st: re: question about Mata programming

[Kit Baum <Baum@bc.edu>]

To more carefully state the point of my last posting:

local macros _are_ dereferenced on demand in Mata's interactive mode.  
Thus

local george 123
mata:
a = J(2,2,`george')
a
end

works. But that is not very useful, as you can't handily update the  
value of the local macro, which must be defined in Stata (or via  
st_local()). Furthermore, you can't use local macros in Mata  
functions in any useful sense. If you do something like

mata: mata clear
mata:
void function eye()
{
a = I(`dim')
a
}
end

Mata will not compile the code because `dim' evals to nothing. If you  
preface this with

local dim 2

Mata will gladly compile the code, but will _hard wire_ the number 2  
into the function, and doing

local dim 3
mata: eye()

will not do what you might think. (This issue---of compile-time  
values---is discussed in the Mata manual). So in summary: don't try  
to use local macros in Mata. Use Mata to access locals and create and  
store new values in Stata's macros, but don't try to use them as  
counter variables as you do in ado-file code.

Kit


Kit Baum, Boston College Economics and DIW Berlin
http://ideas.repec.org/e/pba1.html
An Introduction to Modern Econometrics Using Stata:
http://www.stata-press.com/books/imeus.html

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