Reading 2-line input Haskell

Question

I've finished writing a Haskell program but I have troubles with the I/O and Main parts of it. I want to read input of the following format:

10 3
1 4 8

The first and second numbers are Ints, and the numbers of the second line should be made into an integer list, [Int]. The length of the list is equal to the second number on the first line.

I have the following code to read one Int at a time, however, it can only get it to work for the first line of input.

getInt :: IO Int
getInt = do
    l <- getLine
    return (read l)

What am I doing wrong?

have a look at the words function - something like map read . words should work for you — Random Dev
– Random Dev, Commented Mar 13, 2016 at 13:44
BTW: I am a bit surprised that you claim that you function works on "10 3" - I would have expected this to give you an no parse error — Random Dev
– Random Dev, Commented Mar 13, 2016 at 13:46
I do get a no parse error, although I thought that stemmed from the second input line and an end-of-file-related error. — user5951413
– user5951413, Commented Mar 13, 2016 at 13:54
no - it's because you try to read in two numbers at once - but you can use the map read . words I talked about - you can patternmatch as well if you like: [x, y] <- map read . words <$> getLine should do the trick here for the first line - you get the second (in a list) by just numbers <- map read . words <$> getLine (all in a do block of course!) — Random Dev
– Random Dev, Commented Mar 13, 2016 at 14:45

Random Dev · Accepted Answer · 2016-03-13 14:57:29Z

1

you should be able to get the data shown here by:

readData :: IO (Int, [Int])
readData = do
    [nr, _] <- map read . words <$> getLine
    nrs     <- map read . words <$> getLine
    return (nr,nrs)

this will read both lines and return the 10 as the first component and [1,4,8] as the second component of a tuple if you read in your example

As the second number in the first line will just be the length of the returned list I skipped it here.

of course you probably will want to adapt this to your needs.

answered Mar 13, 2016 at 14:57

Random Dev

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