[Haskell-cafe] List questions

Achim Schneider barsoap at web.de
Thu May 8 10:20:14 EDT 2008

<u4538637 at anu.edu.au> wrote:

> Hi I have a bit of a dilemma.I have a list of lists, eg,
> [[1,2,3],[4,5,6],[7,8,9]]. Imagine they represent a grid with 0-2 on
> the x axis and 0-2 on the y axis, eg, (0,0) is 1, (1,0) is 2, (2,1)
> is 6, etc and (2,3) is 9. I want to be able to put in the list of
> lists, and the (x,y) coordinate, and return the value. 
select x y ys = (ys !! y) !! x

> Also, I need to be able to replace a value in the list. Eg, if I
> wanted to replace (2,3) with 100, then the output of the expression
> would be [[1,2,3],[4,5,6],[7,8,100]].
you can't replace it, you can only construct a new list: Your function
will likely be of the type

replace :: Int -> Int -> a -> [[a]] -> [[a]]

I won't give the answer here, as that would spoil all of the fun
and delight you'll have while browsing through the Prelude and
Data.List and see how much duct tape there's around, waiting to be
used, plus I'd have to open a new file as ghci doesn't support
multi-line functions.

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