Logical operators in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of combined logical operators

What is the output of the following C++ code?

C++

#include <iostream>
int main() {
    bool a = true, b = false, c = true;
    std::cout << (a && b || c) << std::endl;
    return 0;
}

A 0

B 1

C Compilation error

D Runtime error

💡 Hint

Remember the precedence of && and || operators.

The expression (a && b || c) evaluates as (false || true) which is true (1).

❓ Predict Output

intermediate

Logical NOT operator effect

What will be printed by this C++ program?

C++

#include <iostream>
int main() {
    bool x = false;
    std::cout << (!x && x) << std::endl;
    return 0;
}

A 1

B Runtime error

C Compilation error

D 0

💡 Hint

Check how !x and x combine with &&.

(!x && x) means (true && false) which is false (0).

❓ Predict Output

advanced

Short-circuit evaluation in logical AND

What is the output of this C++ code snippet?

C++

#include <iostream>
int main() {
    int i = 0;
    bool result = (false && (++i > 0));
    std::cout << i << std::endl;
    return 0;
}

A 0

B 1

C Compilation error

D Runtime error

💡 Hint

Does ++i get executed when the first operand is false in &&?

Because false && anything short-circuits, ++i is not executed, so i remains 0.

❓ Predict Output

advanced

Logical OR with side effects

What will be the output of this C++ program?

C++

#include <iostream>
int main() {
    int x = 5;
    bool val = (x > 10 || ++x > 5);
    std::cout << x << std::endl;
    return 0;
}

A 6

B Compilation error

C 5

D Runtime error

💡 Hint

Check if ++x is evaluated when x > 10 is false.

Since x > 10 is false, ++x > 5 is evaluated, incrementing x to 6.

🧠 Conceptual

expert

Understanding logical operator precedence

Given the expression: true || false && false, which part is evaluated first in C++?

A true || false && false (evaluated left to right)

B true || false

C false && false

D The expression causes a syntax error

💡 Hint

Remember operator precedence: && has higher precedence than ||.

&& is evaluated before ||, so false && false is evaluated first.

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