I wrote this code in Eclipse to solve the NBody physics equation however there i
ID: 3686606 • Letter: I
Question
I wrote this code in Eclipse to solve the NBody physics equation however there is an unresolved error. In this line "double Fnet = G*mass*mass/(dist*dist);" eclipse underlines G*mass in red and gives me the message "The operator * is undefined for the argument type(s) double, double[]." How do I resolve this issue? The full code is included below.
public class NBody {
public static final String PLANETS_FILE = "planets.txt";
// animation pause (in miliseconds)
public static final int DELAY = 20;
// music (2001 theme)
public static final String MUSIC = "2001theme.wav";
// background image
public static final String BACKGROUND = "starfield.jpg";
// gravitational constant (N m^2 / kg^2)
public static final double G = 6.67e-11;
// parameters from command line
public static double T; // simulate from time 0 to T (s)
public static double dt; // time quantum (s)
// parameters from first two lines
public static int N; // number of bodies
public static double R; // radius of universe
public static double[] rx; // x position (m)
public static double[] ry; // y position (m)
public static double[] vx; // x velocity (m/s)
public static double[] vy; // y velocity (m/s)
public static double[] mass; // mass (kg)
public static String[] image; // name of gif
// TODO: read the planet file, new the parallel arrays, and load
// their values from the file.
public static void loadPlanets(String planetFileName) {
java.util.Scanner input = new java.util.Scanner(System.in);
int N = input.nextInt();
double R = input.nextDouble();
double[] rx = new double[N];
double[] ry = new double[N];
double[] vx = new double[N];
double[] vy = new double[N];
double[] mass = new double[N];
String[] image= new String[N];
for (int i = 0; i < N; i++) {
rx[i] = input.nextDouble();
ry[i] = input.nextDouble();
vx[i] = input.nextDouble();
vy[i] = input.nextDouble();
mass[i] = input.nextDouble();
image[i] = "./images/" + input.next();
}
}
public static void runSimulation() {
// run numerical simulation from 0 to T
for (double t = 0.0; t < T; t += dt) {
// the x- and y-components of force
double[] fx = new double[N];
double[] fy = new double[N];
// calculate forces on each object
for (int i = 0; i < N; i++) {
// reset forces to zero
fx[i] = 0.0;
fy[i] = 0.0;
for (int j = 0; j < N; j++) {
if (i != j) {
double dx = rx[j] - rx[i];
double dy = ry[j] - ry[i];
double dist = Math.sqrt(dx*dx + dy*dy);
double Fnet = G*mass*mass/(dist*dist);
fx[i] += Fnet * dx / dist;
fy[i] += Fnet * dy / dist;
}
}
}
//calculate acceleration, velocities and positions
double ax = 0.0;
double ay = 0.0;
for (int i = 0; i < N; i++) {
ax = fx[i] / mass[i];
ay = fy[i] / mass[i];
vx[i] += dt * ax;
vy[i] += dt * ay;
rx[i] += dt * vx[i];
ry[i] += dt * vy[i];
// draw background and then planets
StdDraw.picture(0, 0, BACKGROUND);
StdDraw.picture(rx[i], ry[i], image[i]);
//loop to plot the N bodies
// pause for a short while, using "animation mode"
StdDraw.show(DELAY);
}
}
}
public static void main(String[] args) {
// TODO: read T and dt from command line.
