lesson11

Welcome to Lesson 11!

Learning Objectives

By the end of today's class, you should know...

What are some pitfalls you might encounter when using logical operators and how can you avoid them
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"?
What is a loop?
What is a while loop?
What is the syntax of a while loop?
How does the syntax of a while loop compare to an if statement?

Announcements

No Lab due Friday due to holiday
Midterms returned Tuesday
Next quiz one week from today

Review Activity

With a partner: Write three equivalent statements to the one below, using the shortcuts described in class:

x = x + 1;

With a partner: Write one statement to print the following value to the console to 5 decimal places:

final double PI = 3.1415927;

Imagine that a local radio station is holding a contest to give away a year's supply of free pet food to one lucky winner.
To win the contest, you must meet the following criteria:

You must be caller 19
The pet your own must be one of the following: dog, cat, rabbit
You must be between the ages of 18 and 65.

Which of the following test conditions will correctly determine the winner of the contest

a. if (numCaller == 19 && (pet == "rabbit" || pet == "cat" || pet == "dog") && (age >= 18 && age <= 65))

System.out.println("You are the winner!");

b. if (numCaller == 19 && (pet == "rabbit" || "cat" || "dog") && (age >= 18 && <= 65))
System.out.println("You are the winner!");

c. if (numCaller == 19 || (pet.equals("rabbit") && pet.equals("cat") && pet.equals("dog")) || (age >= 18 && age <= 65))
System.out.println("You are the winner!");

d. if (numCaller == 19 && (pet.equals("rabbit") || pet.equals("cat") || pet.equals("dog")) && (age >= 18 && age <= 65))
System.out.println("You are the winner!");

e. if (numCaller = 19 && (pet.equals("rabbit") || pet.equals("cat") || pet.equals("dog") && (age >= 18) || age <= 65))
System.out.println("You are the winner!");

What will the following print to the console?

int age = 20;

boolean is_student = true;

if (is_student || age > 21) {

System.out.println("Fi!");

}

if (!is_student && age < 21) {

System.out.println("Fo!");

}

if (is_student && age < 21) {

System.out.println("Fum!");

}

Conditional Pitfalls

Below are some common mistakes when using logical operators.
Fortunately, Java gives you an error message when you make the below mistakes.

Using = Instead of ==

One common mistake is to use = when you meant to use ==

For example, look at the test condition in the following code:

if (guess = 7) {
    System.out.println("*** Correct! ***");
} else {
    System.out.println("Sorry, that is not correct.");
}

Fortunately, if you make this mistake, the Java compiler will not allow your code to run.

However, if you correctly use == then your code will compile
```
if (7 == guess) {
```

Strings of Inequalities

Do NOT use a string of inequalities like the following:

int a = 5, b = 1, c = 10;
if (a < b < c) {
    System.out.println("b is between a and c");
} else {
    System.out.println("b is NOT between a and c");
}

Again, your code will not compile

Instead, the correct way is to use && as follows:

int a = 5, b = 1, c = 10;
if (a < b && b < c) {
    System.out.println("b is between a and c");
} else {
    System.out.println("b is NOT between a and c");
}

Strings of Logical Operators

Logical expressions often read like "normal" English.
However, Java requires more exactness than English

For example, the following code will compile and run but give wrong results:

int guess;
System.out.print("Enter a guess: ");
guess = input.nextInt();
if (guess == 7 || 8) {
    System.out.println("*** Correct! ***");
} else {
    System.out.println("Sorry, that is not correct.");
}

The above produces an error when you try to compile your code.

Instead, the correct way is to use || as follows:

int guess;
System.out.print("Enter a guess: ");
guess = input.nextInt();
if (guess == 7 || guess == 8) {
    System.out.println("*** Correct! ***");
} else {
    System.out.println("Sorry, that is not correct.");
}

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 any type in Java, including ints, doubles, chars, and Strings.
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");
        }
    }

The Case Against Switch Statements

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();
    }

Activity 11.1: Letter Grades Again (10 pts)

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.
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

}

When you are finished revising your program, both partners should submit it to Canvas.

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

While Loops

The simplest looping structure is the while statement
A while statement has a test condition and a body, like an if statement
Before each execution of the loop body, the while statement checks the test condition
If the test condition evaluates to true, then our program executes the body of the loop
When the program reaches the end of the loop body, it automatically returns to the while statement
Here is the syntax of a while loop:

initialization statement;

while (test){

statements to repeat
}

For example:

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

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

repeat = input.next();
}

What does the while loop syntax remind you of?
The following flow chart shows how the while loop operates

Diagram of while Loop Operation

Image source.

Let's look at an example.

Understanding the `while` Loop

The following looping application simulates the play of an exciting game

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!");
}

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:

repeat = input.next();

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.

Group Activity: Candy Crush Loop

As a class, we are going to act out the following while loop:

While Loop :

int num_pieces_candy = 12;
instructor.fill_with_candy(Bag, num_pieces_candy);
instructor.hand_bag(first_student);
while (num_pieces_candy > 0)
{
student.takeOnePieceCandy();
student.passBag(next_student);
num_pieces_candy--;

}

With your partner, discuss for the while loop above:

What is/are the initialization statement(s)?
What is the loop test condition?
What are the statements that repeat?
Which statement will ultimately cause the test condition to fail?

Now, let's act out the While Loop!

Activity 11.2: Counting Down...

Grab a partner for pair programming.
Open up Eclipse and create a new Java project named Countdown.
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:

NASA Mission Control readying for liftoff.
Initializing countdown from 10...
10
9
8
7
6
5
4
3
2
1
We have liftoff!

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 names and section in the block comments.

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:

NASA Mission Control readying for liftoff.

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

Initializing countdown from 10...

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:

System.out.println(countdown);
countdown--;

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.

Part 2: Using a While Loop (10 pts)

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:

while (test will go here) {

//statements
}

What should go inside the test?

countdown > 0

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:

String repeat = "y";

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

// ... statements to repeat

repeat = input.next();
}

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

Wrap Up

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

Upcoming Assignments

Assignment 11 due Tuesday at 11:20am on Canvas

No Lab due Friday

De Anza College CIS

Navigation

Review Activity

What will the following print to the console?

Conditional Pitfalls

Using = Instead of ==

Strings of Inequalities

Strings of Logical Operators

Wrapping Up Conditionals: Switch Statements

Example of a `switch` Statement:

When to Use `switch` Statements

Introducing Loops

Simple Loops

Diagram of `while` Loop Operation

Let's look at an example.

Understanding the `while` Loop

Group Activity: Candy Crush Loop

About those Curly Braces

Program Style: Indent Statements Inside a Loop

Wrap Up

Upcoming Assignments

Assignment 11 due Tuesday at 11:20am on Canvas

No Lab due Friday

lesson11

Review Activity

What will the following print to the console?

Conditional Pitfalls

Using = Instead of ==

Strings of Inequalities

Strings of Logical Operators

Wrapping Up Conditionals: Switch Statements

Example of a switch Statement:

When to Use switch Statements

Introducing Loops

Simple Loops

Diagram of while Loop Operation

Let's look at an example.

Understanding the while Loop

Group Activity: Candy Crush Loop

About those Curly Braces

Program Style: Indent Statements Inside a Loop

Wrap Up

Upcoming Assignments

Assignment 11 due Tuesday at 11:20am on Canvas

No Lab due Friday

Example of a `switch` Statement:

When to Use `switch` Statements

Diagram of `while` Loop Operation

Understanding the `while` Loop