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我正在尝试解决我的 java 类中的 Nbody 示例（这是我在 java 中的第一个学期）。

节目详情如下：

编写一个程序 Nbody.java，它使用 Scanner 从标准输入中读取宇宙，使用上述的跳蛙方案模拟其动态，并使用我们的 StdDraw 对其进行动画处理。维护几个数组来存储数据。要进行计算机模拟，请编写一个无限循环，反复更新粒子的位置和速度。绘图时，考虑使用 StdDraw.setXscale(-R, +R) 和 StdDraw.setYscale(-R, +R) 将物理坐标缩放到屏幕坐标。

我编写了以下代码来编译和加载背景图像、歌曲和正确放置的行星。但是，我无法让植物按应有的方式旋转。

这是我的代码：

import java.util.Scanner;

public class Nbody {

// method dist calculates distance between two points
// it accepts four double values (x1, y1, x2, y2)
// it returns a double value
private static double dist(double x1, double y1, double x2, double y2)

{

     double r;
     r = Math.sqrt((x1-x2)*(x1-x2) + (y1-y2)*(y1-y2));
     return r;
} 


public static void main(String[] args) {

    final double G = 6.67e-11; // constant value G

    final double deltaT = 25000.0; // constant value delta T, which determines animation interval

    // ax and ay are accerlations on x axis and y axis
    double ax;
    double ay;


    Scanner scan = new Scanner (System.in); // define a keyboard object: scan

    int bodyNum = scan.nextInt(); // read in the first integer, which specify how many particles
    double radius = scan.nextDouble(); // read in radius of universe


    // define arrays to save x values and y values of each particle
    double[] x = new double[bodyNum];
    double[] y = new double[bodyNum];

    // define arrays to save velocity of each particle
    double[] vx = new double[bodyNum];
    double[] vy = new double[bodyNum];

    double[] m = new double[bodyNum];  // array stores mass of each particle
    String[] name = new String[bodyNum]; // array stores file name of each particles
    double[] Fx = new double[bodyNum]; // force on X axis
    double[] Fy = new double[bodyNum]; // force on Y axis

    StdDraw.setXscale(-radius, radius); // Set the X scale
    StdDraw.setYscale(-radius, radius); // Set the Y scale
    StdDraw.picture(0, 0, "starfield.jpg"); // Display background pic centered at (0,0)

    // read in inital location values and velocity values for each particle

    for (int i = 0; i < bodyNum; i++)

    {
       x[i] = scan.nextDouble();
       y[i] = scan.nextDouble();
       vx[i] = scan.nextDouble();
       vy[i] = scan.nextDouble();
       m[i] = scan.nextDouble();
       name[i] = scan.next();
       StdDraw.picture(x[i], y[i], name[i]); // display particle pic on screen
    }

    // This following line play background music, uncomment it in lab
    // If you work from a remote location via VNC, comment it
    StdAudio.play("2001.mid");

    // main animation loop
    while(true){

        // array Fx and Fy store net force acting on each body
        // initialize these two arrays to zeros

        for (int n = 0; n<bodyNum; n++)

        {
           Fx[n] = 0.0;
           Fy[n] = 0.0;
        }

         //  for loop to process all bodies
         for(int body = 0; body<bodyNum; body++)

            {

            // calculate the gravitational attraction between current body
            // and all other bodies

            Fx[body] = (m[body] * m[body+1])  / (x[body+1] -x[body])*G;

            Fy[body] = (m[body] * m[body+1]) / (y[body+1] -y[body])*G;

                for (int j = 0; j < bodyNum; j++)

                {

                 // calculate only when two bodies are different
                 // Please fill out the following if statement body                 

                 if(body != j)

                 {

                 Fx[j] = (m[j] * m[j+1]) / (y[j+1] -y[j])*G;
                 Fy[j] = (m[j] * m[j+1]) / (y[j+1] -y[j])*G;


                 }


                }
            }

         // update vleocity value and location value for each particle
         // please fill out the for loop
         for (int j = 0; j < bodyNum; j++)

            {

             // calculate accleration rate
             ax = Fx[j] / m[j];
             ay = Fy[j] / m[j];

             // update vleocity value 

            vx[j] = vx[j] + deltaT * ax;
            vy[j] = vy[j] + deltaT * ay;

            // update location value

            x[j] = x[j] + deltaT * vx[j];
            y[j] = y[j] + deltaT * vy[j];


            } 

         // redraw background
         StdDraw.setXscale(-radius, radius);
         StdDraw.setYscale(-radius, radius);
         StdDraw.picture(0, 0, "starfield.jpg");

         for (int i = 0; i < bodyNum; i++){
             // display the particle           
             StdDraw.picture(x[i], y[i], name[i]);
         }

         // display and pause for 30ms      
         StdDraw.show(30);        
    }
}
}

有什么想法吗？

4

1 回答 1

2

你的力计算对我来说似乎有点不对劲。我会这样做：

Arrays.fill(Fx, 0.0);
Arrays.fill(Fy, 0.0);
for (int body = 0; body < bodyNum - 1; body++) {
    for (int body2 = body + 1; body2 < bodyNum; body2++) {
        double dx = x[body2] - x[body];
        double dy = y[body2] - y[body];
        double d2 = dx * dx + dy * dy;
        double d = Math.sqrt(d2);
        double f = G * m[body] * m[body2] / d2;
        double fx = f * dx / d;
        double fy = f * dy / d;
        Fx[body] += fx;
        Fy[body] += fy;
        Fx[body2] -= fx;
        Fy[body2] -= fy;
    }
}
// then update positions and velocities based on Fx and Fy arrays

这将计算作用于的力的 x 和 y 分量，body然后body2将其添加到 x 和 y 力分量中，body然后从 中减去它们body2。通过从 开始索引body + 1，我们可以在同一个内循环通道中计算两个物体上的相等和相反的力，从而将计算次数减少一半。

对于更新位置，您使用的是时间增量结束时的速度，就好像物体以增量结束时的速度移动整个增量一样。进行某种插值可能更准确，但这应该是二阶效应。

于 2012-10-21T22:04:52.680 回答