• Let's look at an example.

Understanding the while Loop

• The following looping application simulates the play of an exciting game

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public static void main(String[] args) 
{
    Scanner input = new Scanner(System.in);
    String repeat = "y";
    while (repeat.equals("y"))
   {
        System.out.println("\nPlaying an exciting game!");
        System.out.print("Do you want to play again? (y/n): ");
        repeat = input.next();
    }
    System.out.println("\nThanks for playing!");
}

• Notice the structure of the code for the looping application
• First was the statement to correctly initialize the looping condition:

  String repeat = "y";

• Then came the while statement with the test condition

  while (repeat.equals("y"))

• The loop executes the body of the loop if and only if the condition evaluates to true
• Another important part of the loop is the statement: 
  

• Most loops have these parts:
  • Initialization code
  • Loop test condition
  • Loop body with some way to change the test condition
• Question: What would happen if I replaced the while loop with an if statement?
• We will explore the while loop more in the next exercise.

Activity 10.1: Counting Down... (20pts Total)

• Open up Eclipse and create a new Java project named Countdown inside of a project folder named Activity10
  
• Imagine we are at NASA Mission Control.
• A new space ship is about to be launched.
  
• Let's write a program that counts down until take off. See the sample output below:

• We are going to write this program in two ways.

Part 1: Repetition, Repetition, Repetition (10 pts)

• The first program should use 13 System.out statements to achieve the above output.
• Add your name and section in a block comment at the start of the program.
  

• Declare an integer variable called countdown at the top of main and set it equal to 10.
  
• Now, write a System.out statement to output the following:

• Add another System.out statement to print the following to the console:

• Next, in a pair of statements, lets print the contents of countdown and then decrease its value by 1.
• Add the following two statements to your code:

• You will need to repeat these two lines of code 10 times in your program.
• Copy the two lines and then paste them into your program 9 more times.
• You should have a very repetitive program by now!
• Finally, add a System.out statement to print out "We have liftoff!" below the numbers.
• Run your code to make sure that your output is identical to my output above.
• When you are finished and the program is working properly, upload it to Canvas.

• Let's alter the above program to be written more efficiently with a loop.
• First, take a look at your code.
• Which part of the code is repetitive?
• Hint: The repetitive lines will go inside the while loop.
• We need a while loop to count down from the number entered by the user. Create a while loop that looks like this:
  

• What should go inside the test?

• Why do you use > 0 here?
  
• Inside the curly braces of the while loop, we need to print out the contents of the countdown variable and then decrease its value by 1.
• Take one of your pairs of System.out statement and variable decrement and place it in the curly braces of the while loop.
• Now, you can erase the other 9 pairs of statements.
• Finally, below your while loop, there should now be only one System.out statement
• Run your program to make sure it is working the same as before.
• When you are finished, upload this new version to Canvas.

About those Curly Braces

• Like the if-statement, the curly braces of a while loop are optional
• Technically, the while statement affects only the single statement that follows
• We use curly braces to make that one statement into a block of statements
• This allows us to put any number of statements within the body
• Curly braces are not always required, but the best practice is to always include them

Program Style: Indent Statements Inside a Loop

• It is important to format the loop code correctly:

  String repeat = "y";
  while (repeat.equals("y")) {
      // ... statements to repeat
      repeat = input.next();
  }

Also Accepted Style:

• Note how the repeated code is indented inside the loop
• This lets us see easily which code is repeated and which is not
• Also note the placement of curly braces
• Different groups have different practices for placing curly braces in a loop statement

Missing Curly Braces

• The curly braces of a while loop let us associate multiple statements with a while loop
• However, if we leave off the curly braces, only the first line of code after the while statement is interpreted to be part of the loop.
• For example, what is wrong with the following code?
  

public static void main(String args[]) {

Scanner input = new Scanner(System.in);
String repeat = "y";
while (repeat.equals("y"))

System.out.println("\nPlaying an exciting game!");
System.out.println("Do you want to play again? (y/n) ");
repeat = input.next();

System.out.println("\nThanks for playing!");

}

Empty Statements

• Remember that statements are terminated by a semicolon
• Is the following a legal statement?;
• This is known as an empty or null statement
• Empty statements are a common source of infinite loops
• For example, what is wrong with the following code?

public static void main(String args[]) {

Scanner input = new Scanner(System.in);
String repeat = "y";
while (repeat.equals("y")) ;

System.out.println("\nPlaying an exciting game!");
System.out.println("Do you want to play again? (y/n) ");
repeat = input.next();
System.out.println("\nThanks for playing!");

}

}

4. Applications of While Loops

• Let's look at 4 common loop applications:

Indefinite Loops

image source

• An indefinite loop is one where you don't know when the loop will end.
• The loop will end (unlike an infinite loop), but you, the programmer, cannot predict exactly how many times it will run (unlike a counting loop)
  
• Instead, it is usually up to the user to decide when to end the loop by pressing a particular key
  
• Recall the example looping application that simulated the play of an exciting game

public static void main(String args[]) {

Scanner input = new Scanner(System.in);
String repeat = "y";
while (repeat.equals("y")){

System.out.println("\nPlaying an exciting game!");
System.out.println("Do you want to play again? (y/n) ");
repeat = input.next();

}

System.out.println("\nThanks for playing!");

