Unit 3¶

Logical Operators¶

Logical operators are used to performing logical “AND”, “OR” and “NOT” operations, i.e. the function similar to AND gate and OR gate in digital electronics. They are used to combine two or more conditions/constraints or to complement the evaluation of the original condition under particular consideration. One thing to keep in mind is the second condition is not evaluated if the first one is false. For example, using the && comparison, if the first expression is false, the value of the second expression doesn’t matter. Likewise, for the || comparison, if the first expression is true, the second value doesn’t matter. When the result of a logical expression using && or || can be determined by evaluating only the first Boolean operand, the second is not executed or evaluated. This is known as short-circuited. This is used extensively to test for several conditions for making a decision.

AND Operator ( && ) – if( a && b ) [if true execute else don’t]
OR Operator ( || ) – if( a || b) [if one of them is true execute else don’t]
NOT Operator ( ! ) – !(a<b) [returns false if a is smaller than b]

3.1 Boolean Expressions¶

Goals

Understand Boolean expressions.
Use relational operators to compute the value of a Boolean expression.
- I will be able to write a program that utilizes a Boolean expression

George Boole formulated the basic rules of Boolean algebra back in 1847. His endeavor created the mathematical logic that we use today. Below are the relational operators that you will need to know for the AP exam.

Relational Operator	Description	Boolean Value
a < b	a less than b	Equates to ‘true’ if a is less than b,’false’ otherwise
a <= b	a less than or equal to b	Equates to ‘true’ if a is less than or equal to b, ‘false’ otherwise.
a > b	a greater than b	Equates to ‘true’ if a is greater than b, ‘false’ otherwise.
a >= b	a greater than or equal to b	Equates to ‘true’ if a is greater than or equal to b, ‘false’ otherwise.
a == b	a equals b*	Equates to ‘true’ if a has the same value as b.
a != b	a does not equal b	Equates to ‘true’ if a does not have the same value as b, ‘false’ otherwise.

* Sometimes this comparison can be misleading. == can look at the value a and compare it to b to see if it is located at the same memory location.

Boolean_1

A	!A
false	true
true	false

Boolean_2

A	B	A \|\| B
false	false
false	true
true	false
true	true

Given boolean variables a ,b , and c, fill in the truth table for a && (B || c).

a	b	c
T	T	T
T	T	F
T	F	T
T	F	F
F	T	T
F	T	F
F	F	T
F	F	F

Solution

a	b	c	b \|\| c	a && (b \|\| c)
T	T	T	T	T
T	T	F	T	T
T	F	T	T	T
T	F	F	F	F
F	T	T	T	F
F	T	F	T	F
F	F	T	T	F
F	F	F	F	F

In Java logical operators, if the evaluation of a logical expression exits in between before complete evaluation, then it is known as Short-circuit. A short circuit happens because the result is clear even before the complete evaluation of the expression, and the result is returned. Short circuit evaluation avoids unnecessary work and leads to efficient processing.

&& short circuit: In the case of &&, the expression is evaluated until we get one false result because the result will always be false, independent of the further conditions. If there is an expression with &&(logical AND), and the first operand itself is false, then a short circuit occurs, the further expression is not evaluated, and false is returned.

|| short circuit: In the case of ||, the expression is evaluated until we get one true result because the result will always be true, independent of the further conditions. If there is an expression with ||(logical OR), and the first operand itself is true, a short circuit occurs, evaluation stops, and true is returned.

Which of the following expressions will short circuit when happy is true and pink is false? Why?

!happy && pink
happy | | pink
pink | | happy

Solution

An && statement short circuits when the left operand is false. !happy is false, so !happy && pink will short circuit to false.

An || statement short circuits when the left operand is true. happy is true, so happy || pink will short circuit to true.

Activity 3.1.1¶

Write the body of the method given the following requirements.

You have three numbers to work with: ints a, b and c.

If the numbers are all different from each other, print “unique”.
If all of the numbers are the same, print “boring”.
If two of the numbers are the same, print “twins!”.

public void suchFun(int a, int b, int c) {

}

**Solution**

public void checkNumbers(int a, int b, int c) {
    if (a == b && b == c) {
        System.out.println("boring");
    } else if (a == b || a == c || b == c) {
        System.out.println("twins!");
    } else {
        System.out.println("unique");
    }
}

Activity 3.1.2¶

Rent-a-Car:

ask the user to enter their name, age, and how much money they have
write conditional statements using logical operators, to determine whether the user can rent a car
- a person must be 25 or older to rent a car
  - if a person is old enough and has enough money ($999), print they can rent a car
  - if a person is old enough but does not have enough money, tell them to come back with more money
- if a person is not old enough, let them know and tell them when they can come back
take care to match the sample outputs exactly
your program must pass all 3 tests

//starter code
import java.util.Scanner;

class Main 
{
  public static void main(String[] args) 
	{
    Scanner input = new Scanner(System.in);
		//your code goes below
  }
}

Sample Output 1:

Enter your name:
Maxim
Enter your age:
28
How much money do you have?
500.12
Maxim please visit us again when you have more money.

