Welcome to Lesson 10!

Learning Objectives

• What is a switch statement?
• How does a switch statement compare to an if statement?
• What is the syntax of a switch statement?
• What is "fall through"?
• How do you format decimal numbers for printing
  
• What is a final and how do you declare one?
• What is are magic numbers and why should they be avoided?
• What are the short cut assignment operators for add, subtract, multiply, divide, and modulus?
• What are the increment and decrement operators?
• What is the difference between pre and post increment and decrement?
• What is casting?
• How do you cast an integer to a double and a double to an integer?

Announcements

• Return Midterms in last 5 minutes of class - excellent results!
• As: 23 (2 100% scores)
• Bs: 9
• Cs: 3
• Ds: 3
• Fs: 4
• No Shows: 2
• If you are in the C, D, or F range something needs to change about your approach
• You can still turn it around!
• Are you doing the homework?
• Are you paying attention in class? Move to the front.
  
• Are you getting help when you are stuck or something is unclear?
• The class is only going to get harder from here!
  

• Quiz 5 and Lab 5 due on Friday
• Assignment 10 due Thursday
  

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public static void main(String[] args) {
    Scanner input = new Scanner(System.in);

    System.out.print("Enter a magnitude on the Richter scale: ");
    double richter = input.nextDouble();

    if (richter >= 8.0) {
        System.out.println("Most structures fall");
    } else if (richter >= 7.0) {
        System.out.println("Many buildings destroyed");
    } else if (richter >= 6.0) {
        System.out.println("Many buildings considerably damaged, some collapse");
    } else if (richter >= 4.5) {
        System.out.println("Damage to poorly constructed buildings");
    } else if (richter >= 3.5) {
        System.out.println("Felt by many people, no destruction");
    } else if (richter >= 0) {
        System.out.println("Generally not felt by people");
    } else {
        System.out.println("Negative numbers are not valid");
    }

}

public static void main(String[] args) {

	     Scanner input = new Scanner(System.in);
	     String typeOfDay="";

	     System.out.print("Enter a day of the week: ");
	     String dayOfWeek = input.next();

	     switch (dayOfWeek) {
	         case "Monday":
	             typeOfDay = "Weekday";
	             break;
	         case "Tuesday":
	              typeOfDay = "Weekday";
	               break;
	         case "Wednesday":
	              typeOfDay = "Weekday";
	              break;
	         case "Thursday":
	             typeOfDay = "Weekday";
	             break;
	         case "Friday":
	             typeOfDay = "Hurray! End of work week!";
	             break;
	         case "Saturday":
	             typeOfDay = "Weekend";
	             break;
	         case "Sunday":
	             typeOfDay = "Weekend";
	             break;
	         default:
	             System.out.println("Error: Invalid day of the week");
	             
	     }
	     System.out.println(typeOfDay);
	     input.close();
	}

public static void main(String[] args) {
        Scanner input  = new Scanner(System.in);

        System.out.print("I show a grade based on a"
            + " score.\nEnter your score: ");
        int score = input.nextInt();

        switch (score / 10) { //integer division to convert score to an int from 1 to 10
            case 10:
            case 9:
                System.out.println("You got an A");
                break;
            case 8:
                System.out.println("You got a B");
                break;
            case 7:
                System.out.println("You got a C");
                break;
            case 6:
                System.out.println("You got a D");
                break;
            default:
                System.out.println("Sorry, an F");
        }
    }

Numbers, Formatting and More About Operators

Decimal Formatting

• Sometimes programs may not display numbers as you would expect!
• Consider the following program and what it will display:

  public static void main(String[] args) {
      double price = 78.50;
      System.out.println("The price is $" + price);
  }
  

• We must explicitly tell Java how to output numbers in our programs!
• These commands do not produce any output but change how System.out outputs floating-point numbers
• "Magic Formula" to force decimal places:

  System.out.printf("%.2f", value_to_print); 
  

• % is the place holder representing where you wish to place the value to print inside your message
• .2f indicates that the floating point value should be printed to 2 decimal places
  
• Once we set the decimal formatting, it stays set.

public static void main(String[] args) {

    double price = 78.50;
    System.out.printf("The price is $%.2f", price);
}

• For more information about using printf for numerical output:
• Oracle documentation
  

Final Variables

• A final variable is a variable that cannot change after being assigned a value
• To declare a constant, we use the keyword: final

  final int my_final = 1;

• Note that we must assign a value when the final is declared

Magic Numbers

• Imagine that you are a programmer hired to modify a payroll program
• You come across the following section of code:

  double pay;
  pay = hours * 7.5 + (hours / 40)
        * (hours - 40) * 7.5 * 0.5;
  

• The numbers are important to the program, but what do they mean?
• Numbers like these are called Magic Numbers.
  
