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Re: st: can one program a function in Stata?


From   "Martin Weiss" <martin.weiss1@gmx.de>
To   <statalist@hsphsun2.harvard.edu>
Subject   Re: st: can one program a function in Stata?
Date   Wed, 22 Apr 2009 18:57:44 +0200

<>

We can also get a grip on the prob using -cond()- so no heavy machinery required...


****
clear*
set obs 10

gen x=_n
gen myanswer=cond(x>3,(sqrt(x) + 5)^(2/3), (sqrt(x) + 9)^(2/3))

list, noobs
****


HTH
Martin
_______________________
----- Original Message ----- From: "David Kantor" <kantor.d@att.net>
To: <statalist@hsphsun2.harvard.edu>
Sent: Wednesday, April 22, 2009 6:45 PM
Subject: Re: st: can one program a function in Stata?


At 12:21 PM 4/22/2009, Jacob wrote:
I am about to order Kit Baum's Introduction to Stata Programming. But
I'm curious whether his book will answer the following question.

Stata has what I would call plain old functions, of which sqrt() is an example:

. di sqrt(3.1415926535)
1.7724539

. sysuse auto, clear
(1978 Automobile Data)

. gen sqrtTurn=sqrt(turn)

But suppose I want to define a function that's not found under -help
functions-. The following approach (with a hypothetical nonstandard
function) seems clumsy:

capture program drop myownfunction
program define myownfunction, rclass
   if `1' > 3 {
      return scalar myanswer= (sqrt(`1') + 5)^(2/3)
      }
   else {
      return scalar myanswer= (sqrt(`1') + 9)^(2/3)
      }
end

myownfunction 3.1415926535
di r(myanswer)

Besides the awkward business of having to first call the function,
then refer to - r(myanswer) - , this program only works for a single
number (a scalar in the mathematical not Stata sense). If I want to
apply it to an entire column of numbers (a Stata numeric variable),
I'd have to write a different function, then always remember whether
to call the scalar or the vector (variable) version. On the other
hand, we can use sqrt() directly for either purpose.

Thus, is there a way to define -myownfunction()- so that one can call
it just as one calls sqrt()?  Does one do this with -mata-, or ...?
Would Kit Baum's book talk about this?

This gets asked fairly often. To my knowledge, there is no way to do what you asked for. (I don't know enough about mata to comment about that.)

Stata doesn't let you do that, but you can have programs that deposit a value somewhere -- a variable, a scalar, an r() value (as you showed in your example), a global macro, a local macro (though there are issues about that, and it is discouraged). Despite this limitation, Stata users manage fairly well.

A notable example is -egen-, which is really a front end for a large suite of programs that deposit values into variables. -egen- is extensible; you can add your own programs to it. We often refer to these programs as "functions", though they really aren't functions. They just appear in a context and syntax that makes them look like functions. To see how to write an egen "function" see...
viewsource egen.ado
plus, look at one of the Stata-given programs, such as max (_gmax):
viesource _gmax.ado

HTH
--David

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