Ball Bouncing off the sides

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int x;

int y;

int w = 20;

int h = 20;

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int speed = 3;

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void setup (){

  size (200, 200);

  x = 10;          // starting it at 10 since my if statement below wants the ball to never be < 10, 

                   // if we started this at 9, or 8 it would jitter and not move

  y = height/2;

}

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void draw() {

  background (255, 255, 0);

  //guide lines so you can see the edge of screen

  line (0,0, 0, height);

  line (width-5, 0, width-5, height);

  ellipse (x, y, w, h);

  x = x+speed;            // give the ellipse some movement. 

  // I'm using width-15 and 10 instead of full width or 0 so that the ball bounces off it's sides, not it's center. It's an illusion. 

  if ((x > width-15) || (x < 10) ) {   

    speed = speed * -1;   // when the ball touches a side, make it move in the opposit direction

  }

}