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From |
"Nick Cox" <n.j.cox@durham.ac.uk> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
st: RE: can one program a function in Stata? |

Date |
Thu, 23 Apr 2009 12:36:10 +0100 |

The short answer is No. That is, you cannot, as a user, write a Stata function so that it behaves exactly like a StataCorp official function -- short of joining StataCorp, becoming a developer, etc. -- meaning, write -foobar()- so that you can put -foobar()- anywhere that an official function would be legal. Martin and David gave less dogmatic answers with good advice, but more can be said. Note that -egen- functions, whatever their merits, which I have exploited myself, can't be used outside -egen- or to emit constants as output (except inelegantly as variables full of the same constant). A Stata function is part of the executable. As Stata's executable code is not open source, you can not see that or contribute to it. Even if Stata were open source, to write a function, you would need to write some C code, to know where to put it, and to re-compile the executable to be able to do that. However, StataCorp could no doubt find a way to let you mimic that with Stata code, so that you could e.g. put in a do file the definition of a Stata function. It would require some low-level hits on Stata beyond that facility, and I have no idea _exactly_ how difficult that would be. I see this as a trade-off problem. Sometimes you want to write something slightly complicated that just returns a constant. You can write a program that leaves something in its wake or you can write a Mata function which you call from Stata. The first can be a bit awkward, undoubtedly, but the second is often more elegant. However, this kind of need is quite rare. I estimate that less than 1% of my Stata programs have the job of returning a constant (I am not counting programs that act as subroutines). I have no idea whether StataCorp have seriously entertained introducing the facility for users to define Stata functions, but my strong impression is that there are plenty of more urgent tasks. If Stata were being rewritten from the ground up, and suggestions were being solicited about design, I'd suggest that users could define Stata functions too, but it's not 1984 any more. I guess that Jacob's example was just a dopey one, but as Martin pointed out you could use -cond(,)- to define that function. And that need not be within a program: you could use it in-line to produce a constant. Nick n.j.cox@durham.ac.uk Martin Weiss ============ We can also get a grip on the problem 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 David Kantor ============ 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): viewsource _gmax.ado Martin Weiss ============ Just define your very own -egen- function, taking a cue from the existing ones (they all start with an _g, so they are easy to locate on the ado-path). As it happens, Kit`s book has a section (3.4) on this subject... Jacob Wegelin ============= 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? * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**st: can one program a function in Stata?***From:*Jacob Wegelin <jacob.wegelin@gmail.com>

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