haskell - processing list of pairs..

techguy

Hi There,

I'm having some hassle in Haskell atm. Basically i'm trying to get a list of pairs and sum the first elements of each pair and the second elements of each pair.
eg. sumPairs [(1,2),(2,3),(4,5),(6,7)]
=> (13,17)

I am able to sum all of the pairs and return a list of the sums of each pair but thats it.

When I try this I get all kinds of type errors:

addPair (a,b) (c,d) = ((a+c),(b,d))
addPairs (a:b:as) = map addPair a b

In the above code i'm taking the first two pairs from the list as and maping them to addPair. How to I define addPair to take two pairs and return one pair?

I'm competely lost on this..any help appreciated.

UPDATE:

I've just gotten addPair to work on its own with two pairs supplied as arguments. When I try and map that function to a list of pairs i'm getting an error
Type error in application.:
Expression: map addPair a b
Term: map
Type: (e-> f) -> [e] -> [f]
Does not match: a->b->c->d

Here's the code:

addPair :: (Integer,Integer) -> (Integer,Integer) -> (Integer,Integer)
addPair (a,b) (c,d) = (a+c,b+d)
addPairs (a:b:as) = map addPair a b

What does the error mean anyway?

Thanks..

ronivek

So the first problem is that you're using map incorrectly; it's giving you the type for map which is (e-> f) -> [e] -> [f].

This is telling you map expects two arguments; the first a function mapping some e to some f, the second a list of e and the result is a list of f.

The second problem is that map isn't really what you want in this instance; map is simply mapping the elements of some list of length n to some other list of length n. What you're looking to do is to effectively accumulate the values of some list of length n into a single element.

I would suggest taking a look at foldr and/or foldl; go here http://www.haskell.org/onlinereport/standard-prelude.html; and do a search for foldl and foldr to have a look at the definitions and see if that gives you any ideas.

techguy

Here's what I got to work.

addPairs [] = (0,0)
addPairs ((a,b) :as) =
	let(x,y) = addPairs as in
		(a+x,b+y)

It's the first time using let,in. Hadn't thought of it before. Thanks for your help.

ronivek

Here's my solution using foldl just for future reference;

addPairs :: (Num a, Num b) => [(a, b)] -> (a, b)
addPairs ps = foldl (\(x, y) (acc_x, acc_y) -> (x + acc_x, y + acc_y)) (0, 0) ps

Or in an alternative form;

addPairs2 :: (Num a, Num b) => [(a, b)] -> (a, b)
addPairs2 ps = foldl addPairAccum (0, 0) ps

addPairAccum :: (Num a, Num b) => (a, b) -> (a, b) -> (a, b)
addPairAccum (x, y) (acc_x, acc_y) = (x + acc_x, y + acc_y)