4_a_4: Add solution

1 files changed, 58 insertions, 0 deletions

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