Haskell :: Parsing Line of Text into a List of List

Question

Hi guys,

I'm parsing a text file which look like this:

114.474998474121 15.7440004348755 25.806999206543 -873 172 182 188 
114.46199798584 15.7419996261597 25.8799991607666 -1396 180 192 205

And wish it can read as like this:

[[114.475,15.744,25.807,-873.0,172.0,182.0,188.0],
[114.462,15.742,25.88,-1396.0,180.0,192.0,205.0]]

Currently my code for this text parsing doesn't give that. Here is my code:

main = do
    text <- readFile "mytext.txt" 
    let
        pcVal = map read (words text) :: [Float]
    print pcVal
    return ()

This code parsed all the text as a single list like this:

[114.475,15.744,25.807,-873.0,172.0,182.0,188.0,
114.462,15.742,25.88,-1396.0,180.0,192.0,205.0]

I couldn't find how to take the whole single line (in text file) as a list, and the second line as another list till end of the file. Appreciate if somebody have experience in this. Thanks.

I assume you are a beginner - one tipp for the future, when you know a bit more haskell; revisit this exercise and implement a parser instead of just having a listof list of double values. — epsilonhalbe
– epsilonhalbe, Commented Dec 4, 2017 at 15:56
yes @epsilonhalbe, I'm a beginner and start to like this language. I will take your advise for the future. thanks! — Sir DK
– Sir DK, Commented Dec 5, 2017 at 8:54

Dan · Accepted Answer · 2017-12-04 11:24:13Z

7

You can use the lines function; for example map words $ lines text.

This would also be a good time for a helper function, ie

let parse :: String -> [Float]
    parse line = map read $ words line
    pcVal = map parse $ lines text

edited Dec 4, 2017 at 11:24

answered Dec 4, 2017 at 9:28

Dan

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2 Comments

Sir DK Over a year ago

Thanks @Dan, I've tried but some bug in String - Char things. A bit of edit then it works! I post the answer below.

Dan Over a year ago

@SirDK I think the type annotation I've added should help.

Sir DK · Accepted Answer · 2017-12-04 11:10:38Z

0

Thanks Dan for suggesting lines as the solution. Here is the code that works fine with my question:

main = do
    text <- readFile "mytext.txt" 
    let
        readparse txtLines = map read txtLines :: [Float]
        parse txtLines = map (words) (lines txtLines)
        pcval = map readparse (parse text) 
    print pcVal
    return ()

Hope it help others too. Thanks and cheers!

answered Dec 4, 2017 at 11:10

Sir DK

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