lesson8

Welcome to Lesson 8!

Learning Objectives

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

How do you test multiple conditions using logical operators?
How do && and || operate when:

both test conditions evaluate to true?
both test conditions evaluate to false?
one test condition evaluates to true and one to false?

What are some pitfalls you might encounter when using logical operators and how can you avoid them?

1. Lesson 7 Practice Midterm Question

What will the following output to the console?

String student1 = "Tanya";

String student2 = "Tasha";

if (student1.compareTo(student2) < 0) {

System.out.println("Alpha");

} else {

System.out.println("Beta");

}

2. Logical Operators

In certain situations, we may wish to use more than one test condition inside of an if statement.
To do so, we will need to chain the test conditions together using logical operators (either && or ||)
For example, recall our guessing game program:

       System.out.println("I'm thinking of a number between 1 and 10.");
    System.out.println("Can you guess it?\n");
    System.out.print("Enter your guess: ");

    guess = input.nextInt();

    if (guess < 1 || guess > 10) { //if guess is < 1 OR guess > 10
       System.out.println("Invalid entry!");
    } else if (guess < 7) {
       System.out.println("Your guess is too low");
    } else if (guess > 7) {
       System.out.println("Your guess is too high");
    } else {
        System.out.println("*** Correct! ***");
    }

Review of Boolean Variables

Sometimes we need to evaluate a logical condition in one part of a program and use it elsewhere
To store a condition that can only be true or false, we use a Boolean variable
Boolean variables are named after George Boole (1815-1864), a pioneer in the study of logic
We specify a Boolean variable using the boolean type, which can hold just one of two values: true or false
```
boolean isCool = true;
boolean lies = false; 
```

Test Conditions and Boolean Values

Remember that test conditions always evaluate to true or false
```
if (num >= 0)
```
Thus we can use a boolean variable as a test condition
```
boolean isPositive = (num >= 0);
if (isPositive)
```
Note that we do not need to add a relational expression to a boolean variable, like:
```
if (isPositive == true) // avoid!
```
Since the boolean variable already evaluates to true or false, adding the == true is redundant
Likewise, we do not need to use:
```
if (isPositive != false) // avoid!
```
If we want to reverse the test condition, we can use the not (!) operator
```
if (!isPositive)
```
We can see the use of a Boolean variable in the following example

Example Application Using a Boolean Variable

public static void main(String[] args)
{

    int num;
    Scanner input = new Scanner(System.in);
    System.out.print("Enter a number: ");
    num = input.nextInt();

    boolean isPositive = (num >= 0);
    System.out.println("The test evaluated to: " + isPositive);
    if (isPositive) {
        System.out.println("The number was 0 or positive");
    } else {
        System.out.println("The number was negative");
    }
}

Introducing &&, || and !

Logical operators and search engines video

Combining Test Conditions with Logical Operators

A logical operator, or Boolean operator, is an operator that treats operands as Boolean values (true or false)
Java has several logical operators, but we only need to use three to create any possible test condition
These three operators are and, or and not, which are discussed below
These logical operators are traditionally written as && (and), || (or) and ! (not)

Truth Tables for AND, OR and NOT

&& (AND) Operator Truth Table

expr1
expr2
expr1 && expr2
Example Result

true true true 5 < 10 && 5 > 2 true

true false false 5 < 10 && 5 < 2 false

false true false 5 > 10 && 5 > 2 false

false false false 5 > 10 && 5 < 2 false

|| (OR) Operator Truth Table

expr1
expr2
expr1 || expr2
Example Result

true true true 5 < 10 || 5 > 2 true

true false true 5 < 10 || 5 < 2 true

false true true 5 > 10 || 5 > 2 true

false false false 5 > 10 || 5 < 2 false

not (!) Operator Truth Table

If expr is... Then ! expr is... Example Result

true false !true false

false true !(5 < 2) true

Testing Multiple Conditions Using Logical Operators

Sometimes we need to test for multiple conditions using a single if statement
In this case, we will need to chain together the two conditions using a logical operator, either && or ||
Note that one either side of the logical operator there must be a stand alone test condition that resolves to true or false

if (age >= 18 && age <= 25) //Correct!!!

if (age >= 18 && <= 25) //No! Incorrect!!

