如何检查一个数组中的值是否相等而不检查同一件事

Question

I have this array

Ball[] balls = new Ball[7]; // 7 just being an example

In my Ball class, I have getters and setters for x and y values. I'm trying to compare the x and y values to make sure that they don't intersect.

My first thought was to make a loop looking like

for(Ball b1 : balls) {
    for(Ball b2 : balls) {
    if(b1.intersects(b1, b2)) {. . .} // I made intersects, not my issue 
    }
}

But this is no good, as it compares:

1. balls 0 to balls 0
2. balls 1 to balls 1

3. etc.

    for(int i = 0; i < balls.length; i++) { System.out.println(f.getContentPane().getWidth() + "\\n" + f.getContentPane().getHeight()); int radius = 10 + rand.nextInt(20); balls[i] = new Ball(360, radius, rand.nextInt(f.getContentPane().getWidth() - 4 * radius - 5) + radius + 5, rand.nextInt(f.getContentPane().getHeight() - 4 * radius - 5) + radius + 5 ); } for(Ball b1 : balls) { for (Ball b2 : balls) { while (b1.intersects(b1, b2)) { System.out.println("Ball started out inside of another, replacing now."); b1.setX(rand.nextInt(f.getContentPane().getWidth() - 2 * b1.getRadius() - 5) + b1.getRadius() + 5); b1.setY(rand.nextInt(f.getContentPane().getHeight() - 2 * b1.getRadius() - 5) + b1.getRadius() + 5); } } } 

////////////// class change //////////////////

class Ball {
private int direction;
private int radius;
private int x,y;

Ball(int direction, int radius, int x, int y) {
    this.direction = direction;
    this.radius = radius;
    this.x = x;
    this.y = y;
}

// Getters + Setters here

boolean intersects(Ball b1, Ball b2) {
    double x = Math.pow((b2.getX() - b1.getX()), 2);    // Distance formula
    double y = Math.pow((b2.getY() - b1.getY()), 2);    // Distance formula
    double r = b1.getRadius() + b2.getRadius();

    //System.out.println(x + " + " + y + " <= " + r );
    return x + y <= r;
}

}

(Ignore that I didn't put my first hunk of code in a method and class, I've done that in my actual code.)

I, for whatever reason, can't think of a way to do this without a whole lot of if statements

(So I'm asking for the best way to do this)

Answer 1

One way to compare every distinct (ie, no ball with itself) pair of Ball s, without comparing any pair more than once would be:

for (int i = 0; i < balls.length; ++i) {
    Ball b1 = balls[i];
    for (int j = i+1; j < balls.length; ++j) {
        Ball b2 = balls[j];
        if (b1.intersects(b1, b2)) {
            // ...
        }
    }
}

Detecting new collisions introduced in the process of resolving previous ones just means making multiple passes over balls until you no longer have any collisions. A simple, perhaps naive, way of doing this would be something like this:

boolean foundCollision;
int numTries = 0;
int maxTries = 1000000;
do {
    foundCollision = false;
    for (int i = 0; i < balls.length; ++i) {
        Ball b1 = balls[i];
        for (int j = i+1; j < balls.length; ++j) {
           Ball b2 = balls[j];
           if (b1.intersects(b1, b2)) {
               foundCollision = true;
               // resolve collision...
        }
    }
    ++numTries;
} while (foundCollision && numTries < maxTries);
if (numTries >= maxTries)
    System.err.println("Couldn't sort out balls after " + maxTries + "tries: what now?");