[Haskell-cafe] Re: Very fast loops. Now!

[David Roundy]

On Tue, Feb 13, 2007 at 08:18:25AM +1100, Donald Bruce Stewart wrote:
> Now, if we rewrite it to not use the temporary:
> 
>     go :: Double -> Double -> Int -> IO ()
>     go !x !y !i
>         | i == 1000000000 = printf "%.6f\n" (x+y)
>         | otherwise       = go (x*y/3) (x*9) (i+1)
> 
> 
>     for (; i<1000000000; i++) {
>         x = x*y/3.0;
>         y = x*9.0;
>     }
> 
> ------------------------------------------------------------------------
> 
> $ time ./hp
> 3.333333
> ./hp  9.95s user 0.00s system 99% cpu 9.965 total
> 
> $ time ./cc                                                 
> 3.333333
> ./cc  10.06s user 0.00s system 99% cpu 10.110 total

I'm rather curious (if you're sill interested) how this'll be affected by
the removal of the division from the inner loop. e.g.

    go :: Double -> Double -> Int -> IO ()
    go !x !y !i
        | i == 1000000000 = printf "%.6f\n" (x+y)
        | otherwise       = go (x*y*(1.0/3)) (x*9) (i+1)


    for (; i<1000000000; i++) {
        x = x*y*(1.0/3.0);
        y = x*9.0;
    }

My guess is that the code will be far faster, and that the differences
between C and Haskell will therefore be more pronounced.  After all,
division is a slow operation...
-- 
David Roundy
Department of Physics
Oregon State University