problem = replaceBasedIdx "X" ["b","d","f"] "aXcXeXgX" -- "abcdefg*** Exception: Prelude.(!!): index too large
This problem occurs because you never check if 'counter' is a valid index in the 'replaceStrList'. You can solve it by not using the !! operator at all. The solution is to also pass 'replaceStrList' in the recursion. That way you can check whether you have exhausted all strings in that list and as a bonus you do not need your counter anymore:
replaceBasedIdx :: String -> [String] -> String -> String replaceBasedIdx findStr replaceStrList myText = loop replaceStrList myText where loop :: [String] -> String -> String loop rs [] = [] -- Check for empty list of replacements loop [] text = text -- Pattern match on the list of replacements to get both the -- entire list rs, the first element r and the tail rs'. loop rs@(r:rs') text = let (prefix, rest) = splitAt n text in if findStr == prefix then r ++ loop rs' rest else head text : loop rs (tail text)
n :: Int n = length findStr
2013/5/8 Roel van Dijk
I stared at the code some more and deduced what I think is the intented meaning. Occurences of 'findStr' in 'myText' are replaced with the strings in 'replaceStrList'.
So replaceBasedIdx "X" ["b","d","f"] "aXcXeXg" = "abcdefg"
The reason your counter didn't increment is because it was defined as an argument to 'replaceBasedIdxSub'. That means its value is fixed once you evaluate that function. Its value will not change no matter how many times the inner 'loop' function is evaluated. The solution is to pass the counter as an argument to said 'loop' function. Now when 'loop' enters the recursion you can pass a different value, such as counter + 1.
replaceBasedIdx :: String -> [String] -> String -> String replaceBasedIdx findStr replaceStrList myText = replaceBasedIdxSub findStr replaceStrList myText
replaceBasedIdxSub :: String -> [String] -> String -> String replaceBasedIdxSub findStr replaceStrList myText = loop 0 myText where loop :: Int -> String -> String loop counter [] = [] loop counter myText = let (prefix, rest) = splitAt n myText in if findStr == prefix then (replaceStrList !! counter) ++ loop (counter + 1) rest else head myText : loop counter (tail myText)
n :: Int n = length findStr