Programming Praxis – First Unrepeated Character In A String

In today’s Programming Praxis exercise, our goal is to find the find the first unrepeated character in a string, along with its index. Let’s get started, shall we?

import Data.Maybe
import Data.List

We traverse the list from right to left, keeping a list of characters we’ve already encountered and a list of possible options. When we check a character, we first check if it’s in the list of duplicate characters. If not, we then check the list of options. If the letter in question is there already, we remove it from the list of options and add it to the list of duplicates. Otherwise, we add the current character to the list of options. At the end, we return the first unique element (if any).

unrepeated :: String -> Maybe (Integer, Char)
unrepeated = listToMaybe . snd . foldr f ([], []) . zip [0..] where
    f (i,c) (ds,us) = if elem c ds then (ds, us) else
        maybe (ds, (i,c):us) (\(fi,fc) -> (fc:ds, delete (fi,fc) us)) $
        find ((== c) . snd) us

Some tests to see if everything is working properly:

main :: IO ()
main = do print $ unrepeated "aaabc"       == Just (3, 'b')
          print $ unrepeated "aaabbbcddd"  == Just (6, 'c')
          print $ unrepeated "aaaebbbcddd" == Just (3, 'e')
          print $ unrepeated "aabbcc"      == Nothing
          print $ unrepeated "aba"         == Just (1, 'b')

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