On 6/19/06, Sara Kenedy wrote:

Hello,

I want to creat a function sumList as below examples: sumList [("s1",2),("s2",4),("s3",3),("s4",2)] = ("#", 1/2 + 1/4 + 1/3 + 1/2) sumList [("s1",2),("s2",4),("s3",3) = ("#", 1/2 + 1/4 + 1/3)

I attempted it as following:

sumList :: (Fractional a) => [(String,a)] -> (String, a) sumList [] = ??? sumList (x:xs) = ("#", 1/(snd x) + 1/snd(sumList xs))

I know this function cannot give the correct answer (even if I define sumList []), but I did not find the right way. If anyone can give me a suggestion, I really appereciate for that. Thanks.

There are numerous ways to write this of course, but I would suggest a less "direct" appoach. You don't really need to write out the explicit recursive definition. Try to "massage" the data to the form you need it in, after a few transformations of the data the answer is hopefully a lot easier to produce. So: sumList xs = ("#", s) where s = ... Would be a good start. The computation of the sum doesn't need to worry about the final tuple form with the "#", so we do that at the end, and produce the sum in the where-clause. Now s depends only on the the second elements of the tuples in the list (if I understand the problem correctly), so you could "massage" the input to help clean out the bits that aren't needed (that is, we want a list containing the second elements of each tuple): sumList xs = ("#", s) where s = ... ys = map snd xs Now the problem is much simpler, you just need to take one divided by each element of ys and sum the results up to produce s. sumList xs = ("#", s) where s = sum zs ys = map snd xs zs = map (\x ->1/x) ys Or how about this simplification using a list comprehension. sumList xs = ("#", s) where s = sum [ 1/x | (_, x) <- xs ] Even shorter: sumList xs = ( "#", sum [ 1/x | (_, x) <- xs ] ) No recursion required! (at least not visibly, sum uses it in its definition). There are plenty of really useful functions in the Prelude and Data.List (and other modules), so for simple functions you rarely need to write out a full recursive solution, but rather use some of the useful library functions to compose the result in a very readable form. /S -- Sebastian Sylvan +46(0)736-818655 UIN: 44640862