Integer Linear Programming (ILP)

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

\[\begin{aligned} \text{Maximize:} && 2x_0 +5x_1+5x_2\\ \text{Subject to:} && x_0 + 3 x_1 + x_2 &\leq 12 \\ && x_0 + 2x_2 &\leq 5\\ && x_1 + x_2 &\leq 4; \end{aligned}\]

QUBO++ program

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

#include <qbpp/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::EasySolver(f);
  auto sol = solver.search({{"time_limit", 1.0}});
  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.body(sol) = " << c1.body(sol)
            << ", c2.body(sol) = " << c2.body(sol)
            << ", c3.body(sol) = " << c3.body(sol) << std::endl;
}

In this program, x is a vector of three integer variables, 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 and each constraint body (c1.body(sol), c2.body(sol), c3.body(sol)) are printed as follows:

x0 = 2, x1 = 3, x2 = 1
objective = 24
c1.body(sol) = 12, c2.body(sol) = 4, c3.body(sol) = 4

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