Re: [Haskell-cafe] turning an imperative loop to Haskell

6 Sep 2007

      On 06/09/07, Axel Gerstenberger  wrote:
...
module Main where
import System.IO
import Text.Printf
main :: IO ()
main = do
     let all_results1 = take 20000 $ step [1]
     --print $ length all_results1 -- BTW: if not commented out,
                                   --      all values of all_results
                                   --      are already
                                   --      calculated here
     loop [1..50] $ \i -> do
         let x = all_results1!!i
         putStrLn $ show i ++ "  " ++ show x
-- create an infinite list with values u_{n+1} ++ [u_n,u_{n-1},...,u_1]
-- where u_{n+1} = f (u_n)
step history =
     case history of
         []   -> error "no start values"
         xs   -> xs ++ (step [ f (head $ reverse (xs) )])
To create an infinite list where each f(u) depends on the previous u,
with a single seed value, use 'iterate':

Prelude> let us = iterate f 3

That produces your infinite list of values, starting with [f 3, f(f3),
f(f(f 3)), ...]. Pretty neat.

Then all you really need is

main = mapM_ (uncurry (printf "%d %f\n")) (zip [1..50] (iterate f 3))

You can probably shorten this a bit more with arrows but I've got a
cold at the moment and not really thinking straight.

Cheers,

D.

Re: [Haskell-cafe] turning an imperative loop to Haskell

Dougal Stanton