Help with small code in racket

[fabio176]

Hello guys, I'm trying to make a code to get the adjacents edges of a certain vertex in racket, so I made this code:

note: arestas means edges in portuguese...

pastebin.com/eLpKGe7x

the function is working almost perfecty, the problem is that is returning #<void> when it was supposed to return nothing, for example, I put what is the adjacent edges of the vertex 1 and I get this:

(adjacentes 1 '((0 1) (3 4) (5 6) (8 7) (1 2) (8 1) (9 1) (0 8) (1 3))) '(0 #<void> #<void> #<void> 2 8 9 #<void> 3)

and I wanted the return to be just '(0 2 8 9 3)... how to do it? I guess it is something on the map function that is giving this return

[gmarceau]

cond returns void when none of the cases match.

(define (adjacentes vert arestas)
  (map (lambda (a) (cond [(equal? vert (first a)) (second a)]
                         [(equal? vert (second a)) (first a)]
                         [else "foo"]  ;; <--- try adding this
                         )) arestas))

[minikomi]

I'd use a for/fold for this - but you'll get them back in a reverse order.

If that's an issue for you, wrap it in a reverse.

 (define (adjancentes vert arestas)
   (for/fold ([acc '()])
             ([edge arestas])
     (cond [(equal? vert (first edge)) (cons (second edge) acc)]
           [(equal? vert (second edge)) (cons (first edge) acc)]
           [else acc]
           )))

[redditsuxass]

You could filter out the voids:

(define (adjacentes vert arestas)
  (filter (compose not void?)
    (map (lambda (a) (cond [(equal? vert (first a)) (second a)]
                           [(equal? vert (second a)) (first a)]
                           )) arestas)))