CODEWITHSUNDEEP
scheme
Trung Bún
Trung Bún

Reputation: 1147

Loop for-each in Scheme

I'm new to Scheme...

can someone please explain for me why the for-each statement doesn't print out the output??

I have a graph defined:

(define graph '((a (b.c)) (c (d))))

and my test code:

(define testing
  (lambda (a-list)
    (if (null? a-list)
        "size = 0"
        (for-each (lambda (i)
                   (cons (car i) (length (cdr i)))
                   (length a-list)) 
                  a-list))))

when run this (testing graph), output expected is ((a . 2) (c . 1)) but it display nothing...

Upvotes: 0

Views: 904

Answers (1)

Óscar López
Óscar López

Reputation: 236004

The for-each procedure doesn't build a list as output, it just executes a procedure on each of the input list's elements. You're looking for map, which creates a new list with the result of applying a function to each of the elements in the input list. Also notice that there are bugs in your code regarding the creation/traversal of the graph. This should fix the problems:

(define graph
  '((a (b c)) ; fixed a bug here
    (c (d))))

(define testing
  (lambda (a-list)
    (if (null? a-list)
        "size = 0"
        (map (lambda (i)
               (cons (car i) (length (cadr i)))) ; fixed a bug here
             a-list))))

Now it works as expected:

(display (testing graph))
=> '((a . 2) (c . 1))

Upvotes: 4

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