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A better way to map a function that requires IO over a list
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Closed 8 years ago.
I'm practicing with a problem that gives text input on stdin and wants output on stdout.
They give an integer T in the first line that I read, and then T lines follow. For the T following lines I have a function that takes an integer and returns an integer that I must print. So my output should be T lines.
I'm stuck at this point:
main = do
strcases <- getLine
map (calculate) [1..(read strcases)]
calculate _ = do
strp <- getLine
print $ addone (read strp)
addone x = x+1
Of course this doesn't work at the moment. Can you show how to print the T lines of output?
You can use readLn, which combines getLine and read, and replicateM_ from Control.Monad to repeat an IO action (or any monad) a given number of times:
import Control.Monad
main :: IO ()
main = do
n <- readLn
replicateM_ n calculate
calculate :: IO ()
calculate = do
x <- readLn
print $ addone x
addone :: Int -> Int
addone x = x+1
Note, the underscore suffix on replicateM_ indicates that the return value of each executed action be discarded. The alternative function without the underscore suffix will return a list containing the return value of each executed action, which we do not want in this case.
Related
Below if my code right now. I want to be able to take in user input like the following: "6 1 2 3 4 5 6" and the get the sum and print. it would also be cool to understand how to use the first number entered as the total numbers. SO here the first number is 6 and the total numbers inputted is 6.
Thank you in advance for helping me with this. I have been researching for weeks and cannot figure this out.
main = do
putStrLn "Enter how many numbers:" -- clearer
num<-getLine
putStrLn("Enter a number: ")
numberString <- getLine
let numberInt =(read numberString :: Int)
print (numberInt*4)
main
It seems you either need an auxiliary recursive function for reading num integers, or some helper like replicateM, which makes writing the code a little easier.
replicateM num action runs action exactly num times, and collects all the action results in a list.
main = do
putStrLn "Enter how many numbers:" -- clearer
num<-getLine
numbers <- replicateM num $ do
putStrLn("Enter a number: ")
numberString <- getLine
return (read numberString :: Int)
-- here we have numbers :: [Int]
...
You can then continue from there.
If instead you want to use an auxiliary function, you can write
readInts :: Int -> IO [Int]
readInts 0 = return []
readInts n = do
putStrLn("Enter a number: ")
numberString <- getLine
otherNumbers <- readInts (n-1) -- read the rest
return (read numberString : otherNumbers)
Finally, instead of using getLine and then read, we could directly use readLn which combines both.
Construct a list of integers using
let l = map (\x -> read x::Int) (words "6 1 2 3 4 5 6")
in (numNumbers, numbers)
You tried to read the whole string into a single number.
I have a file in the form:
3
1 2
3 4
5 7
Where the first line is the number of lines
I know that:
getInt :: IO Int
getInt = readLn
main = do num <- getInt
print (num)
Reads the first line.
Next, I tried:
readInts :: IO [Int]
readInts = fmap (map read.words) getLine
For read a line and get a list: [a,b].
And I tried to use the above in a recursive loop
loop :: Int -> IO()
loop n = if 1 == n then do num <- readInts
print(num)
else loop (n-1)
I'm getting the first line only:
[5,3]
But I need to read the rest of lines, given N
The format of the input file looks a lot like the ones used in programming contests. Here is my standard setup for programming contests like that:
import Control.Monad
import Text.Printf
main :: IO ()
main = do
n <- readLn
forM_ [1 .. n] $ \i -> do
printf "Case %d: " (i :: Int)
solve
An example of solve might be:
solve :: IO ()
solve = do
nums <- map read . words <$> getLine
print (sum nums)
Adding to this helpful answer and comment, for some challenges you'll need to collect n lines and then emit a single result at the end based on an aggregation of the data. An approach for creating a list might use replicateM as follows:
import Control.Monad
toInt :: String -> Int
toInt x = read x :: Int
lineToInts :: String -> [Int]
lineToInts x = map toInt $ words x
main :: IO ()
main = do
n <- readLn
remainingLines <- replicateM n getLine
let list = map lineToInts remainingLines
print list
Sample run:
3
0 1
3 4
6 8
[[0,1],[3,4],[6,8]]
See also Read n lines input with Haskell
Have you looked into the function lines? It takes a string and returns the same string as a list separated by \n. Using this function you don't even have to have the number of lines.
I have written a Haskell code as:
loop = do
x <- getLine
if x == "0"
then return ()
else do arr <- replicateM (read x :: Int) getLine
let blocks = map (read :: String -> Int) $ words $ unwords arr
putStr "Case X : output = "; -- <- What should X be?
print $ solve $ blockPair blocks;
loop
main = loop
This terminates at 0 input. I also want to print the case number eg. Case 1, 2 ...
Sample run:
1
10 20 30
Case 1: Output = ...
1
6 8 10
Case 2: Output = ...
0
Does anyone know how this can be done? Also, If possible can you suggest me a way to print the output line at the very end?
Thanks in advance.
For the first part of your question, the current case number is an example of some "state" that you want to maintain during the course of your program's execution. In other languages, you'd use a mutable variable, no doubt.
