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RE: st: RE: foreach problem


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: RE: foreach problem
Date   Wed, 17 Sep 2008 13:54:25 +0100

To expand a little on Neil's point: 

Let's flag first that your problem is that Stata didn't understand your
-egen- command. Its placing within a -foreach- loop is incidental. 

When looking at your -egen- command Stata expects an expression as the
argument of          the -egen- function -mean()-.  

As it happens, your expression was intended to be a single variable
name, but -egen- doesn't know that. (-egen- would never know about any
macro references either. When -egen- sees a command line, macro names
have already been replaced by contents.) 

As your expression is a single token, but not a number, the only
interpretation possible is that it must be the name of something (a
variable, a scalar, etc.) -- but that can't be, because the presence of
' ' makes the token an invalid name. 

That's good logic, and Stata found your error. It just did not explain
it in the way that you expected, but its point of view was totally
defensible. 

It so happens that ' ' within your expression are not illegal, by the
way. Your expression could be, in principle, something like 

stringvar == "'a'" 

That is unlikely, but it's legal. 

In wanting Stata to do something more, you raise two very big general
questions. 

First off, Stata can check your syntax, but it is weak on semantics and
useless at inferring users' intentions separately from their commands. 

Second, I guess that almost always when users like you and me type ' '
it is a typo for ` ', but, with some exceptions, Stata's not in the
business of issuing warnings, or trying to second guess what you
probably mean. 

The developers could put a lot of effort in trying to trap things that
are probably mistakes, but it would be a poor use of effort. On the
whole, Stata is built rather on the premise that we are smart enough to
see mistakes more quickly the next time we see them. 

Again, there are exceptions, but I think that's the Stata point of view
on this. 

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

P.S. In my favourite text editor (Vim) when Stata code is being edited
strings `stuff' and 'stuff' are highlighted differently. 

Neil Shephard

Rajesh Tharyan wrote:
> Not an excuse for looking carefully enough, but does stata have
something
> built in to check for this?
   
Yes it's the error message you received stating...

> . abnor'var' invalid name
>   r(198);
>  

 
It's not very informative as the online help indicates, but it does
point 
out that you need to look at _exactly_ how you've typed your variable 
names.  Although in this particular instance it was your 
mis-specification of single-quotes around the local macro that was the 
root cause of the error.

See -search r(198)- for more info.


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