Loqo is a version of Bob Vanderbei's LP solver LOQO that can
be used either "stand-alone" or with AMPL's solve command or with
AMPL's -ob and -g options.
Stand-alone invocations of loqo have the form
loqo stub [-AMPL] [keywd=value ...]
where stub was specified in
ampl -obstub ...
or
ampl -ogstub ...
Such an invocation causes loqo to read from stub.nl and, if -AMPL or
"wantsol" or "wantsol=" appears on the command line, to write stub.sol.
You can use AMPL's solve command to invoke loqo and have the solution
loqo computes made available to your AMPL session. To do this, in the
AMPL session you may need to set the AMPL's solver option to loqo by
giving the command
option solver loqo;
To see the current solver option, use the command
option solver;
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Controlling loqo
----------------
Loqo reads key words and values from the environment (shell) variable
loqo_options and from the command line (in which case each key word
must be followed immediately by an = sign). Execute
loqo -=
for a summary of the keywords loqo recognizes.
For MPS input, invoke
loqo -m [stub] [keywd=value ...]
If stub is present, loqo expects to read files stub.spc and stub.mps .
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Sample Invocations
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If you're using AMPL, just say
option solver loqo;
solve;
If you've executed, say,
ampl -objunk junk.model junk.data
then you could say
loqo junk iterlim=30 dual=
to force loqo to solve the dual problem and to have it run for
at most 30 iterations. Either of the invocations
loqo_options='iterlim 30 dual' loqo junk
or
loqo_options='iterlim 30 dual'
export loqo_options
loqo junk
would have the same effect; within AMPL, specifying
option loqo_options 'iterlim 30 dual', solver loqo;
solve;
would also have this effect.
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solve_result_num values
=======================
Here is a table of solve_result_num values that "loqo" can return
to an AMPL session, along with the text that appears in the associated
solve_message.
Value Message
0 optimal solution
100 suboptimal solution
201 primal infeasible
202 dual infeasible
203 primal and/or dual infeasible
210 primal infeasible -- inconsistent equations
301 primal unbounded
302 dual unbounded
400 iteration limit
500 resource limit
510 ??? LOQO bug
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Questions about this stuff? Contact dmg@ampl.com (David M. Gay).