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RE: st: RE: forvalues problem

From   "Nick Cox" <[email protected]>
To   <[email protected]>
Subject   RE: st: RE: forvalues problem
Date   Thu, 1 Aug 2002 11:21:27 +0100

Fred Wolfe
> Nick writes further
> >A quite different comment is that it's not clear
> >why you need the { } around the macro name in that position.
> It is not clear to me either, but running the program 
> without braces produced:
> - forvalues j = 1 / 2 {
> - forvalues k = 1 / $catvar`j'_totline {
> invalid syntax
>    di "`k'. ${catvar`j'vname"`k'}"
> r(198);

That was a red herring of mine. Fred does need them
for what he is doing. 

Imagine you are Stata. You try to make sense 
of a command line from left to right, but following 
your punctuation rules, including parsing 
on spaces. Given 

forvalues k = 1 / $catvar`j'_totline {

the tokens 

forvalues k = 1 / 

all make sense. So far, so good; this is fun. Now 
the next token starts with "$", evidently the start 
of a global name. To you the global name is maximally 


because "`" signifies the start of a _local_ name: 
you must change your mind about what you are interpreting.
Now the global 


is not known to you, so you just blank it out here. 
That's the standard rule for undefined macros. 
The local 


is known to you. It has contents "1". The next characters

_totline { 

and the line as a whole now is 

forvalues k = 1 / 1_totline {

which is illegal. Squawk! 

To put it more concisely: what we need is to insist that 
Stata evaluates `j' before evaluating $catvar`j'_totline and that 
what's the braces do. 

But as said, Stata can get confused by those braces 
and they should be put on a different line. 

Having said all that, I find that I use globals very 
little in programming. That's partly a question of 
style, but there are some arguments for it. 

forvalues k = 1 / `catvar`i'_totline' { 

would cause none of these problems, I think. 

[email protected] 

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