On Sun, Dec 19, 2010 at 12:27 PM, ender <crazyen...@gmail.com> wrote: > Hi all: > I'v been learning haskell for several months, and now I'm trying > to write some "real word" program in haskell, like finding files under > one directory or something > My problem is that, I dont know the way of writing a loop in > haskell. I searched google and found some code that translate c loop > into haskell like this one: > >> > > I am new to haskell and would look to write a function equivalent >> > > to the following loop in C >> > > >> > > int value = 500000; >> > > int part_stack[4]; >> > > int *part_ptr = part_stack; >> > > for (; value; value /= 10000) >> > > *part_ptr++ = value % 10000; >> > >> > part_stack :: [Int] >> > part_stack = [0,50] >> > >> > Note that I've performed a memoization optimization--this >> > makes the code both smaller, faster and easier to read! :P >> >> Ignore David, he's pulling your leg. Here's the proper translation: >> >> do >> alloca $ \value -> do >> poke value (500000::Int) >> allocaArray 4 $ \part_stack -> do >> alloca $ \part_ptr -> do >> poke part_ptr part_stack >> let loop = do >> val <- peek value >> if val == 0 then return () else do >> p <- peek part_ptr >> poke p (val `rem` 10000) >> poke part_ptr (p `plusPtr` 1) >> poke value (val `quot` 10000) >> loop >> loop > > and I really think that's not a "haskell way", it's just translate c > code into haskell code byte by byte > My question is: how to translate above c code into haskell in "haskell way"
Hi, Usually, loops are translated in recursion or higher order functions (maps, folds, scans). To generate a list from a number, use functions like iterate or repeat. To end the iterate, use a function from the takeWhile family. My translation of the above would be (no type signatures because I'm lazy right now): valueTrim x = div x 10000 valueGet x = mod x 100000 f x = map valueGet $ takeWhile (/= 0) $ iterate valueTrim x (call with f 500000) Reading the last line, you have a very concise description: trim x until it becomes 0 and apply valueGet to each of the results. -- Mihai _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
