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,