Question

I have the following grid:

[["a1","b1",    "c1","d1"],
 ["a2","b2",    "c2","d2"],

 ["a3","b3",    "c3","d3"],
 ["a4","b4",    "c4","d4"]]

and would like to extract a range of values in their ‘squares’ so I end up with a list of square values. The x-values are horizontal and y-values are vertical.

I have the following function started:

type Coordinate = (Int,Int)

return :: [[String]] -> Coordinate -> [String]
return grid (x,y) = .....

where (0,0) is the top left corner of the grid. Using the above function I would like to be able to extract a square so I get like this (if my coordinates are (2,0), (3,0), (2,1), (3,1)):

["c1","d1","c2","d2"]

I have tried some methods including the map function but need some tips on how to proceed.

Answer 1

I suggest the following:

localSquare :: [[a]] -> (Int,Int) -> [[a]]
localSquare xss (x,y) = map (take y . drop n) $ (take x . drop n) xss
  where n = length xss `div` 2

matrix = replicate 10 [1..10]

applied:

> localSquare matrix (5,5)
[[6,7,8,9,10],[6,7,8,9,10],[6,7,8,9,10],[6,7,8,9,10],[6,7,8,9,10]]

If you indeed need a plain list, apply just type concat $ localSquare matrix (5,5)