Lesson 3.5 - The while Loop
Purpose: To learn how to use a conditional loop

Conditional Repetition  Many times we need repetition of unknown number in Java. A for loop is great for known repetition, but what if you don't know how many times you need to do something? That's where the while loop comes in.

Here is the format:

   while(condition)
   {
      list of statments...
   }

You will probably note that a while loop looks a lot like an if statement. In fact a while loop is an if statement that can repeat. In other words, it keeps asking the question until it gets an answer of false. So as long as the condition is true, it will continue to do the list of statements.

Time for an example:

Suppose we want to enter an unknown number of positive integers and then find the average of those integers. We can't use a for loop because we don't know how many numbers there are. We can use a while loop though, and have a negative value represent the signal to stop entering numbers. Here is how we will do it:

int number=0, sum=0, count=0;   
while(number>=0)
{
   System.out.println("Enter a positive integer: ");
   number = keybd.readInt();
   if(number>=0){count++; sum+=number;}
     
}
      double average = sum / (1.0 * count);

In this example, once a negative number is entered, the condition number>=0 becomes false and the loop is terminated. The computer always checks the condition first before entering the loop. If the condition is true, the loop is entered. If not, it skips to the next line of code that follows the while loop.

While in Place of For  If there were no for loop in Java, we could mimic its function with a well-crafted while loop. Take note of the following example:

   int count=1;   
   while(count<=10)
   {
      System.out.println(count);
      count++;      
   }

This while loop is equivalent to the following for loop:

   for(count=1; count<=10; count++) System.out.println(count);

Infinite Loops   It is possible to write a while loop that may not stop - meaning its condition never reaches a false state. Take note of the following example:

   int count=1;   
   while(count!=10)
   {
      System.out.println(count);
      count+=2;      
   }

The value of count starts at 1, then becomes 3, 5, 7, etc.... Thus count is always an odd number, making it impossible to equal 10. So the condition count!=10 will always be true. An always true condition makes this loop go on forever, causing a crash as the computer can only take so much of such nonsense! We call this phenomenon an infinite loop.


In closing, The while loop is useful in situations where repetition is needed, but the number of repetitions is unknown. This usually means that repetition should continue until a given condition is false. Lastly, be careful to ensure the condition can be false or you run the risk of an infinite loop, causing the program to crash