[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 `&&'.


> 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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