outline

Loops

• 
  Allows certain parts of code to repeat based on the status of a condition
  
  The while statement.
  The while statement checks the condition and repeats the code if the condition is TRUE.

Sentry variables and Initialization

• Start by creating a variable
• Giving the variable an initial value is initializing the variable
• Variables used in loops are often known as sentry variables

Example

• //Note that I initialized stillRacing incorrectly
    var stillRacing = "N";
  
  //This condition will only be checked once, and it will be false
    while (stillRacing == "Y"){
     //the body of this loop will NEVER be activated
     alert ("zooming around the track.");
     //see if we're still on
     stillRacing = prompt("Is the race still on? (Y/N)");
    }//end while
  
  alert ("Nice race!!");

Preventing Endless Loops

• Inside a loop, we MUST ensure that there is some way of exiting the loop
• We have to make it possible for the sentry variable to change values

Example

•  var stillRacing = "Y";
  
   while (stillRacing == "Y"){
     alert("zooming around the track");
  
     //forgot to check if we are still racing!!!
  
   }// end while
   alert ("Nice race!!");
  
• This program will go on FOREVER!!!

Endless Loops

• The condition statement checks to see if the condition is TRUE
• In the previous example, the condition is true
• Because the condition is true, the body of the loop still thinks we are "zooming around the track"
• Whenever you make a loop, it is your obligation to ensure that the sentry variable is changed so it is possible to exit the loop

The For Loop

• Variation of the loop we have already used
• Difference - we have added a counter
• Needs four pieces of information:
   - name of counting variable
   - starting value of the variable
   - ending value of the variable
   - how to increment the variable

For Loop

• The for statement needs three parts:
   1.  initialize the variable
   2.  a condition
   3.  how to increment the variable
• Once the loop is set up, we don't have to worry about it in the body - it automatically increments

Example

• var lap = 1;
  
  for (lap = 1; lap <= 10; lap ++){
     alert ("Now on lap " + lap);
  }// end for
  
  alert ("Good race!!");