Re: st: RE: combinatorics

[James Muller <james.muller@internode.on.net>]

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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