (lambda (lst) your_code_here))

I got stuck in the following code and I do not understand how can I use filter.

(define lengths
  (lambda (lst)
    (if (null? lst)
        nil
        (fold-right list (string-length (car lst)) (cdr lst)))))

Answer 1

This seems like a work for map, you just have to pass the right procedure as a parameter:

(define (lengths lst)
  (map string-length lst))

As you should know, map applies a procedure to each of the elements in the input list, returning a new list collecting the results. If we're interested in building a list with the lengths of strings, then we call string-length on each element. The procedure pretty much writes itself!

A word of advice: read the documentation of the procedures you're being asked to use, the code you're writing is overly complicated. This was clearly not a job for filter, although fold-right could have been used, too. Just remember: let the higher-order procedure take care of the iteration, you don't have to do it explicitly:

(define (lengths lst)
  (fold-right (lambda (x a) 
                (cons (string-length x) a))
              '()
              lst))