Hello, I am trying to solve a simple task, but got stuck with double recursion - for some reason not all list elements get processed. Please advice on a simple solution, using plane old recursion :) *** Task:
From a sequence of chars build all possible chains where each chain consists of chars that can be accessed from one to another. For example, given the sequence: "abcde" In table below chars in a left column can access a list of chars in the right column: a | b c d e
b | c d e c | d e d | e Then chains for this example will be: abcde, acde, ade, ae, bcde, bde, be, cde, cd, ce, de *** My incomplete result, which I get running my program (see at the end of this message): ('a','b'),('b','c'),('c','d'),('d','e'), -- abcde, ('c','e'), -- ce, ('b','d'),('d','e'), -- bde, ('b','e') -- be, ('a','c'),('c','d'),('d','e'), -- acde ('c','e'), ('a','d'),('d','e'), -- ade, ('a','e') -- ae Sequence 'abcde' contains 'bcde', 'cde', 'cd', 'de'. How to add all these sub-sequences as a separate sequences in my output? And I have 'ce' sequence duplicated, which I don't need. Also, how to output chains as a list of lists? Such as: [[abcde, acde, ade, ae,] [bcde, bde, be,] [cde, cd, ce,] de]] *** my code test = chains testPairs 'a' [] testPairs = pairs testSeq testSeq = "abcde" -- From a list of '((from,to),1)' pairs build char chains -- Char chain is a list of chars: [from1, to1 = from2, to2 = from3, to3 = from4, ...] -- 'pairList' - a list of pairs chains pairList from chainList = chainWrk (nextTo pairList from) from chainList where chainWrk [] from chainList = chainList chainWrk (to:tos) from chainList = chainWrk tos from (chains pairList to (chainList ++ [(from,to)])) -- Find direct neighbors for 'from' nextTo pairList from = toList (findPairs pairList from) [] -- From a list of '((from,to),1)' pairs build a list of 'to'-s toList [] tos = tos toList (((from,to),len):ps) tos = toList ps (tos ++ [to]) -- From 'pairList' find elements with 'from' equal to 'start' -- 'pairList' is a list of '((from,to),1)' pairs findPairs pairList search = filter (flt search) pairList where flt search ((from, to), len) = search == from -- From a sequence of chars buid a list of '((from,to),1)' pairs pairs [] = [] pairs (x:xs) = pairWrk x xs [] ++ pairs xs pairWrk hd [] pairLst = pairLst pairWrk hd (x:xs) pairLst = pairWrk hd xs (pairLst ++ [((hd,x),1)])