05 ControlFlow - C# Mastery Hub

What is Control Flow?

Control flow is how your program decides which code to run and when. It's like a traffic light for your code - it directs the flow of execution based on conditions.

If/Else
Make decisions

Switch
Multiple choices

Ternary
Short conditions

Think of it like this: When you wake up, if it's raining you take an umbrella, else you don't. That's control flow in real life!

The if Statement

The if statement checks a condition. If the condition is true, it runs the code inside the curly braces.

Basic if

if (age >= 18)
{
    Console.WriteLine("Adult");
}

What if condition is false?

If the condition is false, the code inside is skipped.

// ===== SIMPLE IF =====
int age = 20;

if (age >= 18)
{
    Console.WriteLine("You are an adult.");
}
// Output: "You are an adult."

// ===== WITH MULTIPLE LINES =====
if (isLoggedIn)
{
    Console.WriteLine("Welcome back!");
    Console.WriteLine("You have 3 new messages.");
}

The if-else Statement

The if-else statement gives you two paths. If the condition is true, run the first block. If false, run the second block.

True Path

if (score >= 60) → "Pass"

False Path

else → "Fail"

// ===== IF-ELSE =====
int score = 75;

if (score >= 60)
{
    Console.WriteLine("Pass");
}
else
{
    Console.WriteLine("Fail");
}
// Output: "Pass"

// ===== REAL-WORLD EXAMPLE =====
bool hasPermission = false;

if (hasPermission)
{
    Console.WriteLine("You can delete this file.");
}
else
{
    Console.WriteLine("You don't have permission.");
}

The if-else if-else Statement

When you have multiple conditions, use else if. It checks each condition in order until one is true.

// ===== MULTIPLE CONDITIONS =====
int grade = 85;

if (grade >= 90)
{
    Console.WriteLine("A");
}
else if (grade >= 80)
{
    Console.WriteLine("B");
}
else if (grade >= 70)
{
    Console.WriteLine("C");
}
else if (grade >= 60)
{
    Console.WriteLine("D");
}
else
{
    Console.WriteLine("F");
}
// Output: "B"

// ===== IMPORTANT: Order Matters! =====
// Conditions are checked from top to bottom
// Once one is true, the rest are skipped

Important! The order of conditions matters. Put the most specific conditions first, and the most general last.

The Switch Statement

The switch statement is a cleaner way to check many specific values of a single variable. It's like a multiple-choice question.

When to use Switch

Checking one variable against many values
The variable has a limited set of possibilities
You want cleaner code than multiple if-else

When NOT to use Switch

Complex conditions with ranges
When you need to check multiple variables
When conditions use different operators

// ===== BASIC SWITCH =====
string day = "Monday";

switch (day)
{
    case "Monday":
        Console.WriteLine("Start of week");
        break;  // Don't forget break!
    case "Friday":
        Console.WriteLine("End of week");
        break;
    case "Saturday":
    case "Sunday":
        Console.WriteLine("Weekend");
        break;  // Both Saturday and Sunday run the same code
    default:
        Console.WriteLine("Midweek");
        break;  // default runs when no case matches
}

// ===== SWITCH WITH NUMBERS =====
int month = 3;

switch (month)
{
    case 1:
        Console.WriteLine("January");
        break;
    case 2:
        Console.WriteLine("February");
        break;
    case 3:
        Console.WriteLine("March");
        break;
    default:
        Console.WriteLine("Invalid month");
        break;
}

Remember: Always include a break at the end of each case. Without it, code will "fall through" to the next case!

The Ternary Operator

The ternary operator (? :) is a short way to write an if-else statement in one line. It's like a quick question: "If true, do this, else do that."

