Logical operators in C Sharp (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 snippet?

C Sharp (C#)

bool a = true;
bool b = false;
bool c = true;
Console.WriteLine(a && b || c);

A True

B False

C Compilation error

D Runtime exception

💡 Hint

Remember the precedence of && and || operators in C#.

The expression a && b || c is evaluated as (a && b) || c. Since a && b is false, and c is true, the overall result is true.

❓ Predict Output

intermediate

Result of negation and OR operator

What will be printed by this C# code?

C Sharp (C#)

bool x = false;
bool y = true;
Console.WriteLine(!x || y);

A False

B Runtime exception

C Compilation error

D True

💡 Hint

Check how the NOT operator affects the value of x.

The NOT operator !x changes false to true. So !x || y becomes true || true, which is true.

❓ Predict Output

advanced

Output with short-circuit evaluation

What is the output of this C# code snippet?

C Sharp (C#)

int i = 0;
bool result = false && (++i > 0);
Console.WriteLine(i);

A Compilation error

B 1

C 0

D Runtime exception

💡 Hint

Consider how the && operator short-circuits evaluation.

Because the first operand is false, the second operand (++i > 0) is not evaluated. So i remains 0.

❓ Predict Output

advanced

Value of variable after complex logical expression

What is the value of variable flag after running this code?

C Sharp (C#)

bool flag = true;
flag = flag && false || !flag && true;
Console.WriteLine(flag);

A Compilation error

B False

C True

D Runtime exception

💡 Hint

Break down the expression into parts and evaluate step by step.

Evaluate flag && false → true && false = false.
Evaluate !flag && true → !true && true = false && true = false.
So overall expression is false || false = false.

🧠 Conceptual

expert

Understanding logical operator precedence and evaluation

Given the expression true || false && false in C#, which statement is correct about its evaluation?

A The expression evaluates to true because && has higher precedence than ||, so false && false is evaluated first.

B The expression evaluates to false because || has higher precedence than &&, so true || false is evaluated first.

C The expression causes a runtime error due to ambiguous operator precedence.

D The expression evaluates to false because both operators have the same precedence and are evaluated left to right.

💡 Hint

Recall operator precedence rules in C# for logical operators.

In C#, the && operator has higher precedence than ||. So false && false is evaluated first, resulting in false. Then true || false is evaluated, resulting in true.

Practice

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

easy

A. !

B. ||

C. &&

D. &

Solution

1. Step 1: Understand the meaning of logical AND

   The AND operator returns true only if both conditions it connects are true.

2. Step 2: Identify the C# symbol for AND

   In C#, the logical AND operator is written as &&, which checks both sides for true.

3. Final Answer:

   && -> Option C

4. Quick Check:

   AND operator = && [OK]

Hint: AND is &&, OR is ||, NOT is ! [OK]

Common Mistakes:

• Confusing & (bitwise AND) with && (logical AND)
• Mixing OR (||) with AND (&&)
• Using ! as AND operator

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

easy

A. isActive = !!isActive;

B. isActive = !isActive;

C. isActive = ~isActive;

D. isActive = isActive!;

Solution

1. Step 1: Understand the NOT operator syntax

   The NOT operator in C# is ! and it negates a boolean value.

2. Step 2: Check the correct usage

   Using ! before the variable negates it, so isActive = !isActive; is correct.

3. Final Answer:

   isActive = !isActive; -> Option B

4. Quick Check:

   NOT operator = ! [OK]

Hint: Use ! before variable to negate boolean [OK]

Common Mistakes:

• Using double !! which is invalid in C#
• Using bitwise NOT ~ on booleans
• Placing ! after the variable

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

bool a = true;
bool b = false;
Console.WriteLine(a || b && !a);

medium

A. True

B. False

C. Compilation error

D. Null

Solution

1. Step 1: Evaluate the expression inside WriteLine

   Operator precedence: ! has highest, then &&, then ||.
   Calculate !a = !true = false.
   Then b && !a = false && false = false.
   Finally a || (b && !a) = true || false = true.

2. Step 2: Determine output

   The expression evaluates to true, so Console.WriteLine prints True.

3. Final Answer:

   True -> Option A

4. Quick Check:

   Operator precedence: ! > && > ||, result = True [OK]

Hint: Remember ! > && > || precedence [OK]

Common Mistakes:

• Ignoring operator precedence
• Evaluating && before !
• Assuming output is false due to b being false

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

bool x = true;
bool y = false;
if (x || && y)
{
    Console.WriteLine("Yes");
}

medium

A. No error, code is correct

B. Incorrect use of || operator

C. Missing parentheses around conditions

D. Missing variable before && operator

Solution

1. Step 1: Analyze the if condition syntax

   The condition is written as x || && y, which is invalid because && needs two operands.

2. Step 2: Identify the missing operand

   Between || and && operators, there is no variable or expression before &&, causing syntax error.

3. Final Answer:

   Missing variable before && operator -> Option D

4. Quick Check:

   Logical operators need operands on both sides [OK]

Hint: Each operator needs two conditions [OK]

Common Mistakes:

• Writing operators without operands
• Assuming || and && can be chained without variables
• Missing parentheses but that is not the main error here

5. You want to check if a number n is between 10 and 20 inclusive in C#. Which condition correctly uses logical operators?

hard

A. if (n >= 10 && n <= 20)

B. if (n >= 10 || n <= 20)

C. if (n > 10 && n < 20)

D. if (!(n < 10 && n > 20))

Solution

1. Step 1: Understand the range condition

   To check if n is between 10 and 20 inclusive, n must be greater or equal to 10 AND less or equal to 20.

2. Step 2: Evaluate each option

   if (n > 10 && n < 20) excludes 10 and 20 because it uses > and <.
   if (n >= 10 || n <= 20) uses OR, which is true for almost all numbers.
   if (n >= 10 && n <= 20) correctly uses >= and <= with AND.
   if (!(n < 10 && n > 20)) negates an impossible condition and is incorrect.

3. Final Answer:

   if (n >= 10 && n <= 20) -> Option A

4. Quick Check:

   Inclusive range needs >= and <= with AND [OK]

Hint: Use && with >= and <= for inclusive range [OK]

Common Mistakes:

• Using || instead of && for range checks
• Using > and < instead of >= and <=
• Negating wrong conditions