Integer Linear Programming (ILP)

Integer Linear Programming (ILP) can be converted into a QUBO expression using QUBO++. As an example, consider the following ILP:

QUBO++ program

The following QUBO++ program formulates this ILP as a QUBO expression and solves it using the Easy Solver:

#include "qbpp.hpp"
#include "qbpp_easy_solver.hpp"

int main() {
  auto x = 0 <= qbpp::var_int("x", 3) <= 5;
  auto objective = 2 * x[0] + 5 * x[1] + 5 * x[2];
  auto c1 = 0 <= x[0] + 3 * x[1] + x[2] <= 12;
  auto c2 = 0 <= x[0] + 2 * x[2] <= 5;
  auto c3 = 0 <= x[1] + x[2] <= 4;

  auto f = -objective + 100 * (c1 + c2 + c3);
  f.simplify_as_binary();
  auto solver = qbpp::easy_solver::EasySolver(f);
  solver.time_limit(1.0);
  auto sol = solver.search();
  std::cout << "x0 = " << sol(x[0]) << ", x1 = " << sol(x[1])
            << ", x2 = " << sol(x[2]) << std::endl;
  std::cout << "objective = " << sol(objective) << std::endl;
  std::cout << "*c1 = " << sol(*c1) << ", *c2 = " << sol(*c2)
            << ", *c3 = " << sol(*c3) << std::endl;
}

In this program, x is a vector of three qbpp::VarInt objects, each taking an integer value in the range $[0, 5]$. The objective function and the three constraints are represented by objective, c1, c2, and c3, respectively. They are combined into a single QUBO expression f, where the penalty constant 100 is used to enforce the constraints.

The Easy Solver searches for a low-energy solution of f and returns it as sol. The obtained solution and the values of objective, *c1, *c2, and *c3 are printed as follows:

x0 = 2, x1 = 3, x2 = 1
objective = 24
*c1 = 12, *c2 = 4, *c3 = 4

We observe that a obtained solution with the objective 24 satisfies all constraints.