Haskell file reading and finding values - file

Question

I have recently started learning Haskell and I'm having a hard time figuring out how to interpret text files.
I have following .txt file:
ncols 5
nrows 5
xllcorner 809970
yllcorner 169790
cellsize 20
NODATA_value -9999
9 0 0 0 0
0 1 0 0 0
0 0 0 0 0
0 0 0 0 0
0 2 0 0 3
The first 6 lines just display some information I need when working with the file in a GIS software. The real deal starts when I try to work with the numbers below in Haskell.
I want to tell Haskell to look up where the numbers 9, 1, 2 and 3 are and print back the number of the row and column where those numbers actually are. In this case Haskell should print:
The value 9 is in row 1 and column 1
The value 1 is in row 2 and column 2
The value 2 is in row 5 and column 2
The value 3 is in row 5 and column 5
I tried finding the solution (or at least similar methods for interpreting files) in tutorials and other Haskell scripts without any success, so any help would be greatly appreciated.

Answer 1

Here is an example of a script to do what you want. Note that this will in its current form does not fail gracefully (but given this is a script, I doubt this is a concern). Make sure there is a trailing newline at the end of your file!
import Control.Monad (replicateM, when)
import Data.Traversable (for)
import System.Environment (getArgs)
main = do
-- numbers we are looking for
numbers <- getArgs
-- get the key-value metadata
metadata <- replicateM 6 $ do
[key,value] <- words <$> getLine
return (key,value)
let Just rows = read <$> lookup "nrows" metadata
Just cols = read <$> lookup "ncols" metadata
-- loop over all the entries
for [1..rows] $ \row ->do
rawRow <- words <$> getLine
for (zip [1..cols] rawRow) $ \(col,cell) ->
when (cell `elem` numbers)
(putStrLn ("The value " ++ cell ++ " is in row " ++ show row ++ " and column " ++ show col))
To use it, pass it as command line arguments the numbers you are looking for and then feed as input your data file.
$ ghc script.hs
$ ./script 9 1 2 3 < data.txt
Let me know if you have any questions!
I wasn't really sure if you wanted to look up just a fixed set of numbers, or any non-zero number. As your question asked for the former, that is what I did.