'Converting three lists into one list in Haskell

I'm new to haskell and am working on a function called combineAll.

Its input is going to be three integer lists, and its output will be one integer list.

It's going to return a sum of the three lists in corresponding order.

For example, if the input value is :

[[1,0,0,0],[1,1,1,0],[1,1,0,0]]

Then the output value will be :

[3,2,1,0]

I defined its signature as:

combineAll :: [[Int]] -> [Int]

I tried zipWith (+) but since it takes two inputs, it seems impossible for me to build up the recursion.

Any help would be appreciated!

listhaskell


Solution 1:[1]

You can turn this problem on its side with transpose, and transform the resulting list with map.

The mapping function will be ..... .

To find out what it is, see what will be the type of each element, and what is the value we're interested in producing for them.

Solution 2:[2]

Check out zipWith3 if you just want to zip 3 lists together.

Alternatively, a more generalised solution would be to fold the list by zipping two int lists together at a time.

Solution 3:[3]

This seems like a homework assignment, and you haven't shown code, so I'm not going to present a full solution with code, but generally:

You can map head across your list of lists. If you do that to [[1,0,0,0],[1,1,1,0],[1,1,0,0]] you'll get [1, 1, 1]. You can sum these to get 3.

If you map tail across [[1,0,0,0],[1,1,1,0],[1,1,0,0]] you get [[0,0,0],[1,1,0],[1,0,0]]. If you apply this recursively, you can get the sums of corresponding elements of the lists in your list of lists.

Solution 4:[4]

Another option is to use the transpose function. With your input that will produce:

ghci> transpose [[1,0,0,0],[1,1,1,0],[1,1,0,0]]
[[1,1,1],[0,1,1],[0,1,0],[0,0,0]]

Sources

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Source: Stack Overflow

Solution Source
Solution 1 Will Ness
Solution 2 Will Ness
Solution 3 Chris
Solution 4 Noughtmare

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