Vfrdtyky

Write a program that returns the list of even numbers of a tree

I do this:

(define (list_pares arbol)

  (cond

    [(empty? arbol) 0]

    [(and (es-hoja? arbol) (even? (dato-tree arbol)))

     (list (dato-tree arbol))]

    [else 

     (cond

       [(even? (dato-tree arbol))

        (append (list (dato-tree arbol))

                (list_pares (left-tree arbol))

                (list_pares (right-tree arbol)))]

       [else

        (append

         (list_pares (left-tree arbol))

         (list_pares (right-tree arbol)))])]))

But when run:

(list_pares (list 2 empty (list 5 (list 4 empty empty) (list 9 (list 6 empty empty) empty))))

returns me this error:

append: last argument must be a list, but received 0

how could it?

Your error message comes from this:

 [(empty? arbol) 0]

So when the resursion where you append the result end with an 0 instead of an empty list you get this problem because (append '(something) 0 '(something else)) is not allowed.

What should you really return if you are given an empty tree and should return a list of even values from it?