On Dec 6, 2009, at 4:42 AM, MeAdAstra wrote:
I only started learning Haskell some days ago. Maybe one of you can give me a hint on how to implement a function that needs a character in the range (a,b,...z) and an integer number k and returns the k-next neighbor of the character? For example, fct a 5 would result in output f.
I'm reading between the lines here. I think you want alphabetic_successor :: Int -> Char -> Char {- alphabetic_successor k c requires k >= 0 and c `elem` lower_case_letters ensures result `elem` lower_case_letters where lower_case_letters = "abcdefghijklmnopqrstuvwxyz" -} What's not clear from your description is what's supposed to happen if you ask for alphabetic_successor 5 'z'. I'll assume you want to think of the letters being in a circle. It is possible to hack this using character->integer and integer->character conversion, but I'm going to do it another way that doesn't depend on the characters being contiguous in the Unicode character set. You could use the code to find the successor in a sequence of consonants, for example. A. We want to find the position of c in lower_case_letters. The first thing to do is to read the documentation and see if there is a suitable function. There's one in the List module: elemIndex :: Eq x => x -> [x] -> Maybe Int. This will work for any type x (including characters) that supports equality (==). It takes an item, a list of items, and either returns Nothing (the item wasn't found) or (Just n) -- the item was found at position n, 0 offset. B. We add k to n and reduce it modulo the length of the list. C. Given our new position, we want to find the character at that position in the list. The function for that is list !! index. import List (elemIndex) alphabetic_successor k c = lower_case_letters !! new_index where Just index = List.elemIndex c lower_case_letters new_index = (k + index) `mod` length lower_case_letters lower_case_letters = "abcdefghijklmnopqrstuvwxyz"