Decision Statements: "If" Statements and Functions

Need (library reference):

• working with Primitive.Collided function.  Specifically, the idea that circles are compared based on the distance, and everything else (include the soccer ball-paddle collision) are checked based on the bounding box of each shape.
• must explain the problem of objects colliding and getting stuck
• working with Audio files

References:

• Pre-requisite: it is assumed that you have read through the prior tutorials, and are familiar with the concepts covered in those tutorials.

Goals:

• Throughout this chapter, we will work towards a simple game, with each tutorial adding a little bit more to what we've already done.  In this tutorial, we're going to:
  1. Change the program so that the ball will bounce off the left or right edges of the screen
  2. Change the program so that the ball bounces off the right paddle

1. Obtain the example code

• Here is the zip file to the source files and compiled executable of this example.
• Here is the keyboard mapping for the XBOX 360 controller.  
• Here is the main tutorial page for working with XNACS1Lib, and here is the complete documentation of the library: html-page or compiled html help file.

When the game starts, you'll see a screen that looks similar to this:

The game behaves in a manner that is identical to the what was described for the previous tutorial, with two new changes:

1. If the soccer ball reaches the left edge of the screen, it will 'bounce off' that edge, meaning that it will reverse it's horizontal direction. 

2. If the soccer ball collides with the right paddle, it will 'bounce off' that paddle, meaning that it will reverse it's horizontal direction.  We will also play a sound, in order to provide audio feedback to the user that the player has successfully bounced the ball

Let's examine the C# source code that produces the behavior we see on-screen.  Since the code is nearly identical to the program that was presented in the previous tutorial, we'll leave out everything, except for code that has changed, or code that is new.  That leaves us with:

• UpdateWorld():

  protected override void UpdateWorld() {     <code omitted, for clarity>     // This is the if condition!     if (m_TheSoccer.CenterX > 100.0f)     {         m_TheSoccer.VelocityX = -m_TheSoccer.VelocityX;     }     if (m_TheSoccer.CenterX < 0.0f)     {         m_TheSoccer.VelocityX = -m_TheSoccer.VelocityX;     }     if (m_RightPaddle.Collided(m_TheSoccer))     {         m_TheSoccer.VelocityX = -m_TheSoccer.VelocityX;         PlayACue("Bounce");     }     EchoToTopStatus("Left Paddle Center=" + m_LeftPaddle.Center + " Right Paddle Center=" + m_RightPaddle.Center);     EchoToBottomStatus("Soccer Ball position: " + m_TheSoccer.Center); }

  • The first new if statement should be familiar: it checks if the X value of the coordinates of the center of the soccer ball has gone below zero (meaning that the soccer ball's center has gone off the left side of the screen), and if that's true, the program will reverse the horizontal direction of the ball. 
  • The second new if statement is very similar to what we've seen before, but with a new twist: this one uses functions.
        if (m_RightPaddle.Collided(m_TheSoccer))

        {

            m_TheSoccer.VelocityX = -m_TheSoccer.VelocityX;

            PlayACue("Bounce");

        }

    Let's break this out into separate parts:

    • if (m_RightPaddle.Collided(m_TheSoccer))
      This part asks "has the right paddle collided with the soccer ball?".  It does so by making use of the Collided method on the m_RightPaddle object.  It passes the soccer ball, m_TheSoccer, as the only parameter/argument to the method.  The method will return true if paddle's rectangle and the soccer ball's circle overlap (more or less - there's more detail about this available in the tutorial on collision detection using the XNACS1Lib).  The method will return false if the two shapes do NOT overlap.
      • m_TheSoccer.VelocityX = -m_TheSoccer.VelocityX;
        As you've seen previously, this will reverse the magnitude of the X part of the soccer ball's velocity. 
        If the ball and the right paddle overlap (if they've collided), then we want to make the ball go in the opposite direction, so we'll run this code only if the ball and paddle have overlapped.
      • PlayACue("Bounce");
        This will play a short sound (also known as an audio cue), whose purpose is to alert the player that something has happened.  In this case, we want to provide audio feedback to the player that they've successfully bounced the ball off the paddle.
        (There's more detail about how to play sounds in the tutorial on using audio effects)

FURTHER EXERCISES:: 

• Start from a blank starter project (1000.201, if you need it), and re-do the code from memory as much as possible.  On your first try, do what you can, and keep the above code open so that when you get stuck, you can quickly look up what you forgot (and that after you finish a line, so that you can compare your line to the 'correct' line).  On the next try, do the same thing, but try to use the finished code less.  Repeat this until you can type everything, without referring the tutorial's code.
  • Repeat this exercise daily for several days, so that you really get the hang of this.  As you go on, periodically review this by re-doing this exercise.
• Syntax For "If" Statements
  For this exercise, you should use the same project that was explained in the above tutorial.
  Examine the following lines of code, then answer the questions that follow
  
  

  Line #	Source Code
  69	if (m_TheSoccer.CenterX < 0.0f)
  70	{
  71	m_TheSoccer.VelocityX = -m_TheSoccer.VelocityX;
  72	}
  73
  74	if (m_RightPaddle.Collided(m_TheSoccer))
  75	{
  76	m_TheSoccer.VelocityX = -m_Soccer.VelocityX;
  77	PlayACue("Bounce");
  78	}

  1. If you remove the curly braces on lines 70 and 71 (the ones highlighted in a light tan), will the program still compile, run without crashing, and run correctly?  Why?
  2. If you remove the curly braces on lines 75 and 78 (the ones highlighted in a light green), will the program still compile, run without crashing, and run correctly?  Why?
• Commutativity of Collision detection
  If you took the line
      if (m_RightPaddle.Collided(m_TheSoccer))
  and replaced it with
      if (m_TheSoccer.Collided(m_RightPaddle))
  would the program still function correctly?  Why?
• Familiarizing Yourself With The Syntax For "If" Statements: Adding a vertical check in
  For this exercise, you should use the same project that was explained in the above tutorial.
  For this exercise, try adding in another if statement so that if the CenterY value of the soccer ball is greater than 56 (which is about where the top of the screen is), the vertical velocity of the soccer ball should be flipped.
• Familiarizing Yourself With The Syntax For "If" Statements: Adding another vertical check in
  For this exercise, you should use the same project that was explained in the above tutorial.
  For this exercise, try adding in another if statement so that if the CenterY value of the soccer ball is less than 0 (which is where the bottom of the screen is), the vertical velocity of the soccer ball should be flipped.

	This work is supported in part by a grant from Microsoft Research under the Computer Gaming Curriculum in Computer Science RFP, Award Number 15871 and 16531.
2/8/2010