Thanks, Dean. I understand foldl a lot better now after working through your higher order solution. I checked out the type of "snd" in hugs and basically figured out what it does, but I was wondering where I could find the definition of this function. Is it possible to look at it in hugs? Thanks, Matt On Sunday 23 February 2003 12:16 am, Dean Herington wrote:
On Fri, 21 Feb 2003, M. Parker wrote:
I'm a real newbie to Haskell, and I'm having trouble with a particular problem dealing with higher-order functions.
Exercise 5.9 in Hudak's "School of Expression" asks us to write a function, "makeChange," s.t. it makes change for a given amount using coins in a coin supply (represented by a list of decreasing integers). For example, make Change 99 [5,1] ==> [19,4]
This chapter is about higher order functions, so I'm assuming he wants us to compute the result using the higher order functions defined in the chapter (map, foldl, foldr). I devised two solutions:
{-Solution 1-} makeChange money coinList = zipWith div (scanl mod money coinList) coinList
{-Solution 2-} makeChange' money (coin:coins) = let money' = money `mod` coin numCoins = money `div` coin in (numCoins: makeChange' money' coins) makeChange' 0 _ = [] makeChange' _ [] = []
However, my problem is that neither solution uses the higher-order functions defined in the chapter. So is it possible to solve this problem using map and fold?
Furthermore, Hudak makes the case that we should strive to find the higher-order solutions instead of the recursive ones because the latter leads to clearer and more concise solutions. Although solution 1 is more concise, I feel like Solution 2 is clearer to me than Solution 1, but maybe this is just because I'm new to haskell and higher order functions. It just seems like its easier to understand the actual algorithm in solution 2 than in solution 1.
Thanks, Matt Parker University of North Texas undergrad http://www.cs.unt.edu
Here's what I had come up with for that exercise:
makeChange1 _ [] = [] makeChange1 amt (c:cs) = q : makeChange1 r cs where (q,r) = amt `quotRem` c
makeChange2 amt coins = reverse (snd (foldl f (amt,[]) coins)) where f (amt,cnts) coin = (r,q:cnts) where (q,r) = amt `quotRem` coin
As you said in comparing your two solutions, I'm not convinced the higher-order solution is clearer in this case. (By the way, I do generally like to use higher-order functions.)
Dean
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