The Increment and Decrement Operators

number++;
number--;
++number;
--number;

The while loop

while (condition)
{
    // statements;
}

• Statements will execute repeatedly while condition is true
• The while loop is a pretest loop, which means that it’ll test the value of the condition before executing the loop.
  • So, a while loop executes 0 or more times.
• Loops that don’t end are called infinite loops

Using the while Loop for Input Validation

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();
}

Mixing Calls to nextLine with Calls to Other Scanner Methods

String’s nextInt, nextDouble and next methods don’t consume the newline character, causing the newline to be consumed by whatever reads System.in next.

• This can cause the program to skip over nextLine and other methods than consume newlines

E.g.,

Scanner scanner = new Scanner(System.in);
System.out.println("Enter your name: ");
String name = scanner.nextLine();
System.out.println("Enter your age: ");
int age = scanner.nextInt();
System.out.println("Enter your city: ");
String city = scanner.nextLine();
System.out.println("Hi " + name + ", your age is " + age + " and your city is " + city);

• Output: Hi John, your age is 10 and your city is

Fix:

Scanner scanner = new Scanner(System.in);
System.out.println("Enter your name: ");
String name = scanner.nextLine();
System.out.println("Enter your age: ");
int age = scanner.nextInt();
scanner.nextLine(); // Consume newline
System.out.println("Enter your city: ");
String city = scanner.nextLine();
System.out.println("Hi " + name + ", your age is " + age + " and your city is " + city);

• Output: Hi John, your age is 10 and your city is Pomona

The do-while Loop

do
{
    // statements;
}
while (condition);

• do-while is a posttest loop, which means it’ll execute the loop before testing the condition.
  • This means a do-while loop will always perform at least one iteration.

The for Loop

for (initialization; test; update)
{
    // statement
}

1. Perform initialization expression (done only once)
2. Evaluate the test expression. If it is false, terminate the loop
3. Execute the body of the loop
4. Perform update expression, then go back to step 2

Note: The for loop is a pre-test loop

Two Categories of Loops:

• Conditional Loops: Executes as long as a particular condition exists
  • e.g., while, do-while
• Count-Controlled Loops: Loop that repeats a specific number of times.

Running Totals and Sentinel Values

• Running Total: Sum of numbers that accumulate with each iteration of a loop.
  • Can be used to calculate the total of a series of numbers
  • Accumulator: Variable used to keep the running total.
• Sentinel Value: Used to notify program to stop acquiring input
  • Used because the end point of input data isn’t always known
    • e.g., user prompts where the user types “STOP” to exit a loop, end-of-file markers for file input

double runningTotalSales = 0.0; // Accumulator (Running Total)
double sales = 0.0;
while (sales != -1) // Sentinel value
{
    totalSales += sales;
    sales = scanner.nextDouble();
}

Nested Loops

• Loops can be nested
  • Inner loop will execute all of its iterations each time the outer loop executes once.

The break and continue Statements

• break can be used to terminate a loop
• continue will cause the currently executing iteration of a loop to terminate and the next iteration to begin

Important: break and continue bypass normal mechanisms and can make code hard to read & maintain—use only when needed

Deciding Which Loop to Use

while:

• Pretest

do-while

• Post-test

for:

• Pretest
• Prefer over a while if there is some type of counting variable involved

Generating Random Numbers with the Random class

import java.util.Random;
Random randomNumbers = new Random();

• nextDouble(): Return random double within 0.0 and 1.0
• nextFloat: Returns random float within 0.0 and 1.0
• nextInt: Returns random int within range of int (-2,147,483,648, 2,147,483,648)
• nextInt(int n): Returns random int with range of 0 to n-1
• nextLong(): Returns random long within range of long (-9,223,372,036,854,775,808, 9,223,372,036,854,775,808)