In Haskell, there are several ways to deal with state. One of the simplest (though it is sometimes a little ugly) is to pass the state explicitly as a function parameter, and this will work pretty well given the way you've already structured your code:
main = loop 1
loop n = do
...
putStr ("Case " ++ show n ++ ": Output = ...")
...
loop (n+1) -- update "state" for next loop
The second part of your question is a little more involved. It looks like you wanted a hint instead of a solution. To get you started, let me show you an example of a function that reads lines until the user enters end and then returns the list of all the lines up to but not including end (together with a main function that does something interesting with the lines using mostly pure code):
readToEnd :: IO [String]
readToEnd = do
line <- getLine
if line == "end"
then return []
else do
rest <- readToEnd
return (line:rest)
main = do
lines <- readToEnd
-- now "pure" code makes complex manipulations easy:
putStr $ unlines $
zipWith (\n line -> "Case " ++ show n ++ ": " ++ line)
[1..] lines
Edit: I guess you wanted a more direct answer instead of a hint, so the way you would adapt the above approach to reading a list of blocks would be to write something like:
readBlocks :: IO [[Int]]
readBlocks = do
n <- read <$> getLine
if n == 0 then return [] else do
arr <- replicateM n getLine
let block = map read $ words $ unwords arr
blocks <- readBlocks
return (block:blocks)
and then main would look like this:
main = do
blocks <- readBlocks
putStr $ unlines $
zipWith (\n line -> "Case " ++ show n ++ ": " ++ line)
[1..] (map (show . solve . blockPair) blocks)
This is similar in spirit to K. A. Buhr's answer (the crucial move is still passing state as a parameter), but factored differently to demonstrate a neat trick. Since IO actions are just normal Haskell values, you can use the loop to build the action which will print the output without executing it:
loop :: (Int, IO ()) -> IO ()
loop (nCase, prnAccum) = do
x <- getLine
if x == "0"
then prnAccum
else do inpLines <- replicateM (read x) getLine
let blocks = map read $ words $ unwords inpLines
prnAccumAndNext = do
prnAccum
putStr $ "Case " ++ show nCase ++ " : output = "
print $ solve $ blockPair blocks
loop (nCase + 1, prnAccumAndNext)
main = loop (1, return ())
Some remarks on the solution above:
prnAccum, the action which prints the results, is threaded through the recursive loop calls just like nCase (I packaged them both in a pair as a matter of style, but it would have worked just as fine if they were passed as separate arguments).
Note how the updated action, prnAccumAndNext, is not directly in the main do block; it is defined in a let block instead. That explains why it is not executed on each iteration, but only at the end of the loop, when the final prnAccum is executed.
As luqui suggests, I have removed the type annotations you used with read. The one at the replicateM call is certainly not necessary, and the other one isn't as well as long as blockPair takes a list of Int as an argument, as it seems to be the case.
Nitpicking: I removed the semicolons, as they are not necessary. Also, if arr refers to "array" it isn't a very appropriate name (as it is a list, and not an array), so I took the liberty to change it into something more descriptive. (You can find some other ideas for useful tricks and style adjustments in K. A. Buhr's answer.)
Basically I would like to find a way so that a user can enter the number of test cases and then input their test cases. The program can then run those test cases and print out the results in the order that the test cases appear.
So basically I have main which reads in the number of test cases and inputs it into a function that will read from IO that many times. It looks like this:
main = getLine >>= \tst -> w (read :: String -> Int) tst [[]]
This is the method signature of w: w :: Int -> [[Int]]-> IO ()
So my plan is to read in the number of test cases and have w run a function which takes in each test case and store the result into the [[]] variable. So each list in the list will be an output. w will just run recursively until it reaches 0 and print out each list on a separate line. I'd like to know if there is a better way of doing this since I have to pass in an empty list into w, which seems extraneous.
As #bheklilr mentioned you can't update a value like [[]]. The standard functional approach is to pass an accumulator through a a set of recursive calls. In the following example the acc parameter to the loop function is this accumulator - it consists of all of the output collected so far. At the end of the loop we return it.
myTest :: Int -> [String]
myTest n = [ "output line " ++ show k ++ " for n = " ++ show n | k <- [1..n] ]
main = do
putStr "Enter number of test cases: "
ntests <- fmap read getLine :: IO Int
let loop k acc | k > ntests = return $ reverse acc
loop k acc = do
-- we're on the kth-iteration
putStr $ "Enter parameter for test case " ++ show k ++ ": "
a <- fmap read getLine :: IO Int
let output = myTest a -- run the test
loop (k+1) (output:acc)
allOutput <- loop 1 []
print allOutput
As you get more comfortable with this kind of pattern you'll recognize it as a fold (indeed a monadic fold since we're doing IO) and you can implement it with foldM.
Update: To help explain how fmap works, here are equivalent expressions written without using fmap:
With fmap: Without fmap:
n <- fmap read getLine :: IO [Int] line <- getLine
let n = read line :: Int
vals <- fmap (map read . words) getLine line <- getLine
:: IO [Int] let vals = (map read . words) line :: [Int]
Using fmap allows us to eliminate the intermediate variable line which we never reference again anyway. We still need to provide a type signature so read knows what to do.
