Logical operators in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are the three main logical operators in C++?

The three main logical operators in C++ are:
&& (logical AND), || (logical OR), and ! (logical NOT).

beginner

Explain the behavior of the logical AND (&&) operator.

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

beginner

What does the logical OR (||) operator do?

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

How does the logical NOT (!) operator work?

The logical NOT (!) operator reverses the truth value of a condition. If the condition is true, ! makes it false; if false, ! makes it true.

intermediate

What is short-circuit evaluation in logical operators?

Short-circuit evaluation means that in expressions using && or ||, evaluation stops as soon as the result is known. For &&, if the first condition is false, the second is not checked. For ||, if the first condition is true, the second is not checked.

Which logical operator returns true only if both conditions are true?

A && (AND)

B || (OR)

C ! (NOT)

D ^ (XOR)

The && operator returns true only if both conditions are true.

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

A true

B false

C undefined

D error

true && false is false, and !false is true.

In C++, what does the || operator do?

A Negates the condition

B Returns true if both conditions are true

C Returns false if any condition is true

D Returns true if at least one condition is true

The || operator returns true if at least one condition is true.

What happens in short-circuit evaluation with the expression: false && (someFunction())?

A someFunction() is called

B Both conditions are evaluated

C someFunction() is not called

D Error occurs

Because the first condition is false, && stops and does not call someFunction().

Which operator reverses the truth value of a condition?

A ||

B !

C &&

D ==

The ! operator negates or reverses the truth value.

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

Think about when each operator returns true or false.

Explain what short-circuit evaluation means and why it is useful in logical expressions.

Imagine checking conditions one by one and stopping when you know the answer.

Practice

1. Which logical operator in C++ means "both conditions must be true"?

easy

A. &&

B. ||

C. !

D. &

Solution

1. Step 1: Understand the meaning of logical AND

   The operator that requires both conditions to be true is called logical AND.

2. Step 2: Identify the C++ symbol for logical AND

   In C++, logical AND is represented by &&.

3. Final Answer:

   && -> Option A

4. Quick Check:

   Both true = true means && [OK]

Hint: && means both true; || means one true [OK]

Common Mistakes:

• Confusing & (bitwise AND) with && (logical AND)
• Thinking || means both true
• Mixing ! with AND or OR

2. Which of the following is the correct syntax to negate a boolean variable flag in C++?

easy

A. !flag = flag;

B. flag = flag!;

C. flag != flag;

D. flag = !flag;

Solution

1. Step 1: Recall the negation operator syntax

   The negation operator ! is placed before the variable to flip its value.

2. Step 2: Check correct assignment syntax

   To update flag to its negation, write flag = !flag; which is valid C++ syntax.

3. Final Answer:

   flag = !flag; -> Option D

4. Quick Check:

   Negate with ! before variable [OK]

Hint: Put ! before variable to negate [OK]

Common Mistakes:

• Placing ! after variable (invalid syntax)
• Using != which means not equal
• Trying to assign to !flag (not a variable)

3. What is the output of this C++ code?

bool a = true;
bool b = false;
if (a || b) {
  std::cout << "Yes";
} else {
  std::cout << "No";
}

medium

A. Yes

B. No

C. true

D. false

Solution

1. Step 1: Evaluate the logical OR condition

   a is true, b is false, so a || b is true because at least one is true.

2. Step 2: Determine which branch runs

   Since the condition is true, the if block runs and prints "Yes".

3. Final Answer:

   Yes -> Option A

4. Quick Check:

   true || false = true [OK]

Hint: || is true if any side is true [OK]

Common Mistakes:

• Thinking both must be true for ||
• Confusing output with boolean literals
• Ignoring that std::cout prints strings

4. Find the error in this C++ code snippet:

bool x = true;
bool y = false;
if (x & y) {
  std::cout << "True";
} else {
  std::cout << "False";
}

medium

A. Incorrect use of std::cout

B. Missing semicolon after bool declarations

C. Using & instead of && in if condition

D. Variables x and y not initialized

Solution

1. Step 1: Identify the operator used in condition

   The code uses & which is bitwise AND, not logical AND.

2. Step 2: Explain why && is needed

   For boolean logic in conditions, && should be used to combine true/false values.

3. Final Answer:

   Using & instead of && in if condition -> Option C

4. Quick Check:

   Logical AND needs && not & [OK]

Hint: Use && for logic, & is bitwise [OK]

Common Mistakes:

• Confusing bitwise & with logical &&
• Assuming & works like && in conditions
• Ignoring operator precedence differences

5. Given int a = 5; and int b = 10;, which condition correctly checks if a is less than 10 AND b is greater than 5?

hard

A. if (a < 10 || b > 5)

B. if (a < 10 && b > 5)

C. if (!(a < 10) && !(b > 5))

D. if (a > 10 && b < 5)

Solution

1. Step 1: Understand the condition requirements

   We want both: a less than 10 AND b greater than 5.

2. Step 2: Match condition with logical AND

   Logical AND (&&) combines both conditions to be true simultaneously.

3. Final Answer:

   if (a < 10 && b > 5) -> Option B

4. Quick Check:

   Both true means && [OK]

Hint: Use && to require both conditions true [OK]

Common Mistakes:

• Using || which means either condition true
• Negating conditions incorrectly
• Swapping comparison operators