Question

Using my zip function(see below), write a function addpairs that takes two lists [x0; x1; … ; xn] and [y0; y1; … ; ym] and returns the list [x0 + y0; x1 + y1; … ; xp + yp], where p = min(n; m). Where this is the output from console.

*Recursion> :t addpairs
addpairs :: (Num a) => [a] -> [a] -> [a]
*Recursion> addpairs [1,2,3,4] [40,50,60,90]
[41,52,63,94]

my zip function:

zip [] _ = []
zip _ [] = []
zip (a:aa) (b:bb) = (a,b) : Recursion.zip aa bb

Any help would be great.

I actually solved this problem
This was the solution I came up with:

addpairs xs ys = map f (Recursion.zip xs ys)
  where f (x,y) = x + y

Thanks for all that helped.

Answer 1

If xs and ys are the two lists your function is given, then zip xs ys will be a list of tuples of numbers to add:

GHCi> zip [1,2,3,4] [40,50,60,90]
[(1,40),(2,50),(3,60),(4,90)]

zip xs ys will have the type [(a,a)] for some numeric type a. Next, take a look at the type of map:

map :: (a -> b) -> [a] -> [b]

Your function should look like this:

addpairs xs ys = map f (zip xs ys)
  where f ...

where f should be a function taking an (a, a) to be added and returning the resulting a. That should help you fill in the rest.