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Re: st: Stata 6 vs. 7


From   Roger Newson <roger.newson@kcl.ac.uk>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Stata 6 vs. 7
Date   Sun, 08 Sep 2002 18:38:00 +0100

At 15:22 07/09/02 -0700, HJW wrote:
Hi,

It seems I encountered an inconsistency between Stata 6 and
7, and I would like to ask for help. Say I have a program
like the follows.

   program define myexpl
     version 6.0
     syntax ...., ..... [myopt(varlist) .... ]

   end

In Stata 7, if I issue the command,

  myexpl ....., myopt()   /* empty in -myopt- */

then Stata will return empty for `myopt' in the program,
which is what I want.  However, if the same program runs
under Stata 6, then Stata complains with the message:

   myopt() is invalid

I know Stata 6 probably expects variable names in -myopt-,
so when no name is given it complains. However, I need the
behavior of Stata 7; is there any way to let Stata 6 behaves
like its next generation does? Something like default=none
may do, but this does not seem to work on options. Can
anyone help?  Thanks in advance.

[Just for those curious: I know his situation is odd,
because if an empty string is desired, why specify the
option -myopt- in the first place? Well, this is a part of a
complicated maximum likelihood program, which estimates
several different models. In order to preserve syntax
consistency across models, -myopt- is required if a
particular model is called, no matter whether it eventually
contains a varlist or an empty string. Alright, this may be
my own eccentric programing style, but please spare me with
the eccentricity.]
In the calling program, you can define a local macro equal to -myopt(varlist)- if the varlist is non-empty, and equal to an empty string otherwise. For instance, if the varlist is in a macro -vlist-, then you might have some lines, in your calling program, as follows:

local myoptv ""
if "`vlist'"!="" {
local myoptv "myopt(`vlist')"
}
myexpl,`myoptv'

and -myexpl- will be called with the -myopt- option if, and only if, the macro -vlist- is non-empty.

I hope this helps.

Roger



--
Roger Newson
Lecturer in Medical Statistics
Department of Public Health Sciences
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: roger.newson@kcl.ac.uk

Opinions expressed are those of the author, not the institution.

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