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Re: st: RE: Taking averages, etc.


From   Richard Williams <Richard.A.Williams.5@nd.edu>
To   rar <r.a.reese@Hull.ac.uk>, Stata distribution list <statalist@hsphsun2.harvard.edu>
Subject   Re: st: RE: Taking averages, etc.
Date   Wed, 17 Dec 2003 08:03:49 -0500

At 12:38 PM 12/17/2003 +0000, Allan Reese wrote:
First point is to analyse the logic.  Reforming a problem often speeds up
the execution far more than fancy coding, and leads to insights.  In this
case:
  gen y=5
  replace y=3 if x1==1 & x2==3
  replace y=4 if y==5 & (x3==2 & x4==17)

performs far fewer tests.  The parentheses do not affect the result but
emphasise the logic.
I'm not sure how that performs fewer tests. The if-then-else structure only performs as many tests as are necessary whereas the above would perform 2 tests for each case. In any event, I think I'd be more likely to make errors with syntax like this. But that may just be me!


Stata, as Nick has already replied, has a completely general control
language that includes if/then/else, for and while.  Unlike SPSS, where
only data transformations can be put into DO IF and LOOP constructions,
Stata allows any statements or blocks of statements.  These may
conveniently be written using an editor (provided it doesn't add a file
extension!) and saved as .DO or .ADO on the fly.
You must get Nick's messages faster in the UK! I look forward to what he has to say. Thanks Allan.


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