问题
I have a function 'subsets' which generate all the subsets of a given set:
subsets :: [Int] -> [[Int]]
subsets [] = [[]]
subsets (x:xs) = subsets xs ++ map (x:) (subsets xs)
How can I combine map, foldl and filter in another function to return me all the subsets with elements that sum up to 0?
**Example: **
set = [1,-1,5,2,-2,3]
result = [[1,-1],[2,-2],[-1,-2,3]]
回答1:
You have subsets already. So we need a function
filterSubs :: [[Int]] -> [[Int]]
filterSubs = --remove all subsets which don't sum to 0
So next we'd need a predicate
sumZero :: [Int] -> Bool
sumZero xs = sum xs == 0
Now, using this and filter it's easy to construct filterSubs. I'll leave this to you to figure out exactly how that works. And then our solution is trivial
zeroSubs = filterSubs . subsets
回答2:
The idea is that you want to take all subsets and then only keep the ones that sum 0
subsetsSum0 ls = filter (\ss -> (sum ss) == 0) (subsets ls)
let's make this code point-free
subsetsSum0 = filter ((==0) . sum) . subsets
来源：https://stackoverflow.com/questions/19772427/haskell-generate-subsets