I am having trouble calling map over a list of lists.
01>(define (foldr f accum xs) (if (null? xs) accum (f (car xs) (foldr f accum (cdr xs))))) 02> (map (lambda xs foldr cons '() xs) '(1 2) '(3 4) '(5 6)) ((1 3 5) (2 4 6))this is what we want, but I can't get this to work from a callable function.
EDIT -- (map (lambda x x) '(1 2) '(3 4) '(5 6)) gives the same result!
03> (define (zip . xs) (map (lambda ys foldr cons '() ys) xs))) 04> (zip '(1 2) '(3 4) '(5 6)) (((1 2)) ((3 4)) ((5 6)))I can see the problem that the arguments passed to zip become a list, but I do not know hot to keep treating them as I did with the working version.
I am not sure how apply is used with map
05> (define zip (lambda xs (apply (map (lambda ys foldr cons '() ys)) xs))) 06> (zip '(1 2) '(3 4) '(5 6)) *** ERROR IN zip, (stdin)@546.12 -- Wrong number of arguments passed to procedure (map '#<procedure #49>)this makes sense as map is not calling anything. ;; but then how could we apply map to the same args that map calls for the fold function?
07> (define (zip . args) (apply (lambda xs (map (lambda ys foldr cons '() ys) xs)) args)) 08> (zip '(1 2) '(3 4) '(5 6)) (((1 2)) ((3 4)) ((5 6)))which is just the same a the first attempt. I'm sure (...hope) that I am missing something tiny here.
The zip function is in my Standard Prelude .
> (define (zip . xss) (apply map list xss)) > (zip '(1 2 3) '(a b c)) ((1 a) (2 b) (3 c)) > (zip '(1 2 3) '(a b c) '(x y z)) ((1 a x) (2 b y) (3 c z))
