java - CPLEX Java - 获取 IloNumExpr 和数字之间的最小值

Question

我尝试使用以下简化代码查找 IloNumExpr 和数字之间的最小值:

import ilog.concert.IloException;
import ilog.concert.IloNumExpr;
import ilog.concert.IloNumVar;
import ilog.cplex.IloCplex;

public class Test {

    public static void main(String[] args) {
        try {
            // Define model
            IloCplex cplex = new IloCplex();

            IloNumExpr objective = cplex.numExpr();
            IloNumVar var = cplex.intVar(0, 10);

            objective = cplex.sum(objective, cplex.prod(var, 10));
            cplex.addMinimize(objective);

            IloNumExpr aux1 = cplex.numExpr();
            IloNumExpr aux2 = cplex.numExpr();
            aux1 = cplex.prod(var, 1);
            aux2 = cplex.min(var, 1);

            if (cplex.solve()) {
                System.out.println("var: " + cplex.getValue(var));
                System.out.println("aux1: " + cplex.getValue(aux1));
                System.out.println("aux2: " + cplex.getValue(aux2));
            }

            cplex.end();
        }
        catch (IloException exc) {
            exc.printStackTrace();
        }
    }
}

但是，当我尝试打印 aux2 的值时，此代码返回错误。输出如下:

Tried aggregator 1 time.
LP Presolve eliminated 0 rows and 1 columns.
All rows and columns eliminated.
Presolve time = 0,00 sec. (0,00 ticks)
var: 0.0
aux1: 0.0
ilog.cplex.IloCplex$UnknownObjectException: CPLEX Error: object is unknown to IloCplex
    at ilog.cplex.CpxNumVar.getVarIndexValue(CpxNumVar.java:289)
    at ilog.cplex.EvalVisitor.visitNumVar(EvalVisitor.java:33)
    at ilog.cplex.CpxNumVar.accept(CpxNumVar.java:57)
    at ilog.cplex.IloCplex.getValue(IloCplex.java:8686)
    at Test.main(Test.java:29)

为什么会发生这种情况？

提前致谢

Answer 1

CPLEX 只能返回属于模型一部分的变量的值。您的变量 aux2 不是模型的一部分，因此 CPLEX 不知道它。在求解模型之前尝试cplex.add(aux2)。这应该可以解决问题。