lp_set(+Handle, ++What, +Value)

Handle

Handle to a solver state

What

Option to set

Value

Value being set

Description

This primitive can be used to change some of the initial options even after setup. Handle refers to an existing solver state, What can be one of the following:

method

Set the method that will be used to solve the problem. Value is one of default, auto, primal, dual, net, net(Simplex), barrier, barrier(Crossover), sifting, sifting(SubMethod), concurrent, concurrent_det. Simplex can be one of default, auto, primal, dual, specifying the Simplex method to use in the Network Simplex algorithm. Crossover can be one of default, auto, primal, dual, none, specifying the crossover method to use for the barrier method. SubMethod could be one of default, auto, primal, dual, net, barrier, specifying the method to use for the subproblems in the sifting method. Note that not every method is available on every external solver. In case of MIP solving, this is the start algorithm (the one that is used to solve the initial relaxation). This method setting will override similar settings done via optimizer_param(_).

node_method

Applicable to MIP problems only. Set the method that will be used to solve the problem at the nodes (except the root) of the branch-and-bound tree. Note that the method for solving the root node is controlled by the method option. Value can be set to the same values as in the method option, although there may be more restrictions on what the actual methods/auxiliary methods that are allowed in combination with the root method, due to limitations/restrictions from the external solver. A warning will be given when the problem is solved if this is the case, and the default method used instead. This node_method setting will override similar settings done via optimizer_param(_).

solution

Make the solutions available each time the problem has been (re-)solved successfully. Value is one of the atoms yes or no.

reduced_cost

Make the reduced costs available each time the problem has been (re-)solved successfully. If the problem is a MIP, then depending on the external solver, this is either unavailable or are the values for the optimal LP node. Value is one of the atoms yes or no.

slack

Make the constraint slacks available each time the problem has been (re-)solved successfully. Value is one of the atoms yes or no.

dual_solution

Make the dual solutions available each time the problem has been (re-)solved successfully. If the problem is a MIP, then depending on the external solver, this is either unavailable or are the values for the optimal LP node. Value is one of the atoms yes or no.

keep_basis

Store the basis each time the problem has been solved successfully, and use this basis as a starting point for re-solving next time. Value is one of the atoms yes or no.

mipstart

Use the previous solution values as a warm-start heuristics for the MIP search. This only has an effect for certain solvers (e.g. Gurobi), if there are integrality constraints, and if there is a previous solution available. Possible values are none (no mipstart values, the default), all (use previous solution for all variables), or integers (use previous solution for all variables that are now constrained to be integral).

cache_iis

When a problem is found to be infeasible, compute an IIS for the problem (if supported by the external solver), and store it so that it can bee retrieved by eplex_get_iis/4 or lp_get_iis/5. This will be done before the problem can be modified and make the computing of the IIS impossible. The IIS will oulu be available before the problem is solved again, and before the infeasible solve is backtracked. This option has no effect if the external solver does not support the finding of IIS. Note that if this option is set, eplex will always ask for an IIS to computed for an infeasible problem, even if it is immediately backtracked by the infeasible handler failing, and that the option is only needed if the problem instance in the external solver is modified before eplex_get_iis/4 or lp_get_iis/5 is called. Value is one of the atoms yes or no.

demon_tolerance

Specify how far outside a variable's range an lp-solution can fall before lp_demon_setup/5 re-triggers. Value is a comma-separated pair (RealTol,IntTol) of floating-point values (default (0.00001,0.5)).

use_var_names

Specify if variable names (set using set_var_name/2 of the var_name library) should be passed to the external solver. If a particular variable does not have a name, a solver's default name would be used. Note that for XPRESS-MP, there is a limit on the length of the name, which can be changed between 8 and 64 in steps of 8 with the parameter N_NAMLEN. Variable names longer than this limit are truncated to the limit. Value is one of the atoms yes or no, the default is no.

timeout

Set the external solver to time-out after Value seconds. The solver will abort (in either the abort or suboptimal state, depending on if a suboptimal solution was found) if the optimal solution was not found within the time limit. In cases where the solver expects an integer for the time-out interval, the time given is rounded up to the next integer value. This should be used instead of the solver specific optimizer_param(Param) for setting timeouts, as eplex sets these parameter(s) itself. Note that the exact behaviour of the timeout setting is solver dependent. Value is a positive number.

cutpool_option(Idx,Option)

Set the option Option as specified by Value for the cutpool constraint with index Idx. Option is one of the following:

active

Set the active status for the constraint Idx to Value. Value can be 0 or 1. 0 is non-active, 1 is active.

add_initially

Specify if the constraint Idx should be added to a problem before the solver is invoked. Value can be 0 or 1. 1 is to add initially, 0 not.

Note that the option is not undone on backtracking.

cutpool_group

Value is an atom, and is used to specify the name of a group of cutpool constraints. If Name is not a current group name, a new group with that name is created. Once created, a named group exists until the problem is destroyed. The default group (with the atom nil ([]) as its name) is predefined and does not need to be created with cutpool_group.

suboptimal_handler

Value is a user defined goal to handle the case when the external solver returned a suboptimal solution (because the problem was aborted). Value would replace any existing suboptimal handler, and would also be run in place of raising the default eplex_suboptimal event.

unbounded_handler

Value is a user defined goal to handle the case when the problem is unbounded. Value would replace any existing unbounded handler, and would be run in place of raising the default eplex_unbounded event.

infeasible_handler

Value is a user defined goal to handle the case when the external solver found the problem to be infeasible. Value would replace any existing infeasible handler, and would be run in place of raising the default eplex_infeasible event. Note that the default and logically correct behaviour is to fail, this handler is provided to allow the user to analyse the cause of the infeasibility. It is recommended that the handler should also fail after performing the analysis.

unknown_handler

Value is a user defined goal to handle the case when the external solver was not able to determine if the problem is unbounded Value would replace any existing unknown handler, and would be run in place of raising the default eplex_unknown event.

abort_handler

Value is a user defined goal to handle the case when the external solver aborted without finding any solution. Value would replace any existing abort handler, and would be run in place of raising the default eplex_abort event.

optimizer_param(Param)

Set the external solver's control parameter Param for the problem Handle. If the solver's parameters are global and not problem specific, an unimplemented functionality exception would be raised. See lp_get/3 for more details on the external solver's parameters. Note that the new setting is not undone on backtracking.

write_before_solve

Value can be the pair (Format,File) or the atom no. If (Format,File) is given, Eplex will ask the external solver to dump the problem each time the solver is solved. This allows the problem in an eplex_probe/2 or lp_probe/3 to be dumped. As in lp_write/3, Format is the format of the dumped problem, and File is the filename. See for more details. Note that the problem is dumped each time the external solver is invoked if the problem has cutpool constraints, where there may be multiple invocations of the solver per solver call. `no' for Value will turn off this dumping.

post_equality_when_unified

Value can be the atoms yes or no. Determines if an equality constraint between two solver variables will be posted to the solver when these variables are unified. Setting Value to no means that the constraint will not be posted. Note that this can lead to the solver's problem becoming inconsistent with ECLiPSe's.

All settings apart from optimizer_param(Param) and cutpool settings will be undone on backtracking!

Resatisfiable

See Also