Wrapping Up Conditionals: Switch Statements

• The switch statement gives us an alternative to an if-else-if chain
• Executes a section of code depending on value of a variable
• The general syntax is:

  switch (expression) {
     case label1:
        statements
        break;
     case label2:
        statements
        break;
     ...
     case labeln:
        statements
        break;
     default:
        statements
  }
  

• Where:
  • expression: a value you want to match
    
  • label_x: a numeric constant
  • statements: the statements to execute when the condition is met
• Any number of case labels can be placed in any order
• Any value that does not match starts executing with the statement after default
• Execution continues until the end of the switch statement or a break statement
• The break statement causes an immediate exit from the switch statement
• Just as case identifies possible starting points, break determines end points

Example of a switch Statement:

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();
	}

• Notice that each case other than the last contains a break statement
• Ensures that the switch statement is exited after a matching case is found

When to Use switch Statements

• Switch statements work for exact matches ONLY
  
• They can be used with a limited number of data types in Java:
  
• byte, short, char, and int primitive data types.
  
• It also works with the String class
• A few special classes that wrap certain primitive types, which you will learn about in 36B: Character, Byte, Short, and Integer

• The below example highlights the problem of only being able to look for an exact match
• For example, we are unable to test for scores using >=
  
• Thus, we have to use a clever trick - dividing the score by 10 to retrieve the first digit(s) (1-10)
  

Another example of switch statements involving ints

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");
        }
    }

• Because they can only be used for exact matches (== or .equals()), switch statements are inherently less useful than if-else statements (which can involve test conditions using all 6 comparison operators, including < and >)
  
• Also, the syntax is no clearer than if-else statements
• Note that there is a reason for the limitations of the switch statement
• Many years ago a compiler could generate more efficient code (using jump tables or binary searches) only within the limitations of the switch statement
• However, modern compilers are quite capable of optimizing if-else statements to the same degree
• Thus, we have no reason to ever use a switch statement (depending on your compiler)
• On the other hand, there are reasons to avoid using a switch statement
• Every branch of the switch statement must be terminated by a break statement
• If the break statement is missing, the program falls through and executes the next case without testing
• There are rare uses for this fall through behavior, such as printing the words for the song, The Twelve Days of Christmas
• However, according to a study by Peter van der Linden, reported in his book, Expert C Programming, p. 38, the falling through behavior is needed less than 3% of the time
• Forgetting to type the break statement is a very common error and the source of many bugs
• Therefore, switch statements are less useful than if statements and potentially more likely to cause bugs in your code.
  

Example of Fall Through:

    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":
                
             case "Tuesday":
                 
             case "Wednesday":
                 
             case "Thursday":
                 typeOfDay = "Weekday";
                 break;
             case "Friday":
                 typeOfDay = "Hurray! End of work week!";
                 break;
             case "Saturday":

             case "Sunday":
                 typeOfDay = "Weekend";
                 break;
             default:
                 System.out.println("Error: Invalid day of the week");
                
         }
         System.out.println(typeOfDay);
         input.close();
    }

• Find a partner for pair programming
  
• Open up one partner's Assignment 9.2: Letter Grades from Canvas
• Revise this program to use switch statements, rather than if statements.
• Note that you only need to handle the cases of A, B, C, D, and F for this activity (no + or - signs)
  
• For example, you will need cases that look like this:

switch(grade) {

    case "A":

        System.out.println("Numeric value: 4.0");

        break;

    //add more cases here

}

Introducing Loops

• In your daily life, there are many actions that you repeat until some condition is met.
• For example: 
• You might run on the treadmill at the gym until 30 minutes are up.
• You might wash dishes until the pile of dirty dishes is gone.
• You might take classes every quarter until you receive your degree.
• Can you think of any others?
• When we write code, often times we find ourselves repeating statements.
• Loops allow us to carry out repetitive steps in our code without having to re-write the same statements over and over.
• These loops will run until some test condition is met, just like the examples above.
• You wrote your first loops when you did your Hour of Code lab assignment.
• When did you need loops in this assignment?

Simple Loops

• A loop is a block of code that can execute repeatedly
• Whether or not a program repeats a block of code is determined by a test condition
• The test condition is checked each time the loop executes
• In general, loops have three components:
  • Initialization code
  • Test condition -- evaluated during the loop
  • Loop body with some way to change the test condition
• There are three loop statements in Java:
  • for
  • while
  • do...while
• We will start with the simplest of the three -- the while statement

initialization statement;

while (test){   

    statements to repeat
}

String repeat = "y";
while (repeat.equals("y")) {

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

repeat = input.next();

}

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import java.util.Scanner;

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!");
}

NASA Mission Control readying for liftoff.

Initializing countdown from 10...

10

9

8

7

6

5

4

3

2

1

We have liftoff!

System.out.println(countdown);

countdown--;

Wrap Up

• With your partner, answer the questions from today's learning objectives

Upcoming Assignments

• Assignment 11 due Thursday
  

• Lab 6 due Friday

• Quiz 5 due Friday