}

• Loops of this type are called indefinite loops because you do not know in advance how many time the loop will execute
• This behavior is different from a counting loop where you know how many times the loop will execute before the loop starts
• With an indefinite loop we can solve a new set of problems
• Most problems solved with indefinite loops make use of while statements

Processing a Sequence of Inputs

• Another common use for indefinite loops is to process a sequence of inputs
• As an example, let us add up (sum) a series of numbers
• Every number is added to the sum
• We use a loop to repeat the input until the user decides to stop
• Since we do not know how many number the user will enter, we use an indefinite loop as shown below

image source

Program Processing a Sequence of Numerical Inputs

public static void main(String[] args) {

double number = 1;

double sum = 0;

String repeat = "y";
Scanner input = new Scanner(System.in);
System.out.println("I will add up numbers for you\n");

while (repeat.equals("y")) {

System.out.println("So far, sum = " + sum);

System.out.print("Enter a number: ");

number = input.nextDouble();

sum = sum + number;

System.out.print("Another number? (y/n) ");

repeat = input.next();

}

System.out.println("Ending sum: " + sum);

}

Terminating the Input with a Sentinel

Whenever we read a sequence of input values, we need to have some way of terminating the input loop
We could use a separate variable and input statement as we have done before:

String repeat = "y";

while (repeat.equals("y")) {

// ... statements to repeat

repeat = input.next();

}

image source

• However, when entering numbers (or other data) repeatedly, answering an extra question each time through the loop becomes annoying
• One way to avoid asking an extra question is to use a sentinel value
• A sentinel is guard who watches for something to happen
• Similarly, a sentinel in a program watches for a specific sentinel value that signals termination of a loop
• To use a sentinel value, we must have a special value in the input
• Some commonly used sentinel values for numeric input are 0 or -1
• However, if our application suggests another value, then we should use that value
• The following program is an update of the previous program to use a sentinel value to end the loop

Example Application Using a Sentinel Value for the Loop Test

public static void main(String[] args) {

double number = 1;

double sum = 0;
Scanner input = new Scanner(System.in);
System.out.println("I will add up numbers for you\n");

while (number != 0) {

System.out.println("So far, sum = " + sum);

System.out.print("Enter a number or 0 to exit: ");

number = input.nextDouble(); sum = sum + number;

}

System.out.println("\nEnding sum: " + sum);

}

Input Validation

• Another common use for indefinite loops is input validation
• Input validation combines a loop with one or more if statements
• The input statement is placed inside the loop
• The if-statement tests for an incorrect input value
• The loop repeats while the user's input contains an error
• Since we do not know how many times the loop must execute ahead of time, the loop is indefinite
• For example, the following program uses a loop to ensure a user enters a positive number
• The if statement is used to decide when to output an error message
• Example of Input Validation Using a Loop

image source

public static void main(String[] args) {

double number = 0.0; // initialize value
Scanner input = new Scanner(System.in); 

while (number <= 0) { System.out.print("Enter a positive number: ");

number = input.nextDouble();

if (number <= 0.0) {

System.out.println("You must enter a positive number\n");

}

}

System.out.println("\nYou entered: " + number);

}


We will explore all of these applications in our programs as we continue to write loops.

Activity 10.2: Guessing Game The Final Round (10 pts)

• Copy the following program into a class named Loopy and then compile and run the starter program to make sure you copied it correctly.

import java.util.Scanner;

public class Loopy {

public static void main(String[] args) {

Scanner input = new Scanner(System.in);

int guess = 0;

System.out.print("I'm thinking of a number between"

+ " 1 and 10.\nCan you guess it?\n\n"

+ "Enter your guess: ");

guess = input.nextInt();

if (7 == guess) {

System.out.println("*** Correct! ***\n");

} else {

System.out.println("Sorry, that is not correct.");

System.out.println("Try again.");

}

}

}

• Add the following code after the statement where int guess is declared:

String repeat = "y";

• This is the initialization code that we will use for the test condition that comes next.
• We want to repeat all the rest of the code in our program.
  
• For this we need to add a while statement such as:
  

while (repeat.equalsIgnoreCase("y")) {

// Place the code after the initialization statements

// and before the end of main between these curly braces.

}

• Statements inside the curly braces repeat while the test condition in the parenthesis, (repeat.equalsIgnoreCase("y")), evaluates to true.
• Inside the while loop we need some way to change the test condition.
  
• We change the test condition by letting the user enter a value for the repeat variable by adding the following code at the end of the loop just before the closing curly brace:
  

System.out.print("\nDo you want to play again? (y/n) ");

repeat = input.next();

• Without these two statements our loop would have no way to exit. A loop with no way to exit is known as an infinite loop.
• Formatting a loop is important. Indent all the code within the curly braces of the while loop. 
• As a final part of our program, we add the infamous phrase: "Game Over".
  
• Add the following statement after the closing curly brace of the while loop:
  

System.out.println("Game over");

• Compile and run your program again and verify the output looks like:

I'm thinking of a number between 1 and 10.

Can you guess it?

Enter your guess: 3

Sorry, that is not correct. Try again.

Do you want to play again? (y/n) y

I'm thinking of a number between 1 and 10.

Can you guess it?

Enter your guess: 7

*** Correct! ***

Do you want to play again? (y/n) n

Game over

• Submit your program source code to Canvas.

Completed Program

• When finished, your application should look like the following.
  
• Note especially the extra indentation within the curly braces of the while loop and the if-else statements.

• What is the difference between an infinite loop and an indefinite loop