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st: RE: Programming advice on generating loops


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: Programming advice on generating loops
Date   Wed, 2 Apr 2008 16:44:03 +0100

Tiago Pereira seconded this question. 

Neither Le Wang nor Tiago gave any underlying context or details of what
else they want to do. 

Your question, on the face of it, would be answered by writing a program
to write a program, given the variables concerned. Writing something to
emit scaffolding (i.e. the 
-forval- loops and the closing braces) would be much easier than writing
something that generates code to fit around the scaffolding (any
initialisations and wrap-up code, and what goes inside each loop). It
would probably be _much_ easier just to write the program directly. 

I doubt very much, however, that you really need what you are asking
for. The key here is that you don't know in advance how many variables
you have to do with. That suggests to me that there is no special
structure to your problem(s), but just a desire to crunch through all
the distinct cross-combinations of a set of variables. If that is so,
your problem(s) may well yield to -by:- and/or looping over composite
groups. There is technique at 

FAQ     . . . . . . . . . . Making foreach go through all values of a
variable
        8/05    Is there a way to tell Stata to try all values of a
                particular variable in a foreach statement without
                specifying them?
                http://www.stata.com/support/faqs/data/foreach.html

For example, 

egen group = group(varlist) 
su group, meanonly 
forval i = 1/`r(max)' { 
	...
}

applies regardless of the number of variables in varlist. 

That said, there have been attempts to set up more general tools in this
territory. 
Those by myself predate or do not supersede the direct use of -forval-
or -foreach- (e.g. the -cp- tool in STB-45). 

I would rather know more than the real problem(s) than try to make
further guesses. 

Nick
n.j.cox@durham.ac.uk 

Le Wang

I need to generate loops according to the number of variables (unknown
beforehand) in the ado program.

For example, if one uses two variables, then I want to do the following
loop

forvalues i=1/10{
      forvalues j=1/32{

      .......

      }
}


if one specifies three variables, then the loop looks like this

forvalues i=1/10{
      forvalues j=1/32{
            forvalues k=1/47{

            .......
            }
      }
}

and so on....

How can I do this? Thanks in advance.

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