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Re: st: RE: combinatorics


From   James Muller <james.muller@internode.on.net>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: RE: combinatorics
Date   Thu, 18 Aug 2005 09:23:17 +1000

For sure combinatorial functions can be written quite efficiently in Mata. I'll try a few different things and pick the best performance. Perhaps then other combinatorial-type Stata programs can be re-written to use the more efficient code that does nothing but provide the sets of combinations and permutations.

James


Nick Cox wrote:


In addition to other replies, there is a -selectvars- package on SSC:
----------------------- extract from help -selectvars- produces a list of all possible n-tuples from a list of
variable names. That is, given a list of k variable names, it produces the
empty list; all possible distinct singletons (each individual variable
name); all possible distinct pairs; and so forth. Tuples are bound in " ".
As -selectvars- is intended for programming, the list is not displayed, but
left behind as a saved result.

Note that there is no checking of a possible error with even modest k:
that a full list of (at most 2^k) tuples can not be accommodated in a
macro.
------------------------

Note that -selectvars- long predates Mata. I would be interested to know if it could be speeded up. Mata or not, I have always been queasy about programs like this because I feared that someone would engender a combinatorial explosion (a genuine term in the literature).
Nick n.j.cox@durham.ac.uk
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