Haskell Array(Matrix) Element Access

Question

I'm currently in a project that will need to access elements in an Array-Matrix in Haskell. So, I've tried googling it, searching everywhere.

The funcion is supposed to be like this:

getElementIndex :: Int -> Array (Int,Int) Int -> (Int,Int)

And it must return the I and J indexes of the element in the matrix.

So, given an element in a 2D array, you need to find that element (presumably, unique?) and return the indices you found it at? That's certainly possible (but a bit odd for arrays, since it will mean a linear scan). How far have you got so far? — Don Stewart
– Don Stewart, Commented May 9, 2011 at 18:53
Yes, it's unique.. I think I'll have to iterate through the whole array and check if the value is the expected and then if it is, return it. But I don't know how to access the elements (individually) in the array. — Breno Inojosa
– Breno Inojosa, Commented May 9, 2011 at 19:23

Don Stewart · Accepted Answer · 2011-05-09 19:35:42Z

6

To read elements from Array types in Haskell, you use the (!) operator, as in:

Prelude Data.Array> let v = listArray (0,9) [1..10]
Prelude Data.Array> v ! 3
4

so, now all you need to do is walk the index space, rows and columns. I like list comprehensions for that kind of task:

assocs' x y arr = [ ((i,j), arr ! (i,j))
                  | i <- [0..x-1]
                  , j <- [0..y-1]
                  ]

which is just a specialized version of Data.Array.assocs:

assocs :: Ix i => Array i e -> [(i, e)]

which returns a lazy list of indices and elements. So, call assocs, and then take the first element that matches.

answered May 9, 2011 at 19:35

Don Stewart

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Comments

fuz · Accepted Answer · 2011-05-09 19:05:53Z

4

How about

\x -> map fst . filter ((==x) . snd) . assocs

answered May 9, 2011 at 19:05

fuz

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Simon · Accepted Answer · 2011-12-12 02:44:08Z

0

answered Dec 12, 2011 at 2:44

Simon

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