Logical patterns (and, or, not) in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of combined logical operators

What is the output of this C# code snippet?

C Sharp (C#)

bool a = true;
bool b = false;
bool c = true;

if (a && b || c)
{
    Console.WriteLine("Yes");
}
else
{
    Console.WriteLine("No");
}

A No

B Runtime exception

C Compilation error

D Yes

💡 Hint

Remember that && has higher precedence than || in C#.

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

❓ Predict Output

intermediate

Result of negation with logical operators

What will be printed by this C# code?

C Sharp (C#)

bool x = false;
bool y = true;

if (!(x || y) && y)
{
    Console.WriteLine("True branch");
}
else
{
    Console.WriteLine("False branch");
}

A Compilation error

B No output

C False branch

D True branch

💡 Hint

Evaluate the expression inside the negation first.

The expression !(x || y) && y becomes !(false || true) && true which is !true && true → false && true → false. So the else branch runs.

🔧 Debug

advanced

Identify the logical error in condition

This code is intended to print "Access granted" only if the user is an admin and is active. What is wrong with the condition?

C Sharp (C#)

bool isAdmin = true;
bool isActive = false;

if (isAdmin || isActive)
{
    Console.WriteLine("Access granted");
}
else
{
    Console.WriteLine("Access denied");
}

A The condition uses || instead of &&, so it grants access if either is true.

B The condition should use !isAdmin && !isActive.

C The else block is missing a return statement.

D The variables are not initialized properly.

💡 Hint

Think about when access should be granted.

The condition isAdmin || isActive allows access if either is true. The requirement is both must be true, so it should be isAdmin && isActive.

📝 Syntax

advanced

Identify the syntax error in logical expression

Which option contains a syntax error in the logical expression?

A if (a && (b || !c)) { }

B if (a && b || !c { }

C if (a && b || !c) { }

D if (!(a || b) && c) { }

💡 Hint

Check for missing parentheses.

Option B is missing a closing parenthesis before the opening brace, causing a syntax error.

🚀 Application

expert

Determine the number of true conditions

Given the following code, how many times will "Condition met" be printed?

C Sharp (C#)

bool[] flags = { true, false, true, false, true };
int count = 0;

foreach (bool flag in flags)
{
    if (flag && !(!flag || false))
    {
        Console.WriteLine("Condition met");
        count++;
    }
}

Console.WriteLine(count);

A 3

B 2

C 5

D 0

💡 Hint

Simplify the condition inside the if statement.

The condition flag && !(!flag || false) simplifies to flag && flag which is just flag. So it prints once for each true in the array, which are 3 times.

Practice

1.

Which logical operator in C# means both conditions must be true?

and, or, or not?

easy

A. and

B. or

C. not

D. xor

Solution

1. Step 1: Understand the meaning of logical operators

   The and operator requires both conditions to be true for the whole expression to be true.

2. Step 2: Compare with other operators

   or means one or both conditions are true, not negates a condition, and xor means exactly one condition is true.

3. Final Answer:

   and -> Option A

4. Quick Check:

   and means both true [OK]

Hint: Remember: and = both true, or = any true, not = reverse [OK]

Common Mistakes:

• Confusing 'and' with 'or'
• Thinking 'not' means both true
• Mixing 'xor' with 'and'

2.

Which of these is the correct C# syntax for the logical NOT operator?

?

easy

A. not x

B. x!

C. !x

D. ~x

Solution

1. Step 1: Recall C# logical NOT syntax

   In C#, the logical NOT operator is ! placed before the variable or expression.

2. Step 2: Check other options

   not x is not valid C# syntax, ~x is bitwise NOT, and x! is invalid.

3. Final Answer:

   !x -> Option C

4. Quick Check:

   Logical NOT uses ! before expression [OK]

Hint: Logical NOT is always ! before the variable [OK]

Common Mistakes:

• Using 'not' keyword instead of !
• Confusing bitwise ~ with logical !
• Placing ! after the variable

3.

What is the output of this C# code?

bool a = true;
bool b = false;
if (a && !b) {
    Console.WriteLine("Yes");
} else {
    Console.WriteLine("No");
}

medium

A. No

B. Yes

C. True

D. False

Solution

1. Step 1: Evaluate variables and conditions

   Variable a is true, b is false, so !b is true.

2. Step 2: Evaluate the if condition

   a && !b becomes true && true, which is true, so the if block runs.

3. Final Answer:

   Yes -> Option B

4. Quick Check:

   true and not false = true [OK]

Hint: Remember ! reverses boolean, so !false = true [OK]

Common Mistakes:

• Ignoring the ! operator
• Thinking false && true is true
• Confusing output strings with boolean values

4.

Find the error in this C# code snippet:

bool x = true;
if (x && !x) {
    Console.WriteLine("True");
} else {
    Console.WriteLine("False");
}

medium

A. Using single & instead of &&

B. Missing semicolon after if statement

C. Incorrect use of Console.WriteLine

D. Contradictory condition: x and not x can never be true

Solution

1. Step 1: Analyze the condition logic

   The condition x && !x means x is true AND x is false at the same time, which is impossible.

2. Step 2: Check syntax correctness

   The syntax is correct: && is used properly, semicolons and Console.WriteLine are fine.

3. Final Answer:

   Contradictory condition: x and not x can never be true -> Option D

4. Quick Check:

   True and not true = false always [OK]

Hint: Check if condition can ever be true logically [OK]

Common Mistakes:

• Thinking condition can be true
• Confusing & with &&
• Looking for syntax errors when logic is the issue

5.

You want to write a C# switch expression that returns "Valid" only if age is between 18 and 30 inclusive, and hasID is true. Which pattern correctly uses logical operators?

return (age, hasID) switch {
    ??? => "Valid",
    _ => "Invalid"
};

hard

A. var (a, id) when a >= 18 && a <= 30 && id

B. (>= 18 and <= 30 and true, hasID)

C. var a when a >= 18 && a <= 30 && hasID == true

D. (>= 18 and <= 30, true)

Solution

1. Step 1: Understand switch pattern matching with tuples

   The switch matches a tuple (age, hasID). To check conditions on both, use a when clause with logical operators.

2. Step 2: Analyze each option

   var (a, id) when a >= 18 && a <= 30 && id correctly uses tuple deconstruction var (a, id) and checks a >= 18 && a <= 30 && id (where id is boolean). Others are invalid syntax or misuse logical operators.

3. Final Answer:

   var (a, id) when a >= 18 && a <= 30 && id -> Option A

4. Quick Check:

   Tuple pattern with when and && is correct [OK]

Hint: Use tuple deconstruction with when and && for multiple conditions [OK]

Common Mistakes:

• Trying to use and inside tuple without when
• Misplacing logical operators outside when clause
• Not deconstructing tuple properly