Numbers, Operators and Precision Cont'd

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;
  

Activity 10.1: 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 at the top of main where you declare your variables.
  

  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