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Editorial Team
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Asked: 2026-06-12T14:38:53+00:00

I’m trying to make a function that takes in a list, and if one

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I’m trying to make a function that takes in a list, and if one of the elements is negative, then any elements in that list that are equal to its positive counterpart should be changed to 0. Eg, if there is a -2 in a list, then all 2’s in that list should be changed to 0.

Any ideas why it only works for some cases and not others? I’m not understanding why this is, I’ve looked it over several times.

changeToZero [] = []
changeToZero [x] = [x]
changeToZero (x:zs:y:ws) | (x < 0) && ((-1)*(x) == y) = x : zs : 0 : changeToZero ws
changeToZero (x:xs) = x : changeToZero xs

changeToZero [-1,1,-2,2,-3,3]
-- [-1,1,-2,2,-3,3]

changeToZero [-2,1,2,3]
-- [-2,1,0,3]

changeToZero [-2,1,2,3,2]
-- [-2,1,0,3,2]

changeToZero [1,-2,2,2,1]
-- [1,-2,2,0,1]
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1 Answer

  1. Editorial Team

    I think a list comprehension is both clearer and easier to get right here.

    changeToZero xs = [if x > 0 && (-x) `elem` xs then 0 else x | x <- xs]

    If you need something more efficient, you can build a set of the negative elements and check that instead of using elem.

    import qualified Data.Set as Set

changeToZero' xs = [if (-x) `Set.member` unwanted then 0 else x | x <- xs]
  where unwanted = Set.fromList $ filter (< 0) xs
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