I have a hard time to figure out the functionality of foldr. I'm doing homework and we are supposed to build a fuction (no need of foldr), that zips the elements of two lists (f.e. '(1 2 3) (4 5 6) -> (1 4) (2 5) (3 6).
That worked pretty okay so far:
(define (zip list1 list2)
(cond
[(or (empty? list1) (empty? list2)) '()]
[else (cons (list (car list1) (car list2)) (zip (cdr list1) (cdr list2)))]))
But now I have to build a function, which uses the zip function I made before and combines two lists with a function f and return them as a list. (fe. (zipfunction + '( 1 2) '(3 4) -> (4 6)
I got the basic idea of foldr it will recursively apply a function I supply to a list from right to left, but I cant get behind it how to do so with several lists and several functions and if I need an accumulator variable or not.
Can someone help me with that?
Thank you in advance!
A more natural answer would be to use map:
(map + '(1 2) '(3 4))
Of course, a map can be expressed using foldr:
(foldr (lambda (x y acc)
(cons (+ x y) acc))
'()
'(1 2)
'(3 4))
What you should remember when using foldr is that it consumes one or more lists as input and each one of its elements is passed as a parameter to a procedure, and in that procedure the accumulated result is also passed as a parameter with the specified initial value (an empty list in this case); it's up to you to decide how to combine / accumulate the parameters in the procedure to yield a result.