inits

[Sebastian Sylvan]

On 4/8/06, Tim Toorop <timtoorop at quicknet.nl> wrote:
> Sebastian Sylvan wrote:
> > On 4/7/06, Spencer Janssen <spencerjanssen at gmail.com> wrote:
> >
> >> Earlier today on the #haskell IRC channel, Tim Toorop (bolrod on
> >> #haskell) pointed out that Data.List.inits is rather slow, and
> >> proposed an alternative.  After some collabrative tweaking, we came up
> >> with the following:
> >>
> >>
> >>> inits xs = [] : (zipWith take [1..] $ map (const xs) xs)
> >>>
> >> This function seems to perform significantly better.  For example, the
> >> program below takes about 15 seconds with the old inits, and only 3
> >> seconds with the new version (tested with GHC 6.4.1 and -O2).
> >>
> >>
> >>> main = print $ sum $ map sum $ inits [1..7000]
> >>>
> >> As this version performs much better and will work as a drop in
> >> replacement, I suggest that it be included in the hierarchical
> >> libraries.
> >>
> >>
> >
> > That's quite a bit faster on my machine as well. I think the following
> > slight variation may be a bit clearer, though:
> > inits xs = [] : zipWith take [1..length xs] (repeat xs)
> >
> > --
> > Sebastian Sylvan
> > +46(0)736-818655
> > UIN: 44640862
> > _______________________________________________
> > Libraries mailing list
> > Libraries at haskell.org
> > http://www.haskell.org/mailman/listinfo/libraries
> >
> The length xs wont work on infinite lists.

Ah of course!

/S
--
Sebastian Sylvan
+46(0)736-818655
UIN: 44640862