// ===== TERNARY OPERATOR SYNTAX =====
// condition ? value_if_true : value_if_false

int age = 20;
string status = age >= 18 ? "Adult" : "Minor";
// status = "Adult"

// ===== SAME AS THIS IF-ELSE =====
string status2;
if (age >= 18)
    status2 = "Adult";
else
    status2 = "Minor";

// ===== MORE EXAMPLES =====
bool isLoggedIn = true;
string greeting = isLoggedIn ? "Welcome back!" : "Please login.";

// ===== NESTED TERNARY (Use carefully!) =====
int score = 85;
string grade = score >= 90 ? "A" : 
             score >= 80 ? "B" : 
             score >= 70 ? "C" : 
             score >= 60 ? "D" : "F";

Tip: Ternary operators are great for simple conditions. For complex logic, use if-else - it's easier to read!

When to Use Each

Statement	Best For	Example
if	Single condition check	`if (age >= 18)`
if-else	Two possible outcomes	`if (score >= 60) else`
if-else if	Multiple related conditions	`if (score >= 90) else if (score >= 80)`
switch	Multiple specific values	`switch (day) { case "Monday": ... }`
Ternary	Simple one-line decisions	`age >= 18 ? "Adult" : "Minor"`

Exercise: Grade Calculator

Task: Create a grade calculator that uses all types of control flow.

Instructions:

Create a console application called "GradeCalculator"
Ask the user to enter their test score (0-100)
Use if-else if to calculate the letter grade:
- 90-100: A
- 80-89: B
- 70-79: C
- 60-69: D
- Below 60: F
Use a switch to display a message based on the letter grade
Use the ternary operator to check if the student passed
Add validation to make sure the score is between 0 and 100

Hints:

Use Console.ReadLine() to get user input
Use int.Parse() or Convert.ToInt32() to convert to a number
Use if (score >= 90) for the first condition
In the switch, use case "A": etc.
Ternary example: string result = passed ? "Passed" : "Failed";

Expected Output:

Enter your test score (0-100): 85
Letter Grade: B
Message: Good job!
Result: Passed!

// Grade Calculator
using System;

class Program
{
    static void Main()
    {
        // Ask for user input
        Console.Write("Enter your test score (0-100): ");
        string input = Console.ReadLine();

        // Convert to integer
        int score = int.Parse(input);

        // Validate the score
        if (score < 0 || score > 100)
        {
            Console.WriteLine("Invalid score! Please enter a number between 0 and 100.");
            return;
        }

        // Calculate letter grade using if-else if
        string letterGrade;

        if (score >= 90)
        {
            letterGrade = "A";
        }
        else if (score >= 80)
        {
            letterGrade = "B";
        }
        else if (score >= 70)
        {
            letterGrade = "C";
        }
        else if (score >= 60)
        {
            letterGrade = "D";
        }
        else
        {
            letterGrade = "F";
        }

        // Display grade using switch
        string message;
        switch (letterGrade)
        {
            case "A":
                message = "Excellent work!";
                break;
            case "B":
                message = "Good job!";
                break;
            case "C":
                message = "Keep practicing!";
                break;
            case "D":
                message = "Need more effort!";
                break;
            case "F":
                message = "Please review the material!";
                break;
            default:
                message = "Unknown grade";
                break;
        }

        // Check if passed using ternary operator
        bool passed = score >= 60;
        string result = passed ? "Passed!" : "Failed!";

        // Print results
        Console.WriteLine($"Letter Grade: {letterGrade}");
        Console.WriteLine($"Message: {message}");
        Console.WriteLine($"Result: {result}");

        // Bonus: Print the grade scale
        Console.WriteLine("\nGrade Scale:");
        Console.WriteLine("90-100: A");
        Console.WriteLine("80-89:  B");
        Console.WriteLine("70-79:  C");
        Console.WriteLine("60-69:  D");
        Console.WriteLine("0-59:   F");
    }
}

Key Takeaway

Control flow statements let you control which code runs and when. Use if for simple decisions, if-else for two paths, if-else if for multiple conditions, switch for many specific values, and ternary for quick one-line decisions.

Control Flow