T = 0;
// load planets from file specified in the command line
String planetFileName = "planets.txt";
loadPlanets(planetFileName);
// rescale coordinates that we can use natural x- and y-coordinates
StdDraw.setXscale(-R, +R);
StdDraw.setYscale(-R, +R);
StdAudio.play( MUSIC );
// turn on animation mode
StdDraw.show(0);
// Run simulation
runSimulation();
// print final state of universe to standard output
System.out.printf("%d ", N);
System.out.printf("%.2e ", R);
for (int i = 0; i < N; i++) {
System.out.printf("%11.4e %11.4e %11.4e %11.4e %11.4e %12s ",
rx[i], ry[i], vx[i], vy[i], mass[i], image[i]);
}
}
}
Explanation / Answer
Hi I have fixed your issue.
public class NBody {
public static final String PLANETS_FILE = "planets.txt";
// animation pause (in miliseconds)
public static final int DELAY = 20;
// music (2001 theme)
public static final String MUSIC = "2001theme.wav";
// background image
public static final String BACKGROUND = "starfield.jpg";
// gravitational constant (N m^2 / kg^2)
public static final double G = 6.67e-11;
// parameters from command line
public static double T; // simulate from time 0 to T (s)
public static double dt; // time quantum (s)
// parameters from first two lines
public static int N; // number of bodies
public static double R; // radius of universe
public static double[] rx; // x position (m)
public static double[] ry; // y position (m)
public static double[] vx; // x velocity (m/s)
public static double[] vy; // y velocity (m/s)
public static double[] mass; // mass (kg)
public static String[] image; // name of gif
// TODO: read the planet file, new the parallel arrays, and load
// their values from the file.
public static void loadPlanets(String planetFileName) {
java.util.Scanner input = new java.util.Scanner(System.in);
int N = input.nextInt();
double R = input.nextDouble();
double[] rx = new double[N];
double[] ry = new double[N];
double[] vx = new double[N];
double[] vy = new double[N];
double[] mass = new double[N];
String[] image = new String[N];
for (int i = 0; i < N; i++) {
rx[i] = input.nextDouble();
ry[i] = input.nextDouble();
vx[i] = input.nextDouble();
vy[i] = input.nextDouble();
mass[i] = input.nextDouble();
image[i] = "./images/" + input.next();
}
}
public static void runSimulation() {
// run numerical simulation from 0 to T
for (double t = 0.0; t < T; t += dt) {
// the x- and y-components of force
double[] fx = new double[N];
double[] fy = new double[N];
// calculate forces on each object
for (int i = 0; i < N; i++) {
// reset forces to zero
fx[i] = 0.0;
fy[i] = 0.0;
for (int j = 0; j < N; j++) {
if (i != j) {
double dx = rx[j] - rx[i];
double dy = ry[j] - ry[i];
double dist = Math.sqrt(dx * dx + dy * dy);
double Fnet = G * mass[i] * mass[i] / (dist * dist);
fx[i] += Fnet * dx / dist;
fy[i] += Fnet * dy / dist;
}
}
}
// calculate acceleration, velocities and positions
double ax = 0.0;
double ay = 0.0;
for (int i = 0; i < N; i++) {
ax = fx[i] / mass[i];
ay = fy[i] / mass[i];
vx[i] += dt * ax;
vy[i] += dt * ay;
rx[i] += dt * vx[i];
ry[i] += dt * vy[i];
// draw background and then planets
StdDraw.picture(0, 0, BACKGROUND);
StdDraw.picture(rx[i], ry[i], image[i]);
// loop to plot the N bodies
// pause for a short while, using "animation mode"
StdDraw.show(DELAY);
}
}
}
public static void main(String[] args) {
// TODO: read T and dt from command line.
T = 0;
// load planets from file specified in the command line
String planetFileName = "planets.txt";
loadPlanets(planetFileName);
// rescale coordinates that we can use natural x- and y-coordinates
StdDraw.setXscale(-R, +R);
StdDraw.setYscale(-R, +R);
StdAudio.play(MUSIC);
// turn on animation mode
StdDraw.show(0);
// Run simulation
runSimulation();
// print final state of universe to standard output
System.out.printf("%d ", N);
System.out.printf("%.2e ", R);
for (int i = 0; i < N; i++) {
System.out.printf("%11.4e %11.4e %11.4e %11.4e %11.4e %12s ",
rx[i], ry[i], vx[i], vy[i], mass[i], image[i]);
}
}
}