If-else execution flow in C Sharp (C#)

Introduction

If-else helps your program make choices. It runs different code depending on conditions.

Deciding if a user is old enough to access a website

Checking if a number is positive or negative

Choosing what message to show based on the time of day

Turning on a light if it is dark outside

Syntax

C Sharp (C#)

if (condition)
{
    // code runs if condition is true
}
else
{
    // code runs if condition is false
}

The condition is a true or false question your program asks.

Only one block runs: either the if part or the else part.

Examples

This checks if age is 18 or more. It prints a message based on that.

C Sharp (C#)

int age = 18;
if (age >= 18)
{
    Console.WriteLine("You can vote.");
}
else
{
    Console.WriteLine("You are too young to vote.");
}

This checks if number is positive or not and prints accordingly.

C Sharp (C#)

int number = -5;
if (number > 0)
{
    Console.WriteLine("Positive number");
}
else
{
    Console.WriteLine("Zero or negative number");
}

Sample Program

This program asks the user for their age. It then uses if-else to print if they are an adult or a minor.

C Sharp (C#)

using System;

class Program
{
    static void Main()
    {
        Console.Write("Enter your age: ");
        int age = int.Parse(Console.ReadLine() ?? "0");

        if (age >= 18)
        {
            Console.WriteLine("You are an adult.");
        }
        else
        {
            Console.WriteLine("You are a minor.");
        }
    }
}

Important Notes

Make sure the condition inside if is something that results in true or false.

You can have many if statements, but only one else per if.

Summary

If-else lets your program choose between two paths.

The if block runs when the condition is true.

The else block runs when the condition is false.

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