Writing code to reverse Haskell program

Question

I have written a program that takes a message as a string and returns an anagram by padding the message with X's as needed such that the string length has exactly 4 factors then essentially rearranges the message as if it had been organized in a grid and read down instead of across. For example, inputting, "Haskell" would return the string, "HealslkX". I have written a program that encodes this anagram, but am having trouble writing a program that can reverse the previous program and decode the anagram, particularly with the removeX function that should remove the X padding. Here is what I have:

encode:

import Data.List

factors :: Int -> [Int]
factors n = [x | x <- [1..n], n `mod` x == 0]

split :: Int -> [a] -> [[a]]
split _ [] = []
split n xs =
    let (ys, zs) = splitAt n xs
    in  ys : split n zs

encode :: [Char] -> [Char]
encode (x:xs) = if (length (factors (length xs))) == 4 then concat 
(transpose (split ((factors (length xs))!!2) xs))
else encode (xs ++ ['X'])

decode:

import Data.List

factors :: Int -> [Int]
factors n = [x | x <- [1..n], n `mod` x == 0]

split :: Int -> [a] -> [[a]]
split _ [] = []
split n xs =
    let (ys, zs) = splitAt n xs
    in  ys : split n zs

removeX :: [a] -> [a]
removeX xs = if (last xs) == 'X' then ((init xs) && removeX xs)
 else xs

decode :: [Char] -> [Char]
decode (x:xs) = removeX (concat (transpose (split ((factors (length xs))!!1) xs)))

Daniel Wagner · Accepted Answer · 2017-09-28 17:24:47Z

3

Just use removeX (init xs) instead of init xs && removeX xs. Haskell is not procedural (you don't write down a sequence of changes to make) but functional (you write down functions that produce new results from old). Haven't read the rest of the code to see if there are other errors, though.

Also consider removeX = reverse . dropWhile ('X'==) . reverse for better efficiency. Lists are singly-linked, so accesses and modifications at the end are relatively expensive.

answered Sep 28, 2017 at 17:24

Daniel Wagner

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1 Comment

David Fletcher Over a year ago

dropWhileEnd is in Data.List now.

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