The idiomatic way is to use replicateM:
runAllTests :: [[Int]] -> IO ()
runAllTests = {- ... -}
main = do
numTests <- readLn
tests <- replicateM numTests readLn
runAllTests tests
-- or:
-- main = readLn >>= flip replicateM readLn >>= runAllTests
This code reads the number of lines to process from the first line of stdin, then it loops number_of_lines_to_process times doing some calculations and prints the result.
I want it to print the line number in "Line #" after "#" but I don't know how to obtain it
import IO
import Control.Monad (replicateM)
main :: IO ()
main = do
hSetBuffering stdin LineBuffering
s <- getLine
let number_of_lines_to_process = read s :: Integer
lines <- replicateM (fromIntegral(number_of_lines_to_process)) $ do
line <- getLine
let number = read line :: Integer
result = number*2 --example
putStrLn ("Line #"++": "++(show result)) --I want to print the number of the iteration and the result
return ()
I guess that the solution to this problem is really easy, but I'm not familiar with Haskell (coding in it for the first time) and I didn't find any way of doing this. Can anyone help?
You could use forM_ instead of replicateM:
import IO
import Control.Monad
main :: IO ()
main = do
hSetBuffering stdin LineBuffering
s <- getLine
let number_of_lines_to_process = read s :: Integer
forM_ [1..number_of_lines_to_process] (\i -> do
line <- getLine
let number = read line :: Integer
result = number * 2
putStrLn $ "Line #" ++ show i ++ ": " ++ show result)
Note that because you use forM_ (which discards the results of each iteration) you don't need the additional return () at the end - the do block returns the value of the last statement, which in this case is the () which is returned by forM_.
The trick is to first create a list of all the line numbers you want to print, and to then loop through that list, printing each number in turn. So, like this:
import Control.Monad
import System.IO
main :: IO ()
main = do
hSetBuffering stdin LineBuffering
s <- getLine
let lineCount = read s :: Int
-- Create a list of the line numbers
lineNumbers = [1..lineCount]
-- `forM_` is like a "for-loop"; it takes each element in a list and performs
-- an action function that takes the element as a parameter
forM_ lineNumbers $ \ lineNumber -> do
line <- getLine
let number = read line :: Integer
result = number*2 --example
putStrLn $ "Line #" ++ show lineNumber ++ ": " ++ show result
return ()
Read the definition of forM_.
By the way, I wouldn't recommend using the old Haskell98 IO library. Use System.IO instead.
You could calculate the results, enumerate them, and then print them:
import IO
import Control.Monad (replicateM)
-- I'm assuming you start counting from zero
enumerate xs = zip [0..] xs
main :: IO ()
main = do
hSetBuffering stdin LineBuffering
s <- getLine
let number_of_lines_to_process = read s :: Integer
lines <- replicateM (fromIntegral(number_of_lines_to_process)) $ do
line <- getLine
let number = read line :: Integer
result = number*2 --example
return result
mapM_ putStrLn [ "Line "++show i++": "++show l | (i,l) <- enumerate lines ]
I'm still new at Haskell, so there could be problems with the program below (it does work). This program is a tail recursive implementation. The doLine helper function carries around the line number. The processing step is factored into process, which you can change according to the problem you are presented.
import System.IO
import Text.Printf
main = do
hSetBuffering stdin LineBuffering
s <- getLine
let number_of_lines_to_process = read s :: Integer
processLines number_of_lines_to_process
return ()
-- This reads "max" lines from stdin, processing each line and
-- printing the result.
processLines :: Integer -> IO ()
processLines max = doLine 0
where doLine i
| i == max = return ()
| otherwise =
do
line <- getLine
let result = process line
Text.Printf.printf "Line #%d: %d\n" (i + 1) result
doLine (i + 1)
-- Just an example. (This doubles the input.)
process :: [Char] -> Integer
process line = let number = read line :: Integer
in
number * 2
I'm a haskell rookie, so any critiques of the above are welcome.
Just as an alternative, I thought that you might enjoy an answer with minimal monad mucking and no do notation. We zip a lazy list of the user's data with an infinite list of the line number using the enumerate function to give us our desired output.
import System.IO
import Control.Monad (liftM)
--Here's the function that does what you really want with the data
example = (* 2)
--Enumerate takes a function, a line number, and a line of input and returns
--an ennumerated line number of the function performed on the data
enumerate :: (Show a, Show b, Read a) => (a->b) -> Integer -> String -> String
enumerate f i x = "Line #" ++
show i ++
": " ++
(show . f . read $ x) -- show . f . read handles our string conversion
-- Runover takes a list of lines and runs
-- an enumerated version of the sample over those lines.
-- The first line is the number of lines to process.
runOver :: [String] -> [String]
runOver (line:lines) = take (read line) $ --We only want to process the number of lines given in the first line
zipWith (enumerate example) [1..] lines -- run the enumerated example
-- over the list of numbers and the list of lines
-- In our main, we'll use liftM to lift our functions into the IO Monad
main = liftM (runOver . lines) getContents