On 27 June Daniel Caro posted a question
about -egen, eqany()-, to which I replied.
On 8 July Ronnie Babigumira posted a question
about the same function, to which Thomas Steichen
and I replied.
Both Daniel and Ronnie have the same underlying
problem: selecting a subset of observations when
the selection is specified by a very complicated
-if- condition. -egen, eqany()- in essence offers
a short-cut way of doing that which imposes less
typing than the brute force but tedious method
of typing out a compound -if- condition, so long
as the -if- condition is equality with any of
a set of integer values.
The little history of this little function is
like a fair fraction of Stata: originally, if I recall
correctly, it was written in response to a
question on Statalist; thereafter it was
published and then revised in STB-50 and STB-57;
currently, it has been adopted as part of official
Stata. I was the original author and retain some
kind of parental feeling. After discussions
with Stata Technical Support, I now attempt to
close this thread.
To cut a long and not especially interesting story
short, and omitting red herrings, false alarms
and unjustified aspersions:
1. -egen, eqany()- was crashing when presented
with a very long argument for -values()-. There
will be a limit to what Stata can cope with here,
but the limit imposed by -eqany()- is lower than
necessary because of redundant code which had an
unwanted side-effect. Rewritten code removes the
redundancy and the performance of -eqany()- improves.
I have drafted a fixed version which Stata Corp
will test and hope to include in the next ado
update.
2. The same problem would arise, and the same
kind of fix will apply, to the perhaps less often
used -egen, neqany()-.
In both cases, there are no backward implications
for applications which worked without crashing.
3. More positively, the thread highlighted
a method of selecting observations using -merge-
which for very complicated selection conditions
is both simple and attractive. Sooner or later
I will try to write this up, either as an FAQ submitted
to the Stata website, or in some other form.
The idea of using -merge- here is one I owe to
Kit Baum.
Nick
[email protected]
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