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

[David Kantor <kantor.d@att.net>]

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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