On Sat, Aug 22, 2009 at 01:23, staafmeister<g.c.stave...@uu.nl> wrote: > But in general you have a structure like > > first line -- integer specifying the number of testcases (n) > Then for each testcase > a line with an integer specifying the number of edges (e) > a line with e pairs of string s and int p where p is the number asociated > with string s, etc. > > Such a structure cannot be parsed by map read.lines > What I used is "words" to tokenize and put the list in a State monad with > readInt, readString, etc. functions, to mimic > C code. This seems to be a lot of overkill, so there must be an simpler > way
Although you most certainly can use a State monad, in most problems this isn't necessary. Most algorithms that you need to solve programming contest problems can be written in a purely functional style, so you can limit monadic code to just a few helper functions. For example, this reads input in the style you mention (assuming the strings don't contain whitespace): > import Control.Monad > > answer = id > > parse [] = [] > parse (s:p:r) = (s, (read p) :: Int) : parse r > > run = getLine >> getLine >>= putStrLn . show . answer . parse . words > > main = flip replicateM_ run =<< readLn The answer function would be a pure function that computes the answer for a particular run. This main function is reusable for all problems with many runs. Observe that the number of edges (e), provided as a convenience for memory allocation in many other languages, is not even necessary in Haskell :) (If anyone knows a better way than explicit recursion to map over a list, two elements at a time, or zip its even elements with its odd elements, I'd love to hear! I can imagine a convoluted fold with a boolean in its state, but it's ugly.) Hope that helps, Thomas _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
