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|
-- Write a program that transposes the text in a file. For instance, it should
-- convert "hello\nworld\n" to "hw\neo\nlr\nll\nod\n".
{-- From examples/examples/ch04/InteractWith.hs --}
import System.Environment (getArgs)
interactWith function inputFile outputFile = do
input <- readFile inputFile
writeFile outputFile (function input)
main = mainWith myFunction
where mainWith function = do
args <- getArgs
case args of
[input,output] -> interactWith function input output
_ -> putStrLn "error: exactly two arguments needed"
-- replace "id" with the name of our function below
myFunction = transpose
{-- End of code from examples --}
-- The assignment doesn't mention that the implementation should handle rows of
-- different lengths, and the example input contains only rows of the same
-- length. Thus, for simplicity, we assume that all rows of the input have the
-- same length.
firstColumn :: [String] -> String
firstColumn [] = ""
firstColumn (x:xs)
| null x = ""
| otherwise = (head x):(firstColumn xs)
removeFirstColumn :: [String] -> [String]
removeFirstColumn [] = []
removeFirstColumn (x:xs) = (tail x):(removeFirstColumn xs)
transpose' :: [String] -> [String]
transpose' rows = if null column
then []
else column:(transpose' rest)
where column = firstColumn rows
rest = removeFirstColumn rows
transpose input = unlines (transpose' (lines input))
-- $ stack ghc 4_a_4.hs
-- [1 of 1] Compiling Main ( 4_a_4.hs, 4_a_4.o )
-- Linking 4_a_4 ...
-- $ cat input.txt
-- hello
-- world
-- $ ./4_a_4 input.txt output.txt
-- $ cat output.txt
-- hw
-- eo
-- lr
-- ll
-- od
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