[R] data filtering
Roger D. Peng
rpeng at jhsph.edu
Wed Jun 2 21:33:32 CEST 2004
I think you want to use a single `&' rather than a double `&&'.
-roger
HENRIKSON, JEFFREY wrote:
> I would like to know if there is a way to do the following command in
> one step, primarily for speed on large data (5 million elements), and
> secondarily for readablity.
>
> mean(delta[(intersect(which(x[['class']]==0),which(delta<1)))])
>
>
> Do I really have to rely on an intersect operator? Isn't that
> O(nlg(n))? Can't I just filter in one step? As an R newbie, I would
> have guessed I could write
>
> mean(delta[which((x[['class']]==0) && (delta<1))])
>
> But I guess no such luck since (delta<1), etc are vectors. Are they
> really implemented as vectors? Ie, if I take 5M data points, does it
> allocate 20MB of RAM to make a test that passes most of the elements?
>
> The only thing I can think of is to use closures to write something like
> a Lisp list "filter". Not sure on the readabilty merits, especially if
> there is a direct way to do it. If Matlab had closures I know running
> them in a loop would be a bear on runtime anyway.
>
>
> Jeff Henrikson
>
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