CODEWITHSUNDEEP
stringfilehaskellinput
user2184057
user2184057

Reputation: 964

Parsing an input file file in Haskell

Is there any fast way in Haskell to cast an input file like that into corresponding types? For example a function that takes a string and produces a list of Ints? Or do I need to parse it manually using getLine and parse the string?

10.
10.
[4, 3, 2, 1].
[(5,8,'~'), (6,4,'*'), (7,10,'~'), (8,2,'o')].
[4,0,9,4,7,5,7,4,6,4].
[4,10,0,6,6,5,6,5,6,2].

Upvotes: 2

Views: 3460

Answers (2)

Gabriella Gonzalez
Gabriella Gonzalez

Reputation: 35089

To build on Jeff Burka's answer, here's the specific code you would use for your particular file:

main = do
    [l1, l2, l3, l4, l5, l6] <- fmap (map init . lines) $ readFile "myFile.txt"
    let myVal :: (Int, Int, [Int], [(Int, Int, Char)], [Int], [Int])
        myVal = (read l1, read l2, read l3, read l4, read l5, read l6)
    print myVal

This will print out the parsed tuple.

The init part is to get rid of the trailing period you have at the end of each line.

Upvotes: 4

Jeff Burka
Jeff Burka

Reputation: 2571

Yes, the read function.

Once you read in the file with readFile for example, you can read each line to convert it to the type you want. You'll have to get rid of the periods first, though. So for example:

main = do
    text <- readFile "test.txt"
    let cases = lines text
        -- to get rid of the periods at the end of each line
        strs = map init cases
        lastLine = read $ last strs
    print $ show (map (+5) lastLine)

This will take your example file and read in a list of Ints from the last line, and the add 5 to the whole list and print it.

If every line were the same type, you could just map read over all the lines to get all of them. If there are different types, like in your example, you'd have to put in some logic to figure out what type is on each line, and then call an appropriate function to deal with that type.

Upvotes: 6

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