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Explicitly invoke the external solver.
- Handle to a solver state
- Value of returned solution
A solver that was setup manually with lp_solve/2 needs to be
triggered explicitly using this predicate.
The external solver's LP, QP or MIP solver is applied to the
problem represented by Handle. Precisely which method is used
depends on the options given to lp_setup/4. lp_solve/2 fails if
there is no solution or succeeds if an optimal solution is found,
returning the solution's cost in Cost (unlike with lp_demon_setup/5,
Cost gets instantiated to a number). After a success, various
solution and status information can be retrieved using lp_get/3
Normally, the external solver is invoked once per call in this
predicate, but if the problem contain cutpool constraints (see
lp_add_cutpool_constraints/4), then the external solver may be invoked
repeatedly to produce a solution which does not violate the cutpool
When a solver is triggered repeatedly, each invocation will
automatically take into account the current variable bounds. The set of
constraints considered by the solver is the one given when the solver
was created plus any new constraints that were added (lp_add/3) in the
The user can define their own handlers (per eplex instance/problem) to
handle cases where there was some error condition, or some limits were
exceeded during the solving of the problem. If no handler was defined,
by default an event would be raised. These are:
- suboptimal handler (eplex_suboptimal event)
This means that a solution was found but it may be suboptimal.
The default behaviour is to print a warning and succeed.
- unbounded_handler (eplex_unbounded event)
This means that the problem is unbounded. The default
behaviour is to bind Cost to infinity (positive or negative
depending on the optimisation direction), print a warning and
succeed. CAUTION: No solution values are computed when the
problem is unbounded, so unless the problem was set up with
the solution(no) option, an error will occur when trying to
continue as if the optimisation had succeeded.
- infeasible_handler (eplex_infeasible event)
This means that the problem is infeasible. The default
behaviour is to fail. Redefining this handler allows the
examination of the failed problem, e.g. obtaining an IIS for it.
- unknown_handler (eplex_unknown event)
This means that due to the solution method chosen, it is unknown
whether the problem is unbounded or infeasible. The default
behaviour is to print a warning and fail (even though this
may be logically wrong!).
- abort_handler (eplex_abort event)
Some other error condition occurred during optimisation.
The default behaviour is to print an error and abort.
External solver was unable to find a solution.
- Solution was found, but is possibly suboptimal
- Problem is unbounded, no solution values
- Result is unknown (infeasible or unbounded)
- External solver aborted for some reason
- (40) stale object handle
- Handle not valid: solver state had been destroyed by cleanup
lp_setup / 4, lp_add / 3, lp_get / 3, lp_var_get / 4, lp_add_cutpool_constraints / 4