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From |
"Jann Ben" <ben.jann@soz.gess.ethz.ch> |

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

Subject |
RE: st: [Mata] passing a function to mata (new question) |

Date |
Fri, 26 Aug 2005 23:22:48 +0200 |

Many thanks for the clarifications, Bill. Using pointers is exactly what I did in my program. I will be looking forward to the revised findexternal() function. I will shut up now and stop asking complicated Mata questions. Why is it that I always stumble over the 0.01% of problems. ben > -----Original Message----- > From: owner-statalist@hsphsun2.harvard.edu > [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of > William Gould, Stata > Sent: Friday, August 26, 2005 3:48 PM > To: statalist@hsphsun2.harvard.edu > Subject: Re: st: [Mata] passing a function to mata (new question) > > > Do you want to read this? > ------------------------- > > Mata is new and many of you have not tried it yet. > > I fear that if you read this question and answer, you will > never use Mata. > You will come to the conclusion that Mata is suitable only > for advanced > programming problems and that using Mata requires knowledge > that you do > not have, nor much care to gain. > > That would be unfortunate, because Mata really is easy to use > and is worth > learning. 99.99% of all problems that will ever be > approached using Mata will > not involve the topics discussed below. Do not hold it > against Mata that it > can deal with the 0.01% of problems, too. > > All that said, what follows really is not difficult, it is > merely unlikely > that the majority of Statalisters would ever face the problem. > > > Question and response > --------------------- > > The subject is passing funtions to functions in Mata, and Ben Jann > <ben.jann@soz.gess.ethz.ch> imagines a situation where the > function he wants > to pass is given by a string variable: > > function me1(string scalar he) > { > ... > if (he=="he1") y = me2(x, &he1()) > else if (he=="he2") y = me2(x, &he2()) > else if (he=="he3") y = me2(x, &he3()) > ... > return(...) > } > > Benn wonders if it is possible to code the above in such a > way that the value > of string scalar he is used directly. As Benn says, > > > If Mata had macro expansion, I would type > > > > function me1(string scalar he) > > { > > ... > > y = me2(x, &`he'()) > > ... > > return(...) > > } > > > Comment 1, "If Mata had macro expansion..." > ------------------------------------------- > > My recommendation is to stop thinking like that, although I know that > after years of ado programming, that is difficult. > > What makes Mata faster than ado? It is the fact that Mata > programs are > compiled, whereas ado programs are interpreted. When you > think in terms > of macro expansion, you are thinking interpretive. > > Mata in fact does have macro expansion but you have to > distinguish between > expansion at compile time and expansion at run time. The > little bit of code > Benn produced would expand `he' at compile-time, and that is > *NOT* what Benn > wants. > > Expansion at run-time requires writing lots of code. Don't go there. > > > Comment 2: Can the string scalar be used directly? > --------------------------------------------------- > > No, not right now. I have more to say on this, but let's go > with the "No" > answer right now. > > The most efficient thing to do is just what Benn said, > > function me1(string scalar he) > { > ... > if (he=="he1") y = me2(x, &he1()) > else if (he=="he2") y = me2(x, &he2()) > else if (he=="he3") y = me2(x, &he3()) > ... > return(...) > } > > Now imagine you had 15 places in your code where you wanted > to execute > function `he'() (excuse the macro notation). You could do this: > > > function me1(string scalar he) > { > ... > if (he=="he1") p = &he1() > else if (he=="he2") p = &he2() > else if (he=="he3") p = &he3() > ... > ... > y = me2(x, p) (1st call) > ... > y = me2(x, p) (2nd call) > ... > ... > y = me2(x, p) (15th call) > } > > In the above, p is a pointer. If you felt like delcaring it, you > could delcare it > > pointer(function) p > > The point of the above is that something like &he1() is just > an address, > and pointers hold addresses, and so you can use pointer variables to > hold addresses of functions just as you can use them to hold addresses > of matrices, vectors, and scalars. > > > Comment 3: Can the string scalar be used directly? > --------------------------------------------------- > > "No," I just said. > > Have you seen the function [M-5] findexternal()? Quoting from the > documentation, > > > findexternal(name) returns a pointer to the external global > matrix, vector, or scalr whose name is specified by name; > it returns NULL if the external global is not found. > > > Thus, findexternal("x") returns the address of global x, if it exists. > > findexternal("he1()") ought to return the address of function he1(). > It does not, but it ought to, and I feel so strongly about > that, I have > just recorded it as a bug that it does not. > > So let's imagine the "bug" is fixed. Then Benn could code, > > > function me1(string scalar he) > { > ... > p = findexternal(he+"()") > ... > y = me2(x, p) > ... > } > > Presumably, Benn ought to worry about p being NULL. > > -- Bill > wgould@stata.com > * > * 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/ > * * 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/

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