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RE: st: RE: list in stata8


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: RE: list in stata8
Date   Tue, 25 Feb 2003 18:36:56 -0000

Nick Winter replied to Alan Feiveson
> >
> > I often just type "list in 101/123" or some such other
> > numbers with
> > limited range - even though I may have 50000 observations. I
> > find this less
> > bothersome than going to the editor and searching for
> > observations 101/123.
> > Will list8 also be slow if you give it a limited range even
> > though there are
> > 10000's of observations?
>
> I just tried this out, and the delay before the list begins
> seems to be
> related to the number of observations to be listed.  Thus,
>
> 	. list in 50000/50010
>
> takes 0.01 seconds in my test database, whereas
>
> 	. list
>
> takes over 4 seconds to begin.
>
> The following takes about 0.17 seconds:
>
> 	. list if inrange(_n,20000,20035)
>
> campared with:
>
> 	. generage rand=uniform()
> 	. list if rand>.9999
>
> which takes about 0.18 seconds to list in my sample dataset, and
> produces 35 observations.  So there appears to be no penalty (beyond
> that associated with -if-) for non-contiguous
> observations...the delay
> in listing is purely a function of the number of observations to be
> listed.  This makes sense, given the cause of the delay, as
> explained earlier on this thread.

This points up a general principle, explored
in some detail by Michael Blasnik a while
ago, that

... in 101/123

is typically (?always) going to be much faster
than the equivalent with -if-, so -- if you have a
choice -- go for -in-. Forget your human intelligence which
tells you that

... if inrange(_n, 101, 123)

is obviously false outside the the range
specified. Stata, I believe, has no sense
of "obvious" in this case.

Nick
n.j.cox@durham.ac.uk

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