Re: RE: st: Converting a SAS datastep to Stata

[Daniel Feenberg <feenberg@nber.org>]

On Thu, 16 Dec 2010, William Gould, StataCorp LP wrote:

> I just sent a posting to Statalist concerning Daniel Feenberg's
> <feenberg@nber.org> SAS-to-Stata issue.
>
> Others have replied, too, and many are saying, "Use Mata!".
>
> I'm a little embarrassed because, concerning Mata, I merely ended my
> posting with,
>
> > Daniel could use Mata.  That would address both the readability and
> > efficiency issues.  If I were writing this code for the first time,
> > that is what I would do, probably.  With Mata, I can go through the
> > observations one at a time just as SAS does.

Do you mean a "for" loop over observations?

       : for ( 1; st_nobs();1) {
             if FLPDYR[i] == 2003 {
               (calculations...)
             }
          }

(I don't know any Mata, but I am studying Kit Baum's book).  Wouldn't that 
structure be subject to the complaint you voiced about explicitly looping 
over observations? (following "You would never dream..." in 
http://www.nber.org/plugins/) If that doesn't apply to Mata (perhaps 
because Mata is pseudo-compiled) it would be very attractive.

> I'm embarrassed because I am the author of the "Mata Matters" column
> in the SJ.  I'm supposedly qualified because I'm the author of Mata, too,
> but the editors of the SJ may be reconsidering.
>
> So please pretend I too said, "Use Mata!" and added a lot of useful
> comments about Mata.  Mata is, in fact, very appropriate for
> Daniel's problem.
>
> I didn't push the Mata solution for two reasons:  (1) I pretty much
> ignored efficiency issues and (2) I was unsure of how much Mata
> Daniel knew.  Assuming efficiency is not Daniel's primary concern,
> and assuming time-of-implementation is, I stand behind my Stata solution.


At the moment I am thinking the Mata way would be to separate out the 
conditionally executed block of code into a separate Mata function, and 
specify the affected returns as a view before calling that function with 
the appropriate marksample/touse. This means that much of the code is in 
traditional Stata, but the conditional blocks are in Mata.

Is that the plan? It looks efficient, but would require a great deal of 
structural change in the code for conditional blocks and there would be a 
substantial amount of code for calling each block. Therefore I am inclined 
to follow your initial suggestion of using local macros judiciously in 
regular stata code. If the SAS and Stata codes have similar structure, 
simultaneous maintanance will be simplified.

Thank you for the thoughtful suggestions - I do appreciate all of them and 
may well change my mind after I start translating.

Daniel Feenberg
NBER

> I don't want to be thought of as one-sided.
>
> Cheers,
>
> -- Bill
> wgould@stata.com
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