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?
2. Isn't there a potential tradeoff between this and another rule,
respect the dataset? If you don't preserve, there is a greater burden on
the programmer to make sure all is well at the end of the run. But, isn't
there also a greater risk if, say, the user stops the program before
completion?
I suppose good, careful programmers can guarantee both speed and security
of data. But, I would add the caveat that, if you are not 100% confident
of your programming, you might want to favor safety over speed.
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--
Roger Newson
Lecturer in Medical Statistics
Department of Public Health Sciences
Division of Asthma, Allergy and Lung Biology
King's College London
5th Floor, Capital House
42 Weston Street
London SE1 3QD
United Kingdom
Tel: 020 7848 6648 International +44 20 7848 6648
Fax: 020 7848 6620 International +44 20 7848 6620
or 020 7848 6605 International +44 20 7848 6605
Email: [email protected]
Website: http://phs.kcl.ac.uk/rogernewson/
Opinions expressed are those of the author, not the institution.
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