If-else execution flow in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of an if-else statement in C#?

An if-else statement lets the program choose between two paths based on a condition. If the condition is true, it runs one block of code; if false, it runs another.

beginner

What happens if the condition in an if statement is false and there is no else block?

If the condition is false and there is no else block, the program skips the if block and continues with the next code after the if statement.

beginner

Explain the flow of execution in this code snippet:

if (score >= 50) { Console.WriteLine("Pass"); } else { Console.WriteLine("Fail"); }

If the score is 50 or more, the program prints "Pass". Otherwise, it prints "Fail". Only one message is printed based on the score.

beginner

Can an if-else statement have multiple else blocks?

No, an if-else statement can have only one else block. To check multiple conditions, use else if blocks between if and else.

beginner

What is the difference between if and else if in C#?

An if starts a condition check. Else if checks another condition only if previous if or else if conditions were false. Else runs if all previous conditions are false.

What will this code print if x = 10?

if (x > 10) { Console.WriteLine("Greater"); } else { Console.WriteLine("Not Greater"); }

A Not Greater

B Greater

C Nothing

D Error

Since 10 is not greater than 10, the else block runs and prints "Not Greater".

Which keyword is used to check multiple conditions after an if in C#?

A else

B elseif

C else if

D elif

C# uses "else if" (two words) to check additional conditions after an if.

What happens if the if condition is true and there is an else block?

A Only the if block runs

B Only the else block runs

C Both if and else blocks run

D Neither block runs

Only the if block runs when the condition is true; the else block is skipped.

Can an if statement exist without an else block in C#?

A No

B Yes

C Only if inside a loop

D Only with a switch

An if statement can stand alone without an else block.

What is the output of this code if number = 0?

if (number > 0) { Console.WriteLine("Positive"); } else if (number < 0) { Console.WriteLine("Negative"); } else { Console.WriteLine("Zero"); }

A Positive

B Negative

C No output

D Zero

Since number is 0, both if and else if conditions are false, so else block runs and prints "Zero".

Describe how an if-else statement controls the flow of a program.

Think about how the program decides which code to run.

Explain the difference between if, else if, and else blocks in C#.

Consider the order and exclusivity of these blocks.

Practice

1. What does the else block do in an if-else statement in C#?

easy

A. Runs before the if block

B. Runs when the if condition is true

C. Runs only if there is no if block

D. Runs when the if condition is false

Solution

1. Step 1: Understand the if-else structure

   The if block runs when its condition is true.

2. Step 2: Role of the else block

   The else block runs only when the if condition is false.

3. Final Answer:

   Runs when the if condition is false -> Option D

4. Quick Check:

   else runs if if is false [OK]

Hint: Else runs only if if-condition is false [OK]

Common Mistakes:

• Thinking else runs when if is true
• Confusing else with else if
• Believing else runs before if

2. Which of the following is the correct syntax for an if-else statement in C#?

easy

A. if (condition) { } else { }

B. if condition { } else { }

C. if (condition) then { } else { }

D. if condition then { } else { }

Solution

1. Step 1: Check C# syntax rules

   C# requires parentheses around the condition in an if statement.

2. Step 2: Identify correct syntax

   Only if (condition) { } else { } uses parentheses and curly braces correctly.

3. Final Answer:

   if (condition) { } else { } -> Option A

4. Quick Check:

   Parentheses and braces required in C# if-else [OK]

Hint: Use parentheses around condition and braces for blocks [OK]

Common Mistakes:

• Omitting parentheses around condition
• Using 'then' keyword incorrectly
• Missing curly braces

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

int x = 5;
if (x > 10) {
    Console.WriteLine("Greater");
} else {
    Console.WriteLine("Smaller or equal");
}

medium

A. No output

B. Greater

C. Smaller or equal

D. Compilation error

Solution

1. Step 1: Evaluate the condition x > 10

   Since x is 5, 5 > 10 is false.

2. Step 2: Determine which block runs

   The else block runs because the if condition is false, printing "Smaller or equal".

3. Final Answer:

   Smaller or equal -> Option C

4. Quick Check:

   5 > 10 is false, so else runs [OK]

Hint: Check if condition is true or false to pick output [OK]

Common Mistakes:

• Assuming if block runs when condition is false
• Confusing output strings
• Thinking code has syntax errors

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

int num = 7;
if num > 5 {
    Console.WriteLine("Big");
} else {
    Console.WriteLine("Small");
}

medium

A. Missing parentheses around condition

B. Missing semicolon after if statement

C. Missing curly braces around else block

D. Incorrect variable type

Solution

1. Step 1: Check if statement syntax

   C# requires parentheses around the condition in an if statement.

2. Step 2: Identify the error

   The code lacks parentheses around num > 5, causing a syntax error.

3. Final Answer:

   Missing parentheses around condition -> Option A

4. Quick Check:

   If condition must be in parentheses [OK]

Hint: Always put condition inside parentheses in if [OK]

Common Mistakes:

• Forgetting parentheses around condition
• Adding semicolon after if line
• Misplacing curly braces

5. You want to print "Positive", "Negative", or "Zero" based on an integer variable num. Which if-else structure correctly implements this?

hard

A. if (num > 0) { Console.WriteLine("Positive"); } else if (num <= 0) { Console.WriteLine("Negative"); } else { Console.WriteLine("Zero"); }

B. if (num > 0) { Console.WriteLine("Positive"); } else { if (num < 0) { Console.WriteLine("Negative"); } else { Console.WriteLine("Zero"); } }

C. if (num >= 0) { Console.WriteLine("Positive"); } else if (num <= 0) { Console.WriteLine("Negative"); } else { Console.WriteLine("Zero"); }

D. if (num > 0) { Console.WriteLine("Positive"); } else if (num > 0) { Console.WriteLine("Negative"); } else { Console.WriteLine("Zero"); }

Solution

1. Step 1: Understand the conditions needed

   We need to check if num is positive, negative, or zero.

2. Step 2: Analyze each option's logic

   if (num > 0) { Console.WriteLine("Positive"); } else { if (num < 0) { Console.WriteLine("Negative"); } else { Console.WriteLine("Zero"); } } correctly nests the else-if logic to handle all three cases without overlap or error.

3. Final Answer:

   if (num > 0) { Console.WriteLine("Positive"); } else { if (num < 0) { Console.WriteLine("Negative"); } else { Console.WriteLine("Zero"); } } -> Option B

4. Quick Check:

   Nested else-if covers all cases correctly [OK]

Hint: Use nested else if or else if for multiple conditions [OK]

Common Mistakes:

• Using overlapping conditions
• Missing zero case
• Repeating same condition twice