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| -rw-r--r-- | README.md | 2 | ||||
| -rw-r--r-- | ch04/4_a_4.hs | 58 |
2 files changed, 59 insertions, 1 deletions
@@ -71,7 +71,7 @@ more visible in the list the first exercise of a group is in bold italics. | **_4_a_1_** | yes | 84 | 4. Functional programming | | 4_a_2 | yes | | | | 4_a_3 | yes | | | -| 4_a_4 | | | | +| 4_a_4 | yes | | | | **_4_b_1_** | | 97 | | | 4_b_2 | | 98 | | | 4_b_3 | | | | diff --git a/ch04/4_a_4.hs b/ch04/4_a_4.hs new file mode 100644 index 0000000..1a66d45 --- /dev/null +++ b/ch04/4_a_4.hs @@ -0,0 +1,58 @@ +-- 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 |