Really quick question

Ok, I am following the book "Killer Game Programming" by O'Reilly. This code is from his book, I just have a few questions on what it means, because i get errors when i try and compile. The errors are that the variable period doesn't exist, which it doesn't. Here is the code:

Code:

---

public void run( )
    /* Repeatedly: update, render, sleep so loop takes close
      to period ms */
    {
      long beforeTime, timeDiff, sleepTime;

      beforeTime = System.currentTimeMillis( );

      running = true;
      while(running) {
        gameUpdate( );
        gameRender( );
        paintScreen( );

        timeDiff = System.currentTimeMillis( ) - beforeTime;
        sleepTime = [COLOR="Blue"]period[/COLOR] - timeDiff;  // time left in this loop

        if (sleepTime <= 0)  // update/render took longer than period
          sleepTime = 5;    // sleep a bit anyway

        try {
          Thread.sleep(sleepTime);  // in ms
        }
        catch(InterruptedException ex){}

        beforeTime = System.currentTimeMillis( );
      }

      System.exit(0);
    } // end of run( )

---

"A popular measure of how fast an animation progresses is frames per second (FPS). For GamePanel, a frame corresponds to a single pass through the update-render-sleep loop inside run( ). Therefore, the desired 100 FPS imply that each iteration of the loop should take 1000/100 == 10 ms. This iteration time is stored in the period variable in GamePanel."

Above is the paragraph in the book describing period. So is period simply a variable that you can put whatever time you want in it, or does it need to be something calculated?

period is a variable indeed, and it's the number of milliseconds you expect one frame to take. So for example if you want 25 FPS, your period should be (1000/25)=40. You don't need to calculate it in code.

ok that's what i was thinking, thanks for helping out so fast =]