CODEWITHSUNDEEP
scheme
Talen Kylon
Talen Kylon

Reputation: 1968

Working backwards through a list in scheme

I'm trying to figure out how to work through a list in reverse order (back to front) in Scheme using recursion. My research shows that there is a reverse function that you can use on a list but I would not like to go that route.

I know that the following returns the last element in a list;

(caddr '(1 5 3))
returns 3

I would like to be able to send the remaining 1 5 back through the function, look at the last element again and repeat until the list is exhausted

From left to right is simple enough, you look at the car of the list and send back the cdr.

Upvotes: 1

Views: 383

Answers (1)

uselpa
uselpa

Reputation: 18917

Do the recursive call first, then the processing of the car. Example:

(define (process-in-reverse lst fun)
  (when (not (null? lst))
    (process-in-reverse (cdr lst) fun)
    (fun (car lst))))

then

> (process-in-reverse '(1 5 3) display)
351

If you just want to reverse the result and if subsequent calls are independent of previous calls, an accumulator will do and is tail-recursive:

(define (process-reverse-result lst fun)
  (let loop ((lst lst) (res null))
    (if (null? lst)
        res
        (loop (cdr lst) 
              (cons (fun (car lst)) res)))))

then

> (process-reverse-result '(1 5 3) add1)
'(4 6 2)

Upvotes: 2

Related Questions