Solution is always = 0

[Andreea Ungurean]

Hi,

I have the following transportation problem.

Can you help me understand why do I only get one solution and every time = 0?

Below you can see the source code I'm using.

//input data

   final static int[] demandV = {30, 20, 70, 30, 60};

   final static int[] supplyV = {50, 60, 50, 50};

   final static int[][] costs = {{16, 16, 13, 22, 17}, 

                         {14, 14, 13, 19, 15},

                                 {19, 19, 20, 23, 50}, 

                                 {50, 12, 50, 15, 11}};

   final static int supplyLength = supplyV.length;

   final static int demandLength = demandV.length;

//the model

Model model = new Model();

IntVar[] demand =  model.intVarArray("demand", demandLength, new int[] {30, 20, 70, 30, 60});

IntVar[] supply = model.intVarArray("supply", supplyLength, new int[] {50, 60, 50, 50});

IntVar[][] result = new IntVar[supplyLength][demandLength];

////

for (int i = 0; i < demandLength; i++) {

demand[i] = model.intVar("demand_" + i, demandV[i]);

}

////

for (int i = 0; i < supplyLength; i++) {

supply[i] = model.intVar("supply_" + i, supplyV[i]);

}

for (int i = 0; i < supplyLength; i++) {

for (int j = 0; j < demandLength; j++) {

result[i][j] = model.intVar("result_" + i + j, 0,100);

}

}

//sum of row values < supply

for (int i = 0; i < supplyLength; i++) {

  model.sum(result[i],"<=",supply[i]).post();   

}

//sum of column values < demand

        for(int i = 0; i < demandLength; i++) {

            IntVar[] col = new IntVar[supplyLength];

            for(int j = 0; j < supplyLength; j++) {

                col[j] = result[j][i];

            }

            model.sum(col, "<=", demand[i]).post(); // columns

        }

        

 

IntVar tot_cost = model.intVar("tot_cost", 0, 9999, true);

 

//x[i][j]*costs[i][j]

int[] rows = new int[demandLength * supplyLength];

    IntVar[] resultCost = new IntVar[demandLength * supplyLength];

     

int k = 0;

         for (int i = 0; i < supplyLength; i++) {

for (int j = 0; j < demandLength; j++) {

                    rows[k] = costs[i][j];

                    resultCost[k] = result[i][j];

                    k++;                   

}

}

  model.scalar(resultCost, rows, "=", tot_cost).post();

model.setObjective(Model.MINIMIZE, tot_cost);

Solver solver = model.getSolver();  

solver.showSolutions();

solver.showShortStatistics();

while(solver.solve())

{

    prettyPrint(model, tot_cost, result);

}

}

private void prettyPrint(Model model, IntVar tot_cost, IntVar[][] result) {

    StringBuilder st = new StringBuilder();

    st.append("Solution #").append(model.getSolver().getSolutionCount()).append("\n");

        for (int i = 0; i < supplyLength; i++) {

for (int j = 0; j < demandLength; j++) {

st.append(result[i][j]).append(" ");

}

st.append("\n");

}

    

    st.append("\tTotal C: ").append(tot_cost.getValue());

    System.out.println(st.toString());

}

Thank you!

[cpru...@gmail.com]

Hi,

I suppose `model.sum(col, ">=", demandV[i]).post(); // columns`  (not <=) is what you really want, no?

In addition, you don't need to declare variables for demand an supply.

Hope that helps,

CP