Zap Game: For Loop

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 a for loop to count our way across the screen.

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 identically to the prior version, which was explained in the 160 tutorial .

There are only two differences, code-wise, between this version and the prior version: this version uses a for loop, instead of a while loop, and this one counts slightly differently (instead of adding 1f to HeroBeam.X each time, it use a separate counter variable, and copies that variable into HeroBeam.X)

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 160 tutorial	Version of code in this tutorial
  XNACS1Circle lastCircle = null; Vector2 HeroBeam = m_Hero.Center; while (HeroBeam.X <= m_Enemy.CenterX) {     HeroBeam.X += 1f;     lastCircle = CreateHeroPath(HeroBeam); }	XNACS1Circle lastCircle = null ; Vector2 HeroBeam = m_Hero.Center; for ( float x = m_Hero.CenterX+1; x < m_Enemy.CenterX; x++) {     HeroBeam.X = x;     lastCircle = CreateHeroPath(HeroBeam); }

  • In order to emphasize the differences between the code provided in the 160 tutorial, 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.
  • Notice that in this tutorial, we're using a for loop to count out the creation of the circles.
    
    for ( float x = m_Hero.CenterX+1; x < m_Enemy.CenterX; x++)
    • Note that we use a new variable, x , to count from the left side of the screen, to the right side. We initialize x to have a value that is one greater than the center of the hero's circle. 
      
          float x = m_Hero.CenterX+1
    • We iterate through this loop until x is equal to the enemy's center's X value, similar to what we've done in prior tutorials.
      
          ; x < m_Enemy.CenterX ;  
    • At each iteration, we increase the X value by one:
      
          x++)  
      
      then copy that value into the HeroBeam variable:
      
      

          HeroBeam.X = x;

      • Because we still use the HeroBeam variable to place the circles, it will have a copy of the hero's Y value.  Thus, all the circles will be drawn in a horizontal line, at the same height that the hero is at.

FURTHER EXERCISES::  

1. Go back to the 160 tutorial, and redo those exercises, using for loop(s).

	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