CODEWITHSUNDEEP
haskell
michaeladrian39
michaeladrian39

Reputation: 15

Differences between values within two lists in Haskell with List Comprehension

I'm making a function in Haskell to compute the differences between values in two lists.

For example, I have two lists:

    List A = [1,2,3]
    List B = [2,3,4]

Subtract the first element of A with the first element of B, subtract the second element of A with the second element of B, and so on. The result should be like this:

    Result = [-1,-1,-1]

How to make this function? I tried using this but failed:

diff xs ys = [i-j | i <- xs, j <- ys, length xs == length ys]

And the result of using that wrong function is (I used list A and list B, look above):

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

Someone told me that i <- xs, j <- ys means cross-joining the elements of xs and ys, so the result is like this:

    [(1-2),(1-3),(1-4),(2-2),(2-3),(2-4),(3-2),(3-3),(3-4)

Using list comprehension, and without using i <- xs, j <- ys, complete this function:

diff :: [Int] -> [Int] -> [Int]
diff [] [] = []
diff xs [] = []
diff [] ys = []

Upvotes: 0

Views: 2762

Answers (2)

koki
koki

Reputation: 352

diff :: [Int] -> [Int] -> [Int]
diff a b = map (\(p, q) -> p - q) $ zip a b

Upvotes: 1

Yuriosity
Yuriosity

Reputation: 1918

If you like list comprehensions, you can use an extension:

{-# LANGUAGE ParallelListComp #-}

diff a b = [p - q | p <- a | q <- b]

which is internally translated into zip.

Upvotes: 2

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