Summary of knapsack solve in CPLEX

Question

So I'm have trouble in python version of CPLEX. I would think it would be pretty easy to get like a summary of what the solver did i.e number of branches, etc.

Anyone know how to do this?

import cplex
from cplex.exceptions import CplexError
class knapsack:
    def __init__(self,N,g,square_list):
        self.N = N
        self.square_list= square_list
        self.g = g
    def solve_problem(self):
        try:
            my_prob = cplex.Cplex()
            prob =my_prob
            prob.set_log_stream(None)
            prob.set_error_stream(None)
            prob.set_warning_stream(None)
            prob.set_results_stream(None)
            my_obj = self.g
            my_ctype = "B"
            number_of_one = self.square_list.count(1.0)
            my_ctype = my_ctype*len(self.square_list)
            val = self.N  -number_of_one
            rhs=[val]
            my_sense="L"
            my_rownames = ["r1"]

            counter =0
            variable_list=[]
            coiff_list=[]
            for i in self.square_list:
                if i==0:
                    coiff_list.append(1.0)
                else:
                    coiff_list.append(-1.0)
                variable_list.append("w" + str(counter))
                counter+=1

            rows = [[variable_list, coiff_list]]
            prob.objective.set_sense(prob.objective.sense.minimize)

            prob.variables.add(obj=my_obj, types=my_ctype,
                        names=variable_list)
            prob.linear_constraints.add(lin_expr=rows, senses=my_sense,
                                    rhs=rhs)
            my_prob.solve()
            x = my_prob.solution.get_values()
            print(my_prob.solution.get_status())
            print("---")
            print(my_prob.solution.status())
            return x
        except CplexError as exc:
            print(exc)
            return

When I look at the methods associated with my_prob and myprob.solution I see

['MIP_starts', 'SOS', '_Cplex__copy_init', '__class__', '__del__', '__delattr__', '__dict__', '__dir__', '__doc__', '__enter__', '__eq__', '__exit__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', '_aborter', '_disposed', '_env', '_env_lp_ptr', '_invoke_generic_callback', '_is_MIP', '_is_special_filetype', '_lp', 'advanced', 'cleanup', 'conflict', 'copy_vmconfig', 'del_vmconfig', 'double_annotations', 'end', 'feasopt', 'get_aborter', 'get_dettime', 'get_num_cores', 'get_problem_name', 'get_problem_type', 'get_stats', 'get_time', 'get_version', 'get_versionnumber', 'has_vmconfig', 'indicator_constraints', 'linear_constraints', 'long_annotations', 'objective', 'order', 'parameters', 'populate_solution_pool', 'presolve', 'problem_type', 'pwl_constraints', 'quadratic_constraints', 'read', 'read_annotations', 'read_copy_vmconfig', 'register_callback', 'remove_aborter', 'runseeds', 'set_callback', 'set_error_stream', 'set_log_stream', 'set_problem_name', 'set_problem_type', 'set_results_stream', 'set_warning_stream', 'solution', 'solve', 'start', 'unregister_callback', 'use_aborter', 'variables', 'write', 'write_annotations', 'write_benders_annotation']
['MIP', '__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', '_add_iter', '_add_single', '_conv', '_cplex', '_env', '_get_index', '_get_index_function', 'advanced', 'basis', 'get_activity_levels', 'get_dual_values', 'get_float_quality', 'get_indicator_slacks', 'get_indices', 'get_integer_quality', 'get_linear_slacks', 'get_method', 'get_objective_value', 'get_quadratic_activity_levels', 'get_quadratic_dualslack', 'get_quadratic_slacks', 'get_quality_metrics', 'get_reduced_costs', 'get_solution_type', 'get_status', 'get_status_string', 'get_values', 'infeasibility', 'is_dual_feasible', 'is_primal_feasible', 'method', 'pool', 'progress', 'quality_metric', 'sensitivity', 'status', 'type', 'write']

Answer 1

Found that there is a write function on the solution object

my_prob.solution.write("myanswer")

This contains all relevant information on the CPLEX run.

Answer 2

If you are familiar with the CPLEX interactive, you may be used to seeing something like the following summary after the optimization:

MIP - Integer optimal, tolerance (0.0001/1e-06):  Objective = -2.0183208990e+02
Current MIP best bound = -2.0181209207e+02 (gap = 0.0199978, 0.01%)
Solution time =    1.43 sec.  Iterations = 25361  Nodes = 4335 (21)
Deterministic time = 686.22 ticks  (479.17 ticks/sec)

As suggested in the comments section, all of this information can be queried if you look through the documentation here. Most of it comes from the Cplex.solution interface as you suggested.

For example, consider the following interactive session:

>>> c.problem_type[c.get_problem_type()]
'MILP'
>>> c.solution.get_status_string()
'integer optimal, tolerance'
>>> c.parameters.mip.tolerances.mipgap.get()
0.0001
>>> c.parameters.mip.tolerances.absmipgap.get()
1e-06
>>> c.solution.get_objective_value()
-201.83208990000034
>>> c.solution.MIP.get_best_objective()
-201.8120920681663
>>> c.solution.MIP.get_mip_relative_gap()
9.908152783804216e-05
>>> print(c.solution.get_quality_metrics())
Incumbent solution:
MILP objective                                -2.0183208990e+02
MILP solution norm |x| (Total, Max)            4.65432e+02  2.02051e+02
MILP solution error (Ax=b) (Total, Max)        5.24512e-11  2.34035e-12
MILP x bound error (Total, Max)                0.00000e+00  0.00000e+00
MILP x integrality error (Total, Max)          0.00000e+00  0.00000e+00
MILP slack bound error (Total, Max)            4.54747e-13  4.54747e-13
>>> c.solution.MIP.get_incumbent_node()
4266
>>> c.solution.MIP.get_num_cuts(c.solution.MIP.cut_type.GUB_cover)
3