For example, we want to test if an age is between 18 and 65
We need to test that both parts of are true: is the age >= 18 AND is the age <= 65

int age = 0;
System.out.print("Enter your age: ");
age = input.nextInt();
if (age >= 18 && age <= 65)
{
System.out.println("Adult!");
} else {
System.out.println("Child, Teen or Older Adult!");
}

Another way to use logical operators to test the age is:

int age = 0;
System.out.print("Enter your age: ");
age = input.nextInt();
if (age < 18 || age > 65)
{
    System.out.println("Child, Teen or Older Adult!");
} else {
    System.out.println("Adult!");
}

Many people confuse && and || conditions, especially when learning about logical operators
A value lies between 18 and 65 if the value is at least 18 and at most 65
A value is outside that range if it is less than (but not equal to) 18 or greater than (but not equal to) 65
There is no golden rule; we have to think carefully and test our conditions

Parenthesis

Remember that Boolean expressions in an if statement can be enclosed in parenthesis

Thus, an if statement with && might look like:

if ((guess != GUESS1) && (guess != GUESS2))

However, relational operators have a higher precedence than logical operators, so the extra parenthesis are not necessary
As such, we can remove the inner parenthesis without affecting the logical meaning:
```
if (guess != GUESS1 && guess != GUESS2)
```
However, if using parenthesis is easier to understand then use the extra parenthesis

3. 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.");
}

Activity 8.1: What's Your Generation? (10 pts)

There are 6 generations living in America, side-by-side, today.
Your program will determine to which generation a user belongs.
)pen up a new project and name it Activity8, and add a new class called Generation
You will need to take as input the year of his or her birth.
Then, you will need a series of test conditions (think if - else if - else) to determine the generation of your user.
You will also need to use logical operators (&&, ||, !).
Below is a chart with the range of birth years for each generation.
Note the double quotes around each generation's name. For full credit, you must include the " when you output the generation.

Years of Birth Generation

1900-1927 "The Greatest Generation"

1928-1945 "The Silents"

1946-1964 "The Baby Boomers"

1965-1979 "Generation X"

1980-1999 "The Millennial Generation"

2000-2020 "Generation Z"

Your goal is to prompt the user for his or her year of birth and then print out a message about which generation he or she belongs to.
To start, print the following message to the user:

What's your generation?

Then, you will need a variable to store the user's year of birth:

int year_of_birth;

Next, prompt your user to enter his or her year of birth with a statement like the following:

Please enter the year of your birth: _

Subsequently, you will need 6 if and if else statements like the following:

if ( year_of_birth >= 1900 && year_of_birth < 1928)
{
System.out.println("You belong to the \"Greatest Generation\"");
}
else if ( year_of_birth >= 1928 && year_of_birth < 1946)
{
System.out.println("You belong to the \"The Silents\"");
}

//rest of your else ifs and your else clause go here

Important: Why are we using && here and not ||?
Finally, you will need to do some error checking of the user input.

If the user inputs a date that is either too high or too low, your program must print out the following message:

Invalid entry. Please enter a birth year in the range 1900 - 2020.

The above should go in your else clause
Make sure your output is identical to the sample output below before you submit.
When you are finished, upload Generation.java to Canvas

What's your generation?

Please enter the year of your birth: 1926

You belong to the "Greatest Generation".

Alternately,

What's your generation?

Please enter the year of your birth: 2021

Invalid entry! Please enter a birth year in the range 1900 - 2020.

More Information on Logical Operators

Domino Addition - Numberphile: Logical operator circuits using dominoes

Wrap up

Question 1:

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!");

Question 2:

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!");

}

Upcoming Assignments

Activities 8.1 due Thursday at 11:59pm
Assignment 7 due Friday at 11:59pm
Quiz 4 due Friday at 11:59pm
Assignment 8 due Tuesday at 11:59pm

~Have a Great Weekend!~

Welcome to Lesson 8!

Learning Objectives

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

1. Lesson 7 Practice Midterm Question

2. Logical Operators

Review of Boolean Variables

Test Conditions and Boolean Values

Example Application Using a Boolean Variable

Introducing &&, || and !

Combining Test Conditions with Logical Operators

Truth Tables for AND, OR and NOT

Testing Multiple Conditions Using Logical Operators

Parenthesis

3. Conditional Pitfalls

Using = Instead of ==

Strings of Inequalities

Strings of Logical Operators

Activity 8.1: What's Your Generation? (10 pts)

More Information on Logical Operators

Wrap up

Question 1:

Question 2:

What will the following print to the console?

Upcoming Assignments