Re: st: -stylerules- updated on SSC

["Michael Blasnik" <michael.blasnik@verizon.net>]

In my experience, the trade-off in execution time between preserve / restore 
and a series of if `touse' statements can reach the tipping point fairly 
quickly.  I find that about 10 (+-5)  -if `touse'- statements requires about 
as much time as one set of preserve/ drop if `touse'/restore in fairly large 
datasets (in small datasets, it matters less).  So a programmer may want to 
look at the preserve, drop if !`touse', restore approach if there are more 
than about 10 -if `touse' lines that need to be executed.  These  timings 
are for a machine where temp files are written to the local hard drive.  In 
practice, I tend to avoid the preserve/restore approach unless the speed 
advatage is clear (perhaps 20+ if `touse ' lines).

Michael Blasnik
michael.blasnik@verizon.net


----- Original Message ----- 
From: "Roger Newson" <roger.newson@kcl.ac.uk>
To: <statalist@hsphsun2.harvard.edu>
Sent: Thursday, April 14, 2005 7:19 AM
Subject: Re: st: -stylerules- updated on SSC



> Re Point 1, file input and output is usually MUCH more time-consuming than 
> in-memory operations such as checking `touse'. However, re Point 2, 
> Richard is right, and a block of code which modifies the data 
> *permanently* should indeed start with a single -preserve- and end with a 
> single -restore, not-, just in case the user presses -break-.
>
> Roger
>
>
> At 01:40 14/04/2005, Richard Williams wrote:
>
> > At 04:07 PM 4/13/2005 +0100, Nick Cox wrote:
> > > The package -stylerules- on SSC has been
> > > updated. I started by making a few changes
> > > in the light of Stata 9, but mostly thought
> > > of some other things to mention while
> > > I was doing that.
> > Some very good rules there Nick.  I'd be curious what thoughts people have 
> > about this one:
> >
> > "Avoid preserve if possible. preserve is attractive to the programmer but 
> > can be expensive in time for the user with large data files.  Programmers 
> > should learn to master marksample."
> >
> > Suppose I ignore that advise and instead do something like
> >
> > preserve
> > keep if `touse'
> >
> > 1.  Will I gain some speed, because Stata will be working with a smaller 
> > data set and won't have to repeatedly process -if `touse'- commands?
<snip> 
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