Hi,

thanks for your answers. I'm exercising with different programming techniques, so the use of an accumulator was intentional although not necessary. As Vlad said, my mistake was the stronger binding of the function application. Sigh, ...

Thank you all again.

Alex

On 10/18/2011 02:48 PM, Lorenzo Bolla wrote:
Hi,

On Tue, Oct 18, 2011 at 1:19 PM, Alexander Raasch <info@alexraasch.de> wrote:
Hi,

so I wrote this function to add two vectors represented as lists:

add a b = add' a b [] where
   add' [] [] s = s
   add' (a:as) (b:bs) s ++ [a+b]


I think something mangled your function, as this is not valid Haskell code.

Anyway, I tried to rewrite your function.
The first version works as expected; the second gives reversed output.
Note that there is no need for the accumulator "s".

add a b = add' a b
        where add' [] [] = []
              add' (a:as) (b:bs) = [a+b] ++ (add' as bs)

add a b = add' a b
        where add' [] [] = []
              add' (a:as) (b:bs) = (add' as bs) ++ [a+b] -- reversed output

Obviously, the same function can be written as:
zipWith (+) [1,2,3] [1,2,3]

hth,
L.