Question

我是stanford cs106a公共资源的新学习者。当我读到BouncingBall时，我被困了，下面是完整的代码我从cs106a讲义。

问题是，对于本页底部的checkForCollision方法代码，为什么球应该执行代码ball.move(0, -2 * diff)，尤其如此执行-2 * diff，我无法理解这个数学代码。没关系球做代码ball.move(0, -diff)？有什么不同？什么是反弹逻辑？这里的任何人都可以帮助我理解这一点代码，我不擅长数学。非常感谢你

/*
 * File: BouncingBall.java
 *--------------------------
 * This program graphically simulates a bouncing ball
 */

import acm.program.*;
import acm.graphics.*;

public class BouncingBall extends GraphicsProgram {

/** Size (diameter) of the ball */
private static final int DIAM_BALL = 30;

/** Amount Y velocity is increased each cycle
 * as result of gravity
 */
private static final double GRAVITY = 3;

/** Animation delay or pause time between call moves */
private static final int DELAY = 50;

/** Initial X and Y location of ball */
private static final double X_START = DIAM_BALL;
private static final double Y_START = 100;

/** X Velocity */
private static final double X_VEL = 5;

/** Amount Y Velocity is reduced when it bounces */
private static final double BOUNCE_REDUCE = 0.9;

/** Starting X and Y Velocities */
private doublexVel = X_VEL;
private doubleyVel = 0.0;

/* private instance variable */
private GOval ball;

public void run() {
    setup();

    // Simulation ends when ball goes off right hand
    // end of screen
    while (ball.getX() < getWidth()) {
        moveBall();
        checkForCollision();
        pause(DELAY);
    }

}

/** Create and place a ball */
private void setup() {
    ball = new GOval (X_START, Y_START, DIAM_BALL, DIAM_BALL);
    ball.setFilled(true);
    add(ball);
}

/** Update and move ball */
private void moveBall() {
    // increase yVelocity due to gravity
    yVel += GRAVITY;
    ball.move(xVel, yVel);
}

/** Determine if collision with floor, update velocities
 *  and location as appropriate
 */
private void checkForCollision() {
    if (ball.getY() > getHeight() - DIAM_BALL) {

        // change ball's Y velocity to now bounce upwards.
        yVel = -yVel * BOUNCE_REDUCE;

        // Assume bounce will move ball an amount above
        // the floor equal to the amount it would have dropped
        // below the floor
        double diff = ball.getY() - (getHeight() - DIAM_BALL);
        ball.move(0, -2 * diff);
        }
    }       
}

Answer 1

当ball.move被召唤时，球已经移动到它弹跳的表面之外。因此-diff会将其移回水面，-2 * diff会使其反弹。

stanford cs106a弹跳球代码问题

1 个答案: