i've tried to create a Haskell function that is merging 2 lists into a single list, where even indices in the new list are from list 1 and the odd are from list 2. Fill with 0's if not the same size.
For example:
[1] [10,15,20] => [1,10,0,15,0,20]
[2,3] [4,5] => [2,4,3,5]
I've tried to create a couple of versions but with no luck.
How can I create such a thing?
There is a interleave function, that does something similar, however, not exactly that. It 'merges' lists until one of them ends.
So you can write that function by yourself:
merge :: [Int] -> [Int] -> [Int]
merge (x:xs) (y:ys) = x : y : merge xs ys
merge (x:xs) [] = x : 0 : merge xs []
merge [] (y:ys) = 0 : y : merge [] ys
merge _ _ = []
When we have some elements on both sides, we take both of them. When one of elements is absent, we take 0 instead of it. In all other cases (it is the merge [] [] case) we end up with our recursion and return an empty list.
We can also slightly generalize our function to support any number-like type:
merge :: Num a => [a] -> [a] -> [a]
merge (x:xs) (y:ys) = x : y : merge xs ys
merge (x:xs) [] = x : 0 : merge xs []
merge [] (y:ys) = 0 : y : merge [] ys
merge _ _ = []
Also, we can go further and use def from Data.Default package to get the default value for our type, so we can use this function with not only lists of numbers:
import Data.Default
merge :: Default a => [a] -> [a] -> [a]
merge (x:xs) (y:ys) = x : y : merge xs ys
merge (x:xs) [] = x : def : merge xs []
merge [] (y:ys) = def : y : merge [] ys
merge _ _ = []
Using the idea from this answer of mine, using the transpose :: [[a]] -> [[a]] function,
interweaveWith :: a -> [a] -> [a] -> [a]
interweaveWith def xs ys =
-- 0 [1] [10,15,20] => [1,10,0,15,0,20]
concat $
zipWith const
(transpose [ xs ++ repeat def, -- no limit, padded with def
ys ++ repeat def ])
(transpose [xs, ys]) -- as long as the longest