CODEWITHSUNDEEP
schemeracketcomputer-science
compscilol
compscilol

Reputation: 3

How to Write a Reverse Function in Scheme?

I have to write a scheme function which does the following:

Define a SCHEME function, named (rev p), which takes a pair as an argument and evaluates to another pair with the first and second elements in the pair p in reverse order. For instance,

( rev ( cons 1 2))
> (2 . 1)

Here is my code:

(define (rev p)
  (cond ((null? p) '())
        (not (pair? (car p)) p)
        (else (append (rev (cdr p)) (list (rev (car p))))

However, my code returns (1 . 2) when I test it when it should be returning (2 . 1).

Upvotes: 0

Views: 717

Answers (4)

Amogh Chaubey
Amogh Chaubey

Reputation: 1

All you need to do for this function is use the car and cdr function for your pair p.

(define (rev p)
    (cons (cdr p) (car p))
    )

Upvotes: 0

alinsoar
alinsoar

Reputation: 15813

(define rev
  (lambda (l acc)
    (if (null? l)
        acc
        (rev (cdr l)(cons (car l) acc)))))

(rev '(1 2 3) '())

And here is an apparently obfuscate version, but the ideas may be useful.

(define rev
  (lambda (l k)
    (if (null? l)
        (k (lambda (x) x))
        (rev (cdr l)
             (lambda (k0)
               (k (lambda (r) (k0 (cons (car l) r)))))))))

((rev '(1 2 3) (lambda (x) x)) '())

--

As suggested by Will, here is other variant, non-tail recursive, hence not completely cps'd, it's a combination of classic recursion and cps.

(define rev
  (lambda (l k)
    (if (null? l)
        (k '())
        (rev (cdr l)
             (lambda (r)
               (cons (car l)
                     (k r)))))))

Upvotes: 1

Gwang-Jin Kim
Gwang-Jin Kim

Reputation: 10010

Or the same expressed as:

(define (rev p)
  (if (pair? p)
      (cons (cdr p) (car p))
      p))

Upvotes: 0

Óscar López
Óscar López

Reputation: 236140

If it's just a pair that you want to reverse, it's pretty simple, you don't even need to do recursion! And remember to use cons, not append:

(define (rev p)
  (cond ((not (pair? p)) p)
        (else (cons (cdr p) (car p)))))

For example:

(rev '())
=> '()
(rev 5)
=> 5
(rev (cons 1 2))
=> '(2 . 1)

Upvotes: 0

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