球拍附加了不希望的列表反转

Question

I wrote a small function that takes a list and returns a list composed of only positive numbers. This all works fine, but for some reason, it is reversing the order. More information below on that. Could someone please tell me if this is normal, or if I miss-wrote something? Thank you in advance.

(define (positive-nums-only lst)
  (if (empty? lst)
      '()
      (append (positive-nums-only (cdr lst))
              (if (>= (car lst) 0)
                  (list (car lst))
                  '()))))
(positive-nums-only '(1 2 -4 90 -4))

The above test case returns '(90 2 1)

Answer 1

You did not make a mistake, the program is making what you asked.

See, the program finishes the recursion calls first, before going into resolving the if statement. This causes the (list ... ) to start listing from the last element that is positive, in this example it will be 90 .

Changing the code order will produce the result you want.

(define (positive-nums-only lst)
(if (empty? lst) '()
  (append (if (>= (car lst) 0 )
             (list (car lst))
             '())
          (positive-nums-only (cdr lst)))
)
)

On the other hand, this kind of recursion could be expensive to the computer. I'd use tail recursion, like this:

  (define positive-nums-only-tail
  (λ (lst r)
    (cond ((empty? lst) (reverse r))
          ((positive? (car lst))
           (positive-nums-only-tail (cdr lst)
                                    (cons (car lst) r)))
          (else (positive-nums-only-tail (cdr lst) r))
          )
    )
  )

Answer 2

Have you tried reversing the append?

(define (positive-nums-only lst)
  (if (empty? lst)
      '()
      (append (if (>= (car lst) 0) (list (car lst)) '())
              (positive-nums-only (cdr lst)))))

Personally I find it more natural to write it like this:

(define (positive-nums-only lst)
  (if (empty? lst)
      '()
      (let ((rest (positive-nums-only (cdr lst))))
        (if (>= (car lst) 0)
            (cons (car lst) rest)
            rest))))