Logical operators (&&, ||, !) in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the logical AND operator (&&) do in Kotlin?

The logical AND operator (&&) returns true only if both conditions on its sides are true. If either condition is false, it returns false.

beginner

Explain the logical OR operator (||) in Kotlin.

The logical OR operator (||) returns true if at least one of the conditions is true. It returns false only if both conditions are false.

beginner

What is the purpose of the logical NOT operator (!) in Kotlin?

The logical NOT operator (!) reverses the value of a boolean expression. If the expression is true, ! makes it false, and vice versa.

beginner

Given: val a = true, val b = false, what is the result of a && b?

The result is false because the AND operator requires both sides to be true, but b is false.

intermediate

How does Kotlin evaluate the expression: !(true || false)?

First, true || false evaluates to true. Then, !true becomes false. So the whole expression is false.

What is the result of true && false in Kotlin?

A true

B false

C null

D Compilation error

The AND operator returns true only if both sides are true. Here, one side is false, so the result is false.

Which operator returns true if at least one condition is true?

A ||

B !

C ==

D &&

The OR operator (||) returns true if any one of the conditions is true.

What does the ! operator do to a boolean value?

A Leaves it unchanged

B Converts it to a number

C Throws an error

D Reverses it (true to false, false to true)

The NOT operator (!) reverses the boolean value.

What is the result of !(false && true)?

A null

B false

C true

D Compilation error

false && true is false, then !false is true.

Which of these expressions is true?

A true || false

B true && false

C !true

D false || false

true || false is true because OR returns true if any side is true.

Describe how the logical AND (&&) and OR (||) operators work in Kotlin with simple examples.

Think about when you want both conditions to be true versus when one is enough.

Explain the effect of the logical NOT operator (!) on a boolean expression.

It flips the meaning of a condition.

Practice

1. Which logical operator in Kotlin means AND and returns true only if both conditions are true?

easy

A. ||

B. !

C. &&

D. &

Solution

1. Step 1: Understand the meaning of && operator

   The && operator checks if both conditions are true and returns true only then.

2. Step 2: Compare with other operators

   The || operator means OR, ! means NOT, and & is a bitwise operator, not logical AND.

3. Final Answer:

   && -> Option C

4. Quick Check:

   AND operator = && [OK]

Hint: AND means both true, use && [OK]

Common Mistakes:

• Confusing && with ||
• Using & instead of &&
• Thinking ! means AND

2. Which of the following is the correct syntax to negate a boolean variable isActive in Kotlin?

easy

A. !isActive

B. not isActive

C. isActive!

D. ~isActive

Solution

1. Step 1: Recall Kotlin negation syntax

   In Kotlin, the NOT operator is written as ! before the variable.

2. Step 2: Check each option

   !isActive uses !isActive which is correct. not isActive is invalid syntax, C places ! after variable which is wrong, D uses ~ which is bitwise NOT invalid for booleans.

3. Final Answer:

   !isActive -> Option A

4. Quick Check:

   Negate with ! before variable [OK]

Hint: Put ! before variable to negate [OK]

Common Mistakes:

• Putting ! after variable
• Using 'not' keyword incorrectly
• Using bitwise ~ for negation

3. What is the output of this Kotlin code?

val a = true
val b = false
println(a && !b || b)

medium

A. false

B. true

C. Compilation error

D. null

Solution

1. Step 1: Evaluate !b

   b is false, so !b is true.

2. Step 2: Evaluate a && !b

   a is true, !b is true, so a && !b is true.

3. Step 3: Evaluate (a && !b) || b

   True || false is true.

4. Final Answer:

   true -> Option B

5. Quick Check:

   True AND NOT false OR false = true [OK]

Hint: Evaluate NOT first, then AND, then OR [OK]

Common Mistakes:

• Ignoring NOT precedence
• Mixing AND and OR order
• Assuming false output

4. Find the error in this Kotlin code snippet:

val x = true
val y = false
if x && y {
    println("Both true")
}

medium

A. Missing parentheses around condition in if statement

B. Using && instead of ||

C. Incorrect println syntax

D. Variables x and y not declared

Solution

1. Step 1: Check if statement syntax

   Kotlin requires parentheses around the condition in if statements.

2. Step 2: Identify the error

   The code uses if x && y without parentheses, which causes a syntax error.

3. Final Answer:

   Missing parentheses around condition in if statement -> Option A

4. Quick Check:

   if condition needs parentheses [OK]

Hint: Always put () around if conditions [OK]

Common Mistakes:

• Omitting parentheses in if
• Confusing && with ||
• Wrong println usage

5. You want to check if a user is logged in and has admin rights before allowing access. Which Kotlin condition correctly uses logical operators for this?

val loggedIn = true
val isAdmin = false
// Choose the correct if condition

hard

A. if (loggedIn || isAdmin) { println("Access granted") }

B. if (!loggedIn || !isAdmin) { println("Access granted") }

C. if (!loggedIn && isAdmin) { println("Access granted") }

D. if (loggedIn && isAdmin) { println("Access granted") }

Solution

1. Step 1: Understand the requirement

   Access is allowed only if user is logged in AND has admin rights.

2. Step 2: Analyze each condition

   if (loggedIn && isAdmin) { println("Access granted") } uses && to require both loggedIn and isAdmin true. if (loggedIn || isAdmin) { println("Access granted") } allows access if either is true, which is wrong. The remaining options use negations or incorrect logic, which do not meet requirement.

3. Final Answer:

   if (loggedIn && isAdmin) { println("Access granted") } -> Option D

4. Quick Check:

   Use AND to require both conditions true [OK]

Hint: Use && to require both true for access [OK]

Common Mistakes:

• Using || instead of &&
• Negating conditions wrongly
• Allowing access if only one is true