I am trying to use or-tools in Python to solve a mixed-integer linear program that has multiple optimal solutions. However, NextSolution() always returns False, so I cannot retrieve more than one solution. I understand that this function works using a constraint solver, but I would like to use the MILP solver.
The related or-tools documentation states:
As of 2020-02-10, only Gurobi and SCIP support NextSolution(), see linear_solver_interfaces_test for an example of how to configure these solvers for multiple solutions. Other solvers return false unconditionally.
However, I cannot find any such linear_solver_interfaces_test in the source repo, documentation, or via a web search. I am using ortools version 7.8.7959 and the included SCIP 7.0.1 with Python 3.6.9.
Below is my example code which illustrates a simple example of the type I would like to solve. It should produce three unique solutions, but currently produces zero solutions.
from ortools.linear_solver import pywraplp
def main():
solver = pywraplp.Solver("multiple_solution_test", pywraplp.Solver.SCIP_MIXED_INTEGER_PROGRAMMING)
x = solver.IntVar(0, 2, "x")
y = solver.IntVar(0, 2, "y")
z = solver.IntVar(0, 2, "z")
solver.Add(x + y <= 2)
solver.Maximize(x + y + z) # should be 4, which can be obtained by (2,0,2), (1,1,2), or (0,2,2)
solver.Solve()
print_solutions(solver, x, y, z)
def print_solutions(solver, x, y, z):
count = 0
while solver.NextSolution(): # <-- NextSolution() always returns False!
count += 1
print("x =", x.solution_value(), "y =", y.solution_value(), "z =", z.solution_value())
print("\nNumber of solutions found:", count)
if __name__ == "__main__":
main()