• They are magic because the value or presence is unexplainable without more knowledge
  • Often, no one knows what they mean after 3 months, including the author
• A programmer can often infer the meaning of numbers after reading the code carefully
• A much better coding style is to use named final variables rather than literal numbers
• For example:

  final double wage = 7.5;
  final double over_time_adder = 0.5;
  final int hours_per_week = 40;
  double pay;
  pay = hours * wage + (hours / hours_per_week)
        * (hours - hours_per_week) * wage * over_time_adder;
  

• Now it is much easier to understand the code and see any problems or limitations in it
• Another reason to use named final variables is that it is easier to change the value of the number
• In the above example, we can easily change the wage without making errors in other parts of our code

More Information

• Magic number (programming): about magic numbers in code from Wikipedia

Assignment Operators

• As we discussed before, we assign values to variables using an equal (=) sign

  int sum = 0;

• However, the equal sign is really an assignment operator and does not denote equality
• Thus, unlike math, we can have the same variable on both sides of an equals sign:

  int sum = 25;    // initialize sum to 25
  sum = sum + 10;  // add to sum
  

• Note that the value of the variable is changed in the second line
• Reading variables from memory does not change them
• Values placed into a variable replace (overwrite) previous values:
  
  
  

Shortcut Assignment Operators

• We can use additional operators to calculate values and assign them to the variable on the left all in one statement
  • Known as shortcut assignment operators
• The general syntax is:

  variable op= expression;

• Where op is one of the five arithmetic operators: +, -, *, /, %
• For example, the following two statements create the same result:

  x = x + 3;
  x += 3;
  

• Shown below are the assignment operators with examples of how they are used:

Summary of Assignment Operators

Increment and Decrement Operators

• Adding or subtracting one is a common operation in programming
• Java provides arithmetic shortcuts for these operations with the increment and decrement operators
• The increment operator (++) adds 1 to a variable's value
• Pre-increment adds 1 before evaluating an expression

  ++sum;

• Post-increment evaluates the expression and then adds 1

  sum++;

• The decrement operator works like the increments operator, except it subtracts 1 from the variable:

  --sum
  sum--
  

• Pre- and post- increment matters when the operation is part of a larger expression
• For example, consider the code:

  int x = 5;
  int y = x++;
  System.out.println("x=" + x + " y=" + y);
  

• We may expect y to be 6 after this code executes
• Instead, y has the value of 5
• The reason is that ++ after a variable (post-increment) is equivalent to:

  y = x;
  x = x + 1;
  

• On the other hand, ++ before a variable (pre-increment) is equivalent to:

  x = x + 1;
  y = x;
  

Casting

Activity 10.2: Prices (10pts)

• Find a partner for pair programming
• Open up Eclipse, create a new Java project named Price
• Copy the following program into a class file named Price.java:
  

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  public static void main(String[] args) { String name; double price = 0;   Scanner input = new Scanner(System.in); System.out.print("Enter the product name: "); name = input.nextLine();   System.out.print("Price of the " + name + ": "); price = input.nextDouble(); // Insert new statements here System.out.println("Total price: $" + price); }

• Alter the block comments to include your name and section information to the top of the program.
• Compile and run the starter program to make sure you entered it correctly.

  When you run the program, the output should look like this:

  Enter the product name: iPod Nano
  Price of the iPod Nano: 149.50
  Total price: $149.5
  

  Note the format of the numbers output for the total price. We will address this formatting issue later in the exercise.

• Run the program again for a product with a very high cost, like a Boeing 777:

  Enter the product name: Boeing 777
  Price of the Boeing 777: 212345678
  Total price: $2.12345678E8
  

  Note the format of the numbers output for the total price. This format is called exponential notation. You may have encountered it in some of your math classes.
  

• Let us correct the formatting of the total price. Alter the System.out statement that prints the total price:

  System.out.printf("Total price: $%.2f\n", price);
  

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

  Enter the product name: Boeing 777
  Price of the Boeing 777: 212345678
  Total price: $212345678.00
  

• Let us add a final variable that we will use later in our program. Enter the following code after the printf statement and before the statement that prints the total price:

  final int percent = 100;
  

  We are declaring this variable rather than using a magic number.
  

• Now we will add sales tax to the price of the product. Enter the following code after the percent variable and before the statement that prints the total price:

  double taxRate = 0;
  System.out.print("Enter sales tax rate (%): ");
  taxRate = input.nextDouble();
  double tax = price * taxRate / percent;
  price += tax;
  

  Notice the last statement: price += tax;. This is an alternate way to code the statement: price = price + tax;.

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

  Enter the product name: iPod nano
  Price of the iPod nano: 89.50
  Enter sales tax rate (%): 9.5
  Total price: $98.00
  

• Now we will find the whole dollars and cents of the amount to demonstrate casting. Enter the following code after the statement that prints the total price and before the closing curly brace of main:

  int dollars = (int) price;
  System.out.println("In whole dollars: $" + dollars);
  

  Notice the (int) in the first statement. This is known as a type cast or just cast. 
  

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

  Enter the product name: iPod nano
  Price of the iPod nano: 89.50
  Enter sales tax rate (%): 9.5
  Total price: $98.00
  In whole dollars: $98