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

["Martin Weiss" <martin.weiss1@gmx.de>]

<>

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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