How to create a list of list of numbers in Haskell

Question

I need to create a function that makes a board of size row and column and then populate that with zeros. mkBoard 2 3 would make [[0,0,0],[0,0,0]]

I don't really know where to start as I am new to Haskell programming I was thinking that the function would be something like this:

mkBoard m n= [m | ???? take n (repeat 0)]

But I am not sure if this would be the correct approach.

Thank you for your help.

Answer 1

There is no need to use list comprehension here. We can make use of replicate :: Int -> a -> [a] that will convert an Int n and an a to a list that repeats that value n times.

We thus can construct a list of three 0s with replicate 3 0. We can then use replicate a second time to construct a list that contains that list two times for example.

mkBoard :: Num n => Int -> Int -> [[n]]
mkBoard m n = replicate m (replicate n 0)

Answer 2

As @arrowd already mentioned, there is a replicate function for take n (repeat x). You can create one row of your board with replicate n 0, and then create a board of such rows like mkBoard m n = replicate m (replicate n 0).

Also you can create more generic version of this function to fill the field with any value:

genericMkBoard :: a -> Int -> Int -> [[a]]
genericMkBoard x m n = replicate m (replicate n x)

And define your mkBoard with partial application:

mkBoard = genericMkBoard 0

UPD: Also you can make it like this to be more consistent with replicate (thanks to @chepner):

replicate2D m n x = replicate m (replicate n x)

mkBoard m n = replicate2D m n 0