sum one variable (X[i][j]) in one constraint with choco

I am using choco to solve a CSP , and one of my constraints is that the sum of one variables (X[i][j]) is less than N=10, and i=j=1....N.

How do I accomplish this? thank you for your help.

sum(X[i][j]) = 1 for i=j=1....N

You need a 1d array and call model.sum():

import org.chocosolver.solver.Model;
import org.chocosolver.solver.Solver;
import org.chocosolver.solver.variables.BoolVar;

public class SumBooleans {

    private final static int N = 10;

    public static void main(String[] args) {
        Model model = new Model("Boolean sum");

        BoolVar[][] vars = new BoolVar[N][N];
        for (int i = 0; i < N; i++) {
            for (int j = 0; j < N; j++) {
                vars[i][j] = model.boolVar("vars[" + i + "][" + j + "]");
            }
        }

        BoolVar[] flatArray = new BoolVar[N * N];
        for (int index = 0; index < N * N; index++) {
            int i = index / N;
            int j = index % N;
            flatArray[index] = vars[i][j];
        }

        model.sum(flatArray, "=", 1).post();
        //model.sum(flatArray, "<", N).post();
        //model.sum(flatArray, ">=", 8).post();

        Solver solver = model.getSolver();
        if (solver.solve()) {

            for (int i = 0; i < N; i++) {
                for (int j = 0; j < N; j++) {
                    System.out.print(vars[i][j].getValue() + " ");
                }
                System.out.println();
            }

        }
    }
}

Output:

0 0 0 0 0 0 0 0 0 0 
0 0 0 0 0 0 0 0 0 0 
0 0 0 0 1 0 0 0 0 0 
0 0 0 0 0 0 0 0 0 0 
0 0 0 0 0 0 0 0 0 0 
0 0 0 0 0 0 0 0 0 0 
0 0 0 0 0 0 0 0 0 0 
0 0 0 0 0 0 0 0 0 0 
0 0 0 0 0 0 0 0 0 0 
0 0 0 0 0 0 0 0 0 0