📓3.5: Compound Booleans

Table of Contents

📖 This page is a condensed version of CSAwesome Topic 3.5

📝 Take notes in a Codespace during class, coding along with the instructor.

  1. Go to GitHub and click on your picture in the TOP RIGHT corner
  2. Select Your repositories
  3. Open CS2-Unit-3-Notes
  4. Now on your repository, click and select the Codespaces tab
  5. Click Create Codespace on main (unless you already have one listed there), wait for the environment to load, then you’re ready to code!

Compound Boolean Expressions

And (&&) Operator

What if you want two things to be true before the body of the conditional is executed? Use && as a logical and to join two Boolean expressions and the body of the condition will only be executed only if both are true.

What if you want to go out and your parents say you can go out if you clean your room and do your homework? Test the code below and try different values for cleanedRoom and didHomework and see what they have to be for it to print "You can go out".

boolean cleanedRoom = true;
boolean didHomework = false;

if (cleanedRoom && didHomework) {
  System.out.println("You can go out");
}
else {
  System.out.println("No, you can't go out");
}

Or (||) Operator

What if it is okay if only one of two things is true? Use || as a logical or to join two Boolean expressions and the body of the condition will be executed if one or both are true.

For example, your parents might say you can go out if you can walk or they don’t need the car. Try different values for walking and carIsAvailable and see what the values have to be to print "You can go out".

boolean walking = false;
boolean carIsAvailable = false;

if (walking || carIsAvailable) {
  System.out.println("You can go out");
}
else {
  System.out.println("No, you can't go out");
}

Not (!) Operator

The not (!) operator can be used to negate a boolean value. We’ve seen ! before in != (not equal).

The code below says if homework is not done, you can’t go out. Try different values for homeworkDone.

boolean homeworkDone = false;
if (!homeworkDone) {
  System.out.println("Sorry, you can't go out!");
}

If you use ! in expressions with && and ||, be careful because the results are often the opposite of what you think it will be at first. We’ll see examples of this in the next lesson.

In Java, ! will be executed before &&, and && will be executed before ||, unless there are parentheses. Anything inside parentheses is executed first.

Truth Tables

AND (&&) TRUTH TABLE

The following table (also called a truth table) shows the result for P && Q when P and Q are both expressions that can be true or false. An expression involving logical operators like P && Q evaluates to a boolean value, true or false. As you can see below the result of P && Q is only true if both P and Q are true.

P Q P && Q
true true true
true false false
false true ???
false false false

💬: The truth table above is missing one result. What is the result of P && Q when P = false and Q = true?

OR (||) TRUTH TABLE

The following table shows the result for P || Q when P and Q are both expressions that can be true or false. As you can see below the result of P || Q is true if either P or Q is true. It is also true when both of them are true.

P Q P \|\| Q
true true true
true false true
false true ???
false false false

💬: The truth table above is missing one result. What is the result of P || Q when P = false and Q = true?

Short Circuit Evaluation

image

Both && and || use short circuit evaluation. That means that the second expression (on the right of the operator) isn’t necessarily checked, if the result from the first expression is enough to tell if the compound boolean expression is true or false:

  • If two boolean values/expressions are combined with a logical or (||) and the first expression is true, then the second expression won’t be executed, since only one needs to be true for the result to be true.
  • If two boolean values/expressions are combined with a logical and (&&) and the first expression is false, then the second expression won’t be executed. If the first expression is false, the result will be false, since both sides of the && need to be true for the result to be true.
  1. What is printed when the following code executes and x has been set to 0 and y to 3? 
    if (x > 0 && (y / x) == 3) {
  System.out.println("first case");
}
else {
  System.out.println("second case");
}
  2. What is printed when the following code executes and x has been set to zero and y is set to 3? 
    if (x == 0 || (y / x) == 3) {
  System.out.println("first case");
}
else {
  System.out.println("second case");
}

Venn Diagram Activity

📝 Explore the following problems in a group of 4:

  1. Draw or print a Venn diagram of 4 intersecting circles. Put the names of the 4 people in your group one in each circle. Write down the age of each person in your group in the circles. If two or more people are the same age, put the age in the intersecting parts of their circles. Write a Boolean expression that compares the age of each person in the group using ==, <, >, and &&, for example Ada’s age > Alan’s age && Alan’s age == Grace’s age. Then, ask each person in your group their favorite movie. If two or more people have the same favorite movie, put the movie in the intersecting parts of their circles. Write a Boolean expression that compares the favorite movies in the group using ==, !=, and &&, for example Ada’s movie == Alan’s movie && Alan’s movie != Grace’s movie. Think of 1 more comparison and write it in the circles and as a Boolean expression. Share the Boolean expressions with the class.

  2. Write the sentence “If it’s sunny, OR if the temperature is greater than 80 and it’s not raining, I will go to the beach.” as a Java if statement using an int variable temperature and boolean variables sunny and raining. If the conditional is true, print out “Go to the beach!”. So, you will go to the beach on days that it is sunny in any temperature, or you will go to the beach on days when the temperature is over 80 degrees and it’s not raining.

  3. Complete a truth table for the if statement that you wrote in #2 with columns for sunny, temperature > 80, raining, and go to the beach.

Boolean Game

🎲 Try the game below written to practice Booleans. Click on Booleans, look at the color and number in the block and evaluate the boolean expression to choose true or false. Then, check on Compound for an added challenge. See how high a score you can get!

⭐️ Summary

  • Logical operators ! (not), && (and), and || (or) are used with Boolean values.

  • A && B is true if both A and B are true.

  • A || B is true if either A or B (or both) are true.

  • !A is true if A is false.

  • In Java, ! has precedence (is executed before) && which has precedence over ||. Parentheses can be used to force the order of execution in a different way.

  • When the result of a logical expression using && or || can be determined by evaluating only the first Boolean operand, the second is not evaluated. This is known as short-circuit evaluation.

🛑 When class ends, don’t forget to SAVE YOUR WORK!

  1. Navigate to the Source Control menu on the LEFT sidebar
  2. Type a brief commit message in the box, for example: updated Main.java
  3. Click the button on the LEFT menu
  4. Click the button on the LEFT menu
  5. Finally you can close your Codespace!

Acknowledgement

Content on this page is adapted from Runestone Academy - Barb Ericson, Beryl Hoffman, Peter Seibel.