racket - list even values ​​of a tree in scheme -


write program returns list of numbers of tree

i 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 error:

append: last argument must list, received 0

how it?

your error message comes this:

 [(empty? arbol) 0]

so when resursion append result end 0 instead of empty list problem because (append '(something) 0 '(something else)) not allowed.

what should return if given empty tree , should return list of values it?


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