Unit 3 Notes

Unit 3: Boolean Expressions and If Statements – AP Computer Science A

🔑 Key Concepts

• Relational operators: ==, !=, <, >, <=, >=
• Logical operators: &&, ||, !
• if, else if, and else structures
• Truth tables and short-circuit evaluation
• Nested conditionals and De Morgan’s Laws

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🤔 Boolean Expressions

A boolean expression evaluates to either true or false.

int a = 5;
int b = 10;
System.out.println(a < b); // true

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🧮 Relational and Logical Operators

Operator	Meaning	Example
==	Equal to	x == 10
!=	Not equal to	x != 5
>	Greater than	x > 3
<	Less than	x < 7
>=	Greater than or equal	x >= 4
<=	Less than or equal	x <= 2
&&	Logical AND	x > 0 && x < 10
||	Logical OR	x < 0 || x > 100
!	Logical NOT	!(x == 5)

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🔄 Control Structures

Basic if statement:

if (score >= 90) {
    System.out.println("A");
}

if-else statement:

if (x < 0) {
    System.out.println("Negative");
} else {
    System.out.println("Non-negative");
}

if-else if-else chain:

if (score >= 90) {
    System.out.println("A");
} else if (score >= 80) {
    System.out.println("B");
} else {
    System.out.println("Below B");
}

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🔁 Nested Conditionals

if (x > 0) {
    if (x < 10) {
        System.out.println("1 to 9");
    }
}

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🧠 De Morgan’s Laws

Original	Equivalent
!(A && B)	!A || !B
!(A || B)	!A && !B

These help simplify complex boolean logic.

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🧪 Practice Multiple Choice (5 Questions)

1. What will this print?

int x = 5;
if (x > 0 && x < 10)
    System.out.println("Yes");
else
    System.out.println("No");

(A) Yes
(B) No
(C) YesNo
(D) Error
✅ Answer: (A)

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2. Which of the following is true if x = 3 and y = 7?
(A) x > y && y > 5
(B) x < y || x > 10
(C) !(x < y)
(D) x == y && y < 10
✅ Answer: (B)

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3. Which statement checks if x is between 1 and 100 (inclusive)?
(A) x >= 1 || x <= 100
(B) x > 1 && x < 100
(C) x >= 1 && x <= 100
(D) x <= 1 && x >= 100
✅ Answer: (C)

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4. What is the output?

int a = 4;
int b = 4;
System.out.println(!(a != b));

(A) true
(B) false
(C) 0
(D) 4
✅ Answer: (A)

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5. Which of the following expressions is equivalent to !(x < 5 || x > 10)?
(A) x < 5 && x > 10
(B) x >= 5 && x <= 10
(C) x < 5 || x < 10
(D) x >= 5 || x <= 10
✅ Answer: (B)

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