Determinant.java

package determinant;

import java.util.Scanner;

public class Determinant {

    //======================================
    // ** calculateDeterminant of the matrix
    // ** A - left side matrix
    // ** B - right side matrix
    //======================================

    public static double calculateDeterminant(double[][] A) {
        int n = A.length;
        double[] b = {1,2,3,4,5,6,7,8,9,10};
        int numofswaps=0;


        for (int p = 0; p < n; p++) {

            //======================================
            //** Find pivot row and swap it
            //======================================

            int max = p;
            for (int i = p + 1; i < n; i++) {
                if (Math.abs(A[i][p]) > Math.abs(A[max][p])) {
                    max = i;
                    numofswaps++;
                }
            }
            double[] temp = A[p]; A[p] = A[max]; A[max] = temp;
            double   t    = b[p]; b[p] = b[max]; b[max] = t;

            //======================================
            // ** Do standard GE
            //======================================

            for (int i = p + 1; i < n; i++) {
                double factor = A[i][p] / A[p][p];
                b[i] -= factor * b[p];
                for (int j = p; j < n; j++) {
                    A[i][j] -= factor * A[p][j];
                }
            }
        }

        //======================================
        // ** Do back substitution
        //======================================

        double[] x = new double[n];
        for (int i = n - 1; i >= 0; i--) {
            double sum = 0.0;
            for (int j = i + 1; j < n; j++) {
                sum += A[i][j] * x[j];
            }
            x[i] = (b[i] - sum) / A[i][i];
        }

        //======================================
        // ** After GE, calculate determinant
        //======================================

        System.out.println("Number of swaps: "+numofswaps);
        double det = 1;
        for(int i=0;i<n;i++){
            det = det*A[i][i];
        }
        if(numofswaps%2==0){
            det = -det;
        }

        return det;
    }

    //======================================
    // ** Main
    //======================================

    public static void main(String[] args) {

        //======================================
        // ** Define variables
        //======================================

        double[][] A;
        double[] b;

        //======================================
        // ** Get data from user
        //======================================

        System.out.println("Enter matrix dimension: ");
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        // Initialize vars
        A = new double[n][n];
        b = new double[n];

        System.out.println("Enter left-side matrix: ");
        for(int i=0; i<n; i++)
            for(int j=0; j<n; j++)
                A[i][j] = sc.nextDouble();
        System.out.println("Enter right-side matrix: ");
        for(int j=0;j<n;j++){
            b[j] = sc.nextDouble();
        }

        //======================================
        //** Calculate determinant
        //======================================

        double result = calculateDeterminant(A);

        //======================================
        // ** Round determinant
        //======================================

        result = Math.round(result);

        //======================================
        // ** Print result
        //======================================

        System.out.println("Determinant: "+result);

    }
}