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Question

Write a function in Scheme that receives as argument of entry a binary search tree of instances of the student structure , a list of birth years and returns a list of list of codes that indicate which birth dates are provided each year.

example:

This is the binary tree

(make-árbol-bin  
(make-estudiante 5 "35889188" "Victor" (make-fecha 10 6 1991) "calle 67 con cra 20" "4444444")  (make-
árbol-bin  
(make-estudiante 2 "6457234" "Manuel" (make-fecha 12 10 1992) "calle 56 con cra 33" "5555555")   (make-árbol-bin 
   (make-estudiante 1 "94252688" "Estela" (make-fecha 20 5 1993) "calle 4 con cra 2" "3333333")    empty    empty)   empty) 
 (make-árbol-bin 
(make-estudiante 7 "34987678" "Juan" (make-fecha 25 3 1995) "calle 34 con cra 12" "6666666")   empty 
 empty) 
)

List years

(list 1992 1991 )  

It should return a list of lists of codes:

((list (list 2) (list 5)  ) 

and I did this but I can not make me return the list of the list

(define-struct fecha ( dia mes año))

This is the structure of the student:

(define-struct estudiante ( codigo id nombre fechanaci direccion telefono))

This is the structure of the binary search tree:

(define-struct arbol-bin( estudiante nod-izq nod-der)) 

I did this

(define (elimina X L)
        (if(null? L) '()
        (if (equal? X (car L))
            (elimina X (cdr L))
            (cons (car L) (elimina X (cdr L))))))

(define (busca d tree)
(if (null? tree)
    #f
    (let ((node (arbol-bin-estudiante tree)))
      (let ((n (estudiante-codigo node))) 
      (let ((fe (estudiante-fechanaci node)))
      (if (or(equal? d (fecha-año fe))
             (busca d (arbol-bin-nod-der tree)))
          n
            (busca d (arbol-bin-nod-izq tree))))))))

(define (pert lst tree)
(elimina false (map (lambda (d) (busca d tree)) lst)))

and this

(pert (list 1992   1991 )  tree)

return

(list 2 5)

As I can do to make me return this:

((list (list 2) (list 5)  )

help me please!

Answer 1

You cannot make it return ((list (list 2) (list 5))) , because that's not a correct Scheme expression. You can make it return (list (list 2) (list 5)) though:

> (list (list 2) (list 5))
'((2) (5))

To do so, in busca , just replace n with (list n) :

(define (busca d tree)
  (if (null? tree)
      #f
      (let ((node (árbol-bin-estudiante tree)))
        (let ((n (estudiante-codigo node))) 
          (let ((fe (estudiante-fechanaci node)))
            (if (or (equal? d (fecha-año fe))
                    (busca d (árbol-bin-nod-der tree)))
                (list n) ; <------
                (busca d (árbol-bin-nod-izq tree))))))))

or

> (list (list (list 2) (list 5)))
'(((2) (5)))

by changing the above line to (list (list n)) .