The previous sections have explained how to design good models for various problems. However, if errors slip in when you implement the constraints of the problem, it can be very difficult to understand why the application finds wrong "solutions," even though the model is very good.

There is a simple way to fix such cases: use an instance of the problem with a known solution to test against the constraints. For testing a solution, you just have to assign the values corresponding to a solution before posting the constraints. Then no failure should happen within this function. If a failure occurs with the known solution, you know there is a problem among the constraints.

For example, let's test a known solution of the map-coloring problem.

#include <ilsolver/ilosolver.h>
 
ILOSTLBEGIN
 
const char* Names[] = {"blue", "white", "red", "green"};
 
int main(){
  IloEnv env;
  try {
    IloModel model(env);
    IloIntVar Belgium(env, 0, 3), Denmark(env, 0, 3),
      France(env, 0, 3), Germany(env, 0, 3),
      Netherlands(env, 0, 3), Luxembourg(env, 0, 3);
 
    // Test a solution
    model.add(Belgium == 0);
    model.add(Denmark == 1);
    model.add(France == 1);
    model.add(Germany ==  0);
    model.add(Netherlands == 1);
    model.add(Luxembourg == 2);
 
    // Constraints
    model.add(Belgium != France);
    model.add(Denmark != Germany );
    model.add(France != Germany);
    model.add(Belgium != Netherlands);
    model.add(Germany != Netherlands);
    model.add(France != Luxembourg);
    model.add(Luxembourg != Germany);
    model.add(Luxembourg != Belgium);
 
    IloSolver solver(model);
    // Search for a solution
    if (solver.solve()) {
      solver.out() << solver.getStatus() << " Solution" << endl;
      solver.out() << "Belgium:     "
                   << Names[(IloInt)solver.getValue(Belgium)]     << endl;
      solver.out() << "Denmark:     "
                   << Names[(IloInt)solver.getValue(Denmark)]     << endl;
      solver.out() << "France:      "
                   << Names[(IloInt)solver.getValue(France)]      << endl;
      solver.out() << "Germany:     "
                   << Names[(IloInt)solver.getValue(Germany)]     << endl;
      solver.out() << "Netherlands: "
                   << Names[(IloInt)solver.getValue(Netherlands)] << endl;
      solver.out() << "Luxembourg:  "
                   << Names[(IloInt)solver.getValue(Luxembourg)]  << endl;
    }
  }
  catch (IloException& ex) {
    cerr << "Error: " << ex << endl;
  }
  env.end();
  return 0;
}