Re: st: .ado file questions

[David Kantor <dkantor@jhu.edu>]

At 11:16 AM 5/18/2005 -0700, Florence wrote:
> Hello everyone, I am writing a .ado file that is supposed to mimic what
> would be done by a "function" in other languages.  The problems I'm
> running into are:
>
> [...]
>
> (2) Is there a way to FORCE stata to return a function after the .ado file
> completes?  So far, I have just been creating scalars and matrices inside
> of the .ado file, but I think this is probably not that efficient for my
> purposes since I really want the function ITSELF to take on a value or a
> matrix.
>
> [...]
>
> THANKS SO MUCH!!!
> Florence
You've already gotten a response from Nick Cox.  I'd like to add to this 
regarding (2).

In Stata, you cannot define a function in the usual sense, as the term is 
used in general programming languages.  That is, you cannot create 
something that can be invoked in an expressions context -- and which yields 
a value which is substituted for the function name in that context.  Stata 
has plenty of these, but they are all built-in.  (E.g., ln(). )

But what you can do is create a program that computes something and leaves 
the results somewhere for you to pick up later.  This can be a variable, 
matrix, scalar, global macro, or a returned value.  (Also, it can be a 
local macro, but that is rarely done, is not recommended, and requires an 
undocumented feature. So I won't tell you how.)

I suggest you learn about returned values.  See -help return- and -help 
program-.  In the latter, pay attention to the rclass, eclass, and sclass 
options.  You probably want rclass to start with.

I would divide programs into two categories: those that are made for a 
specific situation, and those that are general.  The former may reference 
and create specifically-named objects.  The latter should not; I would 
include any ado program in this category.

If your program is to leave something in a variable, and this is to be a 
generalized purpose program, then your variable can have its name passed in 
via an option.  Typically this is done as in...
   myprog .... , gen(newvar)

(Within the program, you would have
 syntax ... , gen(name)
)
The program then deposits the computed values into `gen'.  The calling 
context, after the call, knows it as newvar.

And other to-be-created objects can be passed in that way: matrices, etc.

But a good way to pass a matrix back is to use -return matrix ...-.  Even 
if the matrix had a tempname in within the program, it still stays around, 
under the returned name.

I hope this helps.
-- David


David Kantor
Institute for Policy Studies
Johns Hopkins University
dkantor@jhu.edu
410-516-5404

*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/