Re: [Haskell-cafe] Incrementation fails
Hi John, Can you tell us what your function is supposed to do? It appears to be some kind of search-and-replace for strings. What is the relationship between 'findStr', 'replaceStrList' and 'myText'? 2013/5/8 John knowledge1...@gmail.com Hello All, I'm in a big trouble with incrementation of a counter in this code. It doesn't increment. Could you please tell me where the problem ist and how can I solve it? replaceBasedIdx:: String - [String] - String - String replaceBasedIdxfindStr replaceStrList myText = replaceBasedIdxSub findStr replaceStrList myText 0 replaceBasedIdxSub :: String - [String] - String - Int - String replaceBasedIdxSub findStr replaceStrList myText counter = loop myText where loop [] = [] loop myText = let (prefix, rest) = splitAt n myText in if findStr == prefix-- found an occurrence? then (replaceStrList !! (counter+1)) ++ loop rest -- yes: replace it else head myText : loop (tail myText) -- no: keep looking n = length findStr Thank you very mutch! Greetings! ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Incrementation fails
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Incrementation fails
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 vandijk.r...@gmail.com 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Incrementation fails
Hello All, I'm in a big trouble with incrementation of a counter in this code. It doesn't increment. Could you please tell me where the problem ist and how can I solve it? replaceBasedIdx:: String - [String] - String - String replaceBasedIdxfindStr replaceStrList myText = replaceBasedIdxSub findStr replaceStrList myText 0 replaceBasedIdxSub :: String - [String] - String - Int - String replaceBasedIdxSub findStr replaceStrList myText counter = loop myText where loop [] = [] loop myText = let (prefix, rest) = splitAt n myText in if findStr == prefix-- found an occurrence? then (replaceStrList !! (counter+1)) ++ loop rest -- yes: replace it else head myText : loop (tail myText) -- no: keep looking n = length findStr Thank you very mutch! Greetings! -- View this message in context: http://haskell.1045720.n5.nabble.com/Incrementation-fails-tp5729905.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