Sample Output 2:

Enter your name:
Jorge
Enter your age:
30
How much money do you have?
1000
Jorge we would be happy to rent you a car.  

Sample Output 3:

Enter your name:
Erika
Enter your age:
17
How much money do you have?
542364
Erika you are too young to rent a car, please visit us again when you are 25 years old.

Activity 3.1.2¶

Instructions Write a program that will take 2 doubles from the user. Use Boolean Expression to compare the two values using: <, <=, =, !=, >, and >=. Evaluate if the expression is true or false.

Input a number: 4
Input a second number: 5
0 is LESS THAN 5.0: true
0 is LESS THAN/ EQUAL to 5.0: true
0 is EQUAL to 5.0: false
0 is NOT EQUAL to 5.0: true
0 is GREATER THAN 5.0: false
0 is GREATER THAN / EQUAL to 5.0: false

3.2 If Statements and Control Flow¶

Goals

Gain an understanding of how to control program flow with if statements.
Start to implement control flow in more complex programs.
- I will be able to write a program that uses if statements to control how a program will operate

In Java, decision driven statements are used in conjunction with mathematical logical operators to navigate complex programs. We use these logical operators below to differentiate solutions based on specific information.

Less than: a < b
Less than or equal to: a <= b
Greater than: a > b
Greater than or equal to: a >= b
Equal to a == b
Not Equal to: a != b

You can use these conditions to perform different actions based on the decisions from user input.

The Java if statement is the most simple-decision making statement. Java has the following conditional statements:

Use the if keyword to specify a block of code to be executed, if a condition is true
Use the else if keyword to specify a new condition to test, if the first condition is false
Use the else keyword to specify a block of code to be executed, if the above conditions are false

Basic if statement format

if(condition) 
{
   // Statements to execute if
   // condition is true
}

Example 1

class Main {
    public static void main(String args[]) {
	int i = 1;
	if (i < 5) {
  	    System.out.println("1 < 5");
            }
	System.out.println("This statement is outside of the if statement.");
  }
}

Example 2

class Main {
    public static void main(String args[]) {
        String str = "Your Name Here!";
        int i = 1;
	if (i == 1) {
            System.out.println(str);
            i++;
            }
        System.out.println("i = " + i);
  }
}

Example 3

public class Main {
    public static void main(String[] args) {
        boolean Yes = true;
        boolean No = false;
		
        if (Yes) {
            System.out.println("a is true"); }
        else {
            System.out.println("a is false"); }
		
        if (No) {
            System.out.println("b is true"); }
        else {
            System.out.println("b is false"); }
  }
}

Advantages of the if-else statement –

Conditional execution: The if-else statement allows code to be executed conditionally based on the result of a Boolean expression. This provides a way to make decisions and control the flow of a program based on different inputs and conditions.

Readability: The if-else statement makes code more readable by clearly indicating when a particular block of code should be executed. This makes it easier for others to understand and maintain the code.

Reusability: By using if-else statements, developers can write code that can be reused in different parts of the program. This reduces the amount of code that needs to be written and maintained, making the development process more efficient.

Debugging: The if-else statement can help simplify the debugging process by making it easier to trace problems in the code. By clearly indicating when a particular block of code should be executed, it becomes easier to determine why a particular piece of code is not working as expected.

Flexibility: The if-else statement provides a flexible way to control the flow of a program. It allows developers to handle different scenarios and respond dynamically to changes in the program’s inputs.

Example 4

if (condition1) {
  // if condition1 is true then the blocks will be executed. 
} else if (condition2) {
  // if the condition1 is false, but condition2 is true then the blocks for block 2 will be executed. 
} else {
  // if both condition1 and condition2 are false, the else statement will be executed
}

public class Main {
    public static void main(String[] args) {
	int age = 21;
	if (age < 12) {
             System.out.println("You are a child.");}
	else if (age < 19) {
             System.out.println("You are a teenager."); }
	else {
             System.out.println("You are an adult."); }

  }
}

Check your understanding

Program 1

Take a look at the code below:

if(age >= 18);
{
System.out.println( "You are a high school student." );
}