proj0介绍

Planet类构造器

public class Planet {

    public double xxPos;
    public double yyPos;
    public double xxVel;
    public double yyVel;
    public double mass;
    public String imgFileName;

    public Planet(double xP, double yP, double xV, double yV, double m, String img) {
        xxPos = xP;
        yyPos = yP;
        xxVel = xV;
        yyVel = yV;
        mass = m;
        imgFileName = img;
    }

    public Planet(Planet p) {
        xxPos = p.xxPos;
        yyPos = p.yyPos;
        xxVel = p.xxVel;
        yyVel = p.yyVel;
        mass = p.mass;
        imgFileName = p.imgFileName;
    }
}

物理理解

引力常数：

\[G = 6.67·10^{-11} \frac {Nm^2}{kg^2} \]

引力：

\[F = \frac {G·m_1·m_2} {r^2} \]

编写Planet类

1. 计算行星之间的距离 calcDistance

       public double calcDistance(Planet p) {
           double distanceX = p.xxPos - xxPos;
           double distanceY = p.yyPos - yyPos;
           double r = Math.sqrt(distanceX * distanceX + distanceY * distanceY);
           return r;
       }
   

   开方：Math.sqrt()

2. 计算行星之间的引力 calcForceExertedBy

3. 计算x方向和y方向的引力 calcForceExertedByX和calcForceExertedByY

4. 计算x方向和y方向的合力 calcNetForceExertedByX 和calcNetForceExertedByY

       public double calcNetForceExertedByX(Planet[] allPlanets) {
           double netForceX = 0.0;
           for (Planet planet : allPlanets) {
               if (!(this.equals(planet))) {
                   netForceX += this.calcForceExertedByX(planet);
               }
           }
           return netForceX;
       }
   
       public double calcNetForceExertedByY(Planet[] allPlanets) {
           double netForceY = 0.0;
           for (Planet planet : allPlanets) {
               if (!(this.equals(planet))) {
                   netForceY += this.calcForceExertedByY(planet);
               }
           }
           return netForceY;
       }
   

5. 更新 传入时间dt、x方向的力、y方向的力

   计算加速度、新速度、新位置

       public void update(double dt, double fX, double fY) {
           double accX = fX / mass;
           double accY = fY / mass;
           xxVel += dt * accX;
           yyVel += dt * accY;
           xxPos += dt * xxVel;
           yyPos += dt * yyVel;
       }
   

增强for循环

模拟器NBody.java

1. readRadius 返回半径

2. readPlanets 返回对象列表

   返回类型：Planet[]

   创建对象列表：Planet[] planets = new Planet[number]

   创建对象：planets[i] = new Planet(xP, yP, xV, yV, m, img)

       public static Planet[] readPlanets(String fileName) {
           In in = new In(fileName);
           int number = in.readInt();
           double radius = in.readDouble();
           Planet[] planets = new Planet[number];
           for (int i = 0; i < number; i++) {
               double xP = in.readDouble();
               double yP = in.readDouble();
               double xV = in.readDouble();
               double yV = in.readDouble();
               double m = in.readDouble();
               String img = in.readString();
               planets[i] = new Planet(xP, yP, xV, yV, m, img);
           }
           return planets;
       }
   

   Checking readPlanets...
   PASS: readPlanets(); Congrats! This was the hardest test!
   

绘制宇宙初始状态 (main)

类型转换：double x = Double.parseDouble()

1. 收集输入

2. 绘制背景

   将starfield.jpg 传入StdDraw.picture()

3. 绘制一个行星

   在Planet.java中添加draw方法

4. 绘制所有行星

   通过for循环调用draw方法，可以得到下图

创建动画

1. 创建时间变量time，默认为0，while循环直到time达到T
2. 创建xForces数组和yForces数组
3. 计算每个行星受到的引力合力
4. 调用update方法更新每个行星的位置速度和加速度
5. 绘制背景图
6. 绘制所有行星
7. 调用StdDraw.show()
8. 调用StdDraw.pause(10)
9. 将时间变量增加dt

动态截图：

打印最终状态

用于打分（provided）：

StdOut.printf("%d\n", planets.length);
StdOut.printf("%.2e\n", radius);
for (int i = 0; i < planets.length; i++) {
    StdOut.printf("%11.4e %11.4e %11.4e %11.4e %11.4e %12s\n",
                  planets[i].xxPos, planets[i].yyPos, planets[i].xxVel,
                  planets[i].yyVel, planets[i].mass, planets[i].imgFileName);   
}

提交

问题：引力常数需要设置为private

修改后通过：

github: https://github.com/ikventure/cs61b-sp18-logs/tree/master/proj0