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Baha

Member

Registered: 2008-09-07

Posts: 52

[Haskell]How to read in haskell and solve simple problems

Hello,

I'm an experienced C/C++ programmer and I would like to use Haskell to solve some problems.

But it is hard to me to write a simple way to read input from a file and analyze it, so I ask you how to do this. I didn't find how to do this around google or this forum, that's why I came to here.

Suppose we are given this problem:

"We're analyzing numbers and we want to know which numbers are even. The input consists on a number, N, and then N lines which contain a single integer. You are to say what numbers are even."

INPUT EXAMPLE:

3
1234
5555555
123044390581349287182

OUTPUT EXAMPLE:

yes
no
yes

I wrote the module that returns a string depending on if the number is even or not... But I would like to know how to repeat that function for N numbers. I don't want a superoptimized way or a strange way, I just want a simple and readable one...

In C++ i would do:

for (int i = 0; i < cases; i++)
{
    cin >> number;
    cout << analyze(number) << endl; // analyze returns a string
}

Can you iluminate with your knowledge, archers?

Thank you.

kazuo

Member

From: São Paulo/Brazil

Registered: 2008-03-18

Posts: 413

Website

Re: [Haskell]How to read in haskell and solve simple problems

http://lmgtfy.com/?q=haskell+read+input+from+a+file

Baha

Member

Registered: 2008-09-07

Posts: 52

Re: [Haskell]How to read in haskell and solve simple problems

http://lmgtfy.com/?q=haskell+read+input+from+a+file

Sorry, but I didn't want that, I read the first results for that search and I know how to get the input, but have no idea to solve the sample problem.

In the example above, I want to apply the funcion N times, i don't want a "while" style loop.

brisbin33

Member

From: boston, ma

Registered: 2008-07-24

Posts: 1,799

Website

Re: [Haskell]How to read in haskell and solve simple problems

you'd probably want to put the lines of the file (the numbers) into a list, then map a simple odd/even check on that list creating a new list of yes/no's.  then just print `take N $ thatList`.  if you set it up right, haskell's laziness will only evaluate the map N times.

Last edited by brisbin33 (2010-05-10 13:57:02)

---

//github/

dvolk

Member

Registered: 2010-05-16

Posts: 6

Re: [Haskell]How to read in haskell and solve simple problems

Try this if you're still having trouble

module Main where

import System.IO

main = do
    let test x = if even (read x) then "yes" else "no"

    withFile "test" ReadMode $ \handle -> do
        content <- hGetContents handle
        mapM_ ( putStrLn . test )
              . drop 1    -- first line is pointless
              . lines
                  $ content

jxy

Member

Registered: 2008-12-03

Posts: 133

Re: [Haskell]How to read in haskell and solve simple problems

I'd like more pure functions.

module Main where

act :: [Int] -> [String]
act (l:xs) = map test $ take l xs
  where
    test x | even x = "yes"
           | otherwise = "no"
act _ = error "no input"

main :: IO ()
main = putStr . unlines . act . map read . lines =<< getContents

Baha

Member

Registered: 2008-09-07

Posts: 52

Re: [Haskell]How to read in haskell and solve simple problems

Thanks for the posts!

However, it looks like you need to get involved a lot in this language to solve problems like that. I looked that dvolk's script would compute all numbers, and not the first N numbers.

jxy posted a nice algorithm, but I can't understand it at all because of that operator (=<<).

I was thinking about this language and I agree it's very good and readable, but I don't like the way to solve these kinds of simple problems so "forced".

Thanks for all the reaplies except the first one

Salut!

dvolk

Member

Registered: 2010-05-16

Posts: 6

Re: [Haskell]How to read in haskell and solve simple problems

Well, you said that the first line has the number N, followed by N lines. Given your description the program would always test the whole file.

If you have a file with X lines and you'd only like to test N <= X  lines, this should do it:

module Main where

import System.IO
import Control.Applicative

main = do
    let test x = if even (read x) then "yes" else "no"

    withFile "test" ReadMode $ \handle -> do
        nlines  <- read <$> hGetLine handle
        content <- hGetContents handle

        mapM_ ( putStrLn . test )
              . take nlines
              . lines
                  $ content

Baha

Member

Registered: 2008-09-07

Posts: 52

Re: [Haskell]How to read in haskell and solve simple problems

Thanks for modifying your program again in order to clarify me about my question