[OS] Common expressions -- user defined variables

[Robert Fourer]

Kipp & Leo,

These are often called "defined variables" in descriptions of AMPL.  An nl file
gives statistics for the number of defined variables appearing

     b   in both the objective and constraints
     c   in two or more constraints but not any objectives
     o   in two or more objectives but not any constraints
     c1  in only one constraint and no objectives
     o1  in only one objective and no constraints

Presumably these distinctions make a difference to some aspects of the ASL,
such as such as the function evaluation routines.

The b, c, o variables can be regarded as defining "common subexpressions" in
the objectives and/or constraints.  I think that's the origin of the term
"common exprs" in the comments of the nl file.

-- Bob


> -----Original Message-----
> From: optimizationservices at googlegroups.com
> [mailto:optimizationservices at googlegroups.com] On Behalf Of Kipp Martin
> Sent: Monday, March 10, 2008 12:31 PM
> To: optimizationservices at googlegroups.com
> Subject: Re: [OS] Common expressions -- user defined variables
> 
> 
> Hi Leo:
> 
> > What do you mean by common expressions? Definitions using a := at
> > declaration time?
> 
> A common expression is a variable defined in terms of other variables.
> Here is an example from
> 
> http://www.gerad.ca/~orban/drampl/def-vars.html
> 
> var x {1..n};
> var v = sum {i in 1..n} x[i]^2;
> var u = v-x[n];
> 
> In the past I have always called v and u, "definitional variables" and
> have no idea where the term "common expression" comes from.  If you look
> at an AMPL nl file you will see, for example, a line:
> 
>   0 0 0 3 0	# common exprs: b,c,o,c1,o1
> 
>  From playing around, I think c1 is the number of common expression in
> the constraints and o1 is the number of common expressions in the
> objective function.  I have no idea as to what (b,c,and o) are. There is
> no documentation on this in "Hooking Your Solver to AMPL"  Working with
> the common expressions is tough because of the lack of documentation and
> trying to figure out what the variables are.
> 
> Cheers,
> 
> 
> 
> 
> >
> > Thanks,
> > Leo.
> >
> > On Sun, Mar 9, 2008 at 11:40 PM, Kipp Martin <Kipp.Martin at chicagogsb.edu>
> wrote:
> >>  Hello:
> >>
> >>  The current trunk version (1701) of OS now supports AMPL common
> >>  expressions (user-defined variables).  If your nl file has user defined
> >>  variables
> >>
> >>  OSSolverService -solver solverName -nl path/to/file/fileName.nl
> >>
> >>  should now work instead of throwing an exception.
> >>
> >>  Please report any problems with this.
> >>
> >>  Thanks
> >>
> >>
> >>  --
> >>  Kipp Martin
> >>  Professor of Operations Research
> >>  and Computing Technology
> >>  Graduate School of Business
> >>  University of Chicago
> >>  5807 South Woodlawn Avenue
> >>  Chicago, IL 60637
> >>  773-702-7456
> >>  kipp.martin at chicagogsb.edu
> >>  http://gsbkip.chicagogsb.edu
> >>  http://www.coin-or.org
> >>  _______________________________________________
> >>  OS mailing list
> >>  OS at list.coin-or.org
> >>  http://list.coin-or.org/mailman/listinfo/os
> >>
> >>  >
> >>
> >
> >
> >
> 
> 
> --
> Kipp Martin
> Professor of Operations Research
> and Computing Technology
> Graduate School of Business
> University of Chicago
> 5807 South Woodlawn Avenue
> Chicago, IL 60637
> 773-702-7456
> kipp.martin at chicagogsb.edu
> http://gsbkip.chicagogsb.edu
> http://www.coin-or.org
> 
> 

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