Generate 2D list with given dimensions in Haskell

Question

I am trying to generate a list of lists with a specified dimension. the data type of this list looks something like this:

data A = X | Y | Z

so the list is of type [[A]]. (A is an instance of the Show type class so don't worry about that).

The user gives in a certain dimension (lets say width = 3 and height = 4), so the content could look like this:

[[X,Y,Z],
 [Y,Y,X],
 [Y,X,Z],
 [X,Z,Z]]

How can I generate a width X height 'matrix', the values aren't all that important at the moment.

thanks in advance.

EDIT: (for clarity reasons)

I just want to know how to generate a 'matrix' of type [[A]] with the width and height as user input. So width = number of elements in the inner list, height = number of lists in the outer list.

that other guy · Accepted Answer · 2014-10-27 19:19:13Z

4

To generate a 3x4 nested list filled by a certain element, you can use:

data A = X | Y | Z deriving (Show)
generate width height = replicate height . replicate width

main = print $ generate 3 4 X

to get [[X,X,X],[X,X,X],[X,X,X],[X,X,X]].

Note that nested lists are not a great substitute for a 2D array in C/Java if the goal is to do frequent point updates. In those cases, use Data.Map or Data.Array.

answered Oct 27, 2014 at 19:19

that other guy

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