Repetition: Counting Down Using A 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 a simple for loop, which is used to repeat some action a large number of times

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 program for this tutorial is almost exactly identical to the program used in the the previous tutorial.  The difference is that for this tutorial, we're using a for loop to count down rather than up (to count backwards rather than forwards)

2. Examining The Program:

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

• UpdateWorld():

  // here is the for loop     for (float xPos = World.WorldMax.X; xPos >= 0.0f; xPos -= 1)     {         float radian = ComputeRadianFromXPos(xPos);         yPos = ComputeSineYPos(radian) ;         CreateABallAt(xPos, yPos, "SoccerBall");     }

  • Let's examine the details of how the for loop operates by tracing through the execution of the first couple of iterations in detail
    1. Just like we did for the normal while loop, the initialization part of the for loop is executed exactly once.  It is always executed, and it it is executed before the first check of the condition:
           for (float xPos = World.WorldMax.X; xPos >= 0.0f; xPos -= 1)
       

       Variable Name	Value
       xPos	100.0f

    2. Next, the first iteration of the loop begins:
       1. At the start of each iteration of the loop, the condition is checked:
              for (float xPos = World.WorldMax.X; xPos >= 0.0f; xPos -= 1)

          This asks "Is the current value of xPos (which is 100.0f) greater than (or equal to) 0.0f?"  This is true, so the program will execute the body of the loop.

       2. The body of the loop consists of the following lines of code:
                  float radian = ComputeRadianFromXPos(xPos);

                  yPos = ComputeSineYPos(radian) ;

                  CreateABallAt(xPos, yPos, "SoccerBall");

          The above code will place a soccer ball at the appropriate place along the sine wave.

       3. After the body of the loop has finished executing, then the counting expression is executed
              for (float xPos = World.WorldMax.X; xPos >= 0.0f; xPos -= 1)

          which decrementsthe value stored in counter by one:

          
          

          Variable Name	Value
          xPos	99.0f

  • Next, the second iteration of the loop begins:
    1. At the start of each iteration of the loop, the condition is checked:
           for (float xPos = World.WorldMax.X; xPos >= 0.0f; xPos -= 1)

       This asks "Is the current value of xPos (which is 99.0f) greater than (or equal to) 0.0f?"  This is true, so the program will execute the body of the loop.

    2. The body of the loop consists of the following lines of code:
               float radian = ComputeRadianFromXPos(xPos);

               yPos = ComputeSineYPos(radian) ;

               CreateABallAt(xPos, yPos, "SoccerBall");

       The above code will place a soccer ball at the appropriate place along the sine wave.

    3. After the body of the loop has finished executing, then the counting expression is executed
           for (float xPos = World.WorldMax.X; xPos >= 0.0f; xPos -= 1)

       which decrementsthe value stored in counter by one:

       
       

       Variable Name	Value
       xPos	98.0f

  • The loop continues to iterate through many, many more iterations.  We will not be tracing through all the iterations here.

  • The loop continues to iterate.  The final iteration looks like this:

  1. At the start of each iteration of the loop, the condition is checked:
         for (float xPos = World.WorldMax.X; xPos >= 0.0f; xPos -= 1)

     This asks "Is the current value of xPos (which is now 0.0f, in the final iteration of the loop) greater than (or equal to) 0.0f?"  This is true (because they're equal), so the program will execute the body of the loop.

  2. The body of the loop consists of the following lines of code:
             float radian = ComputeRadianFromXPos(xPos);

             yPos = ComputeSineYPos(radian) ;

             CreateABallAt(xPos, yPos, "SoccerBall");

     The above code will place a soccer ball at the appropriate place along the sine wave.

  3. After the body of the loop has finished executing, then the counting expression is executed
         for (float xPos = World.WorldMax.X; xPos >= 0.0f; xPos -= 1)

     which decrementsthe value stored in counter by one:

     
     

     Variable Name	Value
     xPos	-1.0f

  • The program goes back to check the condition
        for (float xPos = World.WorldMax.X; xPos >= 0.0f; xPos -= 1)

    and asks "Is the current value of xPos (which is now -1.0f) greater than (or equal to) 0.0f?"  This is NOT true, so the program will NOT execute the body of the loop.

FURTHER EXERCISES:: 

1. 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 refering 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.
2. Go back through the previous tutorials, and re-implement the code, using loops that count down, rather than up.  Your instructor may have a specific list of tutorials (and exercises from those tutorials) that you will implement.

	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