Lecture 5-2: `while` loops

After reading through Chapter 5, you should be able to

Use the ++ and -- operators
Explain the difference between prefix and postfix modes
Explain what a loop is
Write while loops
Write simple count-controlled loops
Use loops that validate user input
Use loops that calculate the running total of a series of numbers
Write loops that are terminated by a sentinel value
Write loops using do-while, and for
Use nested loops
Determine which loops are best for different circumstances
Write code that saves data to text files and reads data from text files

Today's Learning Objectives

Know the three type of looping constructs in Java
Know how to write loops in Java using while statements
Know how to validate user input using a loop

Loops

Loop statements allow us to execute a Java statement(s) multiple times repetitively. Like if statements, they are controlled by boolean expressions

Java has three kinds of loop statements:

while loop
do-while loop
for loop

The programmer must choose the right kind of loop for the situation

The `while` Statement

The while statement has the following syntax:

   while ( boolean expression ) 
   {

      statement(s); 
   }

while is a reserved word in Java.

The boolean expression inside the parenthesis is called the loop condition
The statement(s) inside the block are called the loop body

Each time the loop body is executed, the condition is checked. If the loop condition is true, the loop continues to execute. If the loop condition is false, the loop terminates.

The loop body is executed repetitively until the condition becomes false. One single cycle of the loop is called an iteration.

Simple count-controlled while loop

   int count;
   count = 1;

   while (count <= 5) 
   {

      System.out.println(count);
      count = count + 1;
   }

   System.out.println ("Done");

Infinite Loops

Care must be taken to set the condition to false somewhere in the loop so the loop will end.
Loops that do not end are called infinite loops.
A while loop executes 0 or more times since if the condition is false, the loop will not execute.

Error checking user input

while loops are often used to check for valid user input, looping repetitively until the user gets it right!

   int number;   
   Scanner keyboard = new Scanner (System.in);
      
   System.out.print("Enter a number in the "
                    + "range of 1 through 100: ");
   number = keyboard.nextInt();
   

// Validate the input
   while ((number < 1) || (number > 100))
   {

      System.out.println("That number is invalid.");
      System.out.print("Enter a number in the "
                       + "range of 1 through 100: ");
      number = keyboard.nextInt();
   }

Lecture 5-2: while loops