Zap Game: Nested For loops

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:

• In this tutorial, we will:
  • Examine how to use nested for loops in order to get the hero's laser to 'slide around' blocks

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:

This version of the 'zap' game looks and plays very similarly to the prior version, which was explained in the 380 tutorial .  The major change in this version is that the game will detect when the player's zapping beam has collided with BlockA, and if it has collided, it will cause the laser beam to go up the wall, and then continue on towards the enemy.  Note that collisions with all the other blocks are still unhandled

Notice that we will also adjust the hero's laser beam so that there are no spaces in it.

2. Examining The Program:

Let's examine the C# source code that produces the behavior we see on-screen

• Drawing the Hero's Laser in UpdateWorld():

  Version of code in 315 tutorial

  Version of code in this tutorial

  XNACS1Circle lastCircle = null ; Vector2 HeroBeam = m_Hero.Center; for (float x = m_Hero.CenterX+1;            x < m_Enemy.CenterX;            x+=(3*HERO_PATH_SIZE) ) {     HeroBeam.X = x;     lastCircle = CreateHeroPath(HeroBeam); }   if (lastCircle.Collided(m_Enemy)) {     m_Enemy.Texture = null ;     PlayACue(@"EnemyZapped");     m_HeroScore++; }

  XNACS1Circle lastCircle = null ; Vector2 HeroBeam = m_Hero.Center; for (float x = m_Hero.CenterX + 1;            x < m_Enemy.CenterX;            x += HERO_PATH_SIZE ) {     HeroBeam.X = x;     lastCircle = CreateHeroPath(HeroBeam);       if (lastCircle.Collided(m_BlockA))     {         for ( float y = HeroBeam.Y + 1;                    y <= m_BlockA.MaxBound.Y;                    y++)         {             HeroBeam.Y = y;             lastCircle = CreateHeroPath(HeroBeam);         }     } }   if (lastCircle.Collided(m_Enemy)) {      m_Enemy.Texture = null ;     PlayACue(@"EnemyZapped");     m_HeroScore++; }

  • In order to emphasize the differences between the code provided in the 315 tutorial (which was the last tutorial that changed how the hero's laser behaved) and the code provided in this tutorial, we have aligned the loop from the 'hero zapping' region side-by-side, with the new loop in this tutorial on the right.
    • First, you'll notice that the outer loop's condition has changed slightly.  Instead of moving X over by 3 times the size of an individual circle, we move it over by just one such length:
      
      for (float x = m_Hero.CenterX + 1; x < m_Enemy.CenterX; x += HERO_PATH_SIZE )
    • Second, you'll notice that inside the outer for loop, we first move the current X location over a little bit, then add the current location to the beam (as we have been doing).  After that, we then check to see if the current circle is overlapping the 'A' block, with this line:
      

          if (lastCircle.Collided(m_BlockA))

      
      if it is, then we use a nested for loop to draw the beam 'up' the side of the 'A' block:
      

              for ( float y = HeroBeam.Y + 1; y <= m_BlockA.MaxBound.Y; y++)

              {

                  HeroBeam.Y = y;

                  lastCircle = CreateHeroPath(HeroBeam);

              }

FURTHER EXERCISES:

	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