I have a (in theory) simple method in Scheme, that has to use recursion. The problem is that I can't seem to figure out the recursion part... I have a procedure that takes a 'pointer' and arbitrary many arguments, and I need to pass the arguments one by one to another method. Here is what I got so far:
(define (push-elements ptr . args)
(define (push-all ptr list)
(if (null? list)
'()
(ptr 'push-elements (car list))))
(push-all ptr list))
I know there is no recursion here, but I can't understand where to put it/how to do it. The thought is clear, inside 'push-all' I need to call:
(push-all ptr (cdr list))
If anyone could help me (and I would be very grateful for an explanation on how to go about making such a recursive method), that would be great.
Basically you are implementing a variant of for-each, which is like map just for the side effects:
(define (for-each procedure lst)
(let loop ((lst lst))
(if (null? lst)
'undefined-value
(begin
(procedure (car lst))
(loop (cdr lst))))))
(for-each display (list 1 2 3 4)) ; prints 1234
But you wanted to be able to specify the values as arguments so you need to change lst to be a rest argument:
(define (for-each-arg procedure . lst) ; changed to dotted list / rest here
(let loop ((lst lst))
(if (null? lst)
'undefined-value
(begin
(procedure (car lst))
(loop (cdr lst))))))
(for-each-arg display 1 2 3 4) ; prints 1234
If you want a more functional approach you are indeed making either a map or a fold. Perhaps a fold-left would be preferred:
(define (fold-args join init . lst)
(let loop ((acc init) (lst lst))
(if (null? lst)
acc
(loop (join (car lst) acc) (cdr lst)))))
(fold-args cons '() 1 2 3 4) ; ==> (4 3 2 1)
You can actually implement for-each-arg with this:
(define (for-each-arg procedure . lst)
(apply fold-args
(lambda (x acc)
(procedure x)
'undefined)
'undefined
lst))
(for-each-arg display 1 2 3 4) ; => undefined-value; prints 1234