🎮 Control Flow in C#

Imagine you’re giving someone directions. You might say, “If the light is red, stop. Otherwise, go.” Or, “Keep walking until you reach the park.” Programs need to make similar decisions and repeat actions too!

This lesson introduces Control Flow – how we guide the order in which instructions run in our C# programs. We’ll learn how to make simple decisions using if and else based on true/false conditions.

(Prerequisites: Ensure you understand the concepts from “Variables and Data Types”.)

Lesson: Making Simple Decisions

True or False: Boolean Logic

At the heart of decision-making is the concept of true and false. In C#, we use the bool data type, which can hold only one of two values: true or false.

How do we get these true or false values? Often, we use comparison operators to compare other values:

• == (Is Equal To?): Checks if two values are the same.
  • 5 == 5 is true
  • 5 == 6 is false
• != (Is Not Equal To?): Checks if two values are different.
  • 5 != 6 is true
  • "hello" != "hello" is false
• > (Greater Than?): Checks if the left value is bigger than the right.
  • 10 > 5 is true
• < (Less Than?): Checks if the left value is smaller than the right.
  • 3 < 8 is true
• >= (Greater Than or Equal To?)
  • 7 >= 7 is true
• <= (Less Than or Equal To?)
  • 4 <= 5 is true

int myAge = 20;
bool canVote = myAge >= 18; // canVote becomes true

int score = 50;
bool isPerfectScore = score == 100; // isPerfectScore becomes false

Console.WriteLine("Can I vote? " + canVote); // Output: Can I vote? True

These true/false results are what we use to make decisions.

The if Statement: Doing Something Conditionally

The simplest way to make a decision is with the if statement. It checks if a condition is true. If it is, the code block inside the curly braces {} runs. If the condition is false, the block is skipped.

// Syntax:
// if (condition)
// {
//     Code to run if condition is true
// }

int temperature = 15;

if (temperature < 20)
{
    Console.WriteLine("It's a bit chilly, wear a jacket!"); // This line runs
}

Console.WriteLine("Weather advice complete."); // This line always runs afterwards

The if-else Statement: Choosing Between Two Paths

What if you want to do one thing if the condition is true, and a different thing if it’s false? Use if-else.

// Syntax:
// if (condition)
// {
//     Code to run if condition is true
// }
// else
// {
//     Code to run if condition is false
// }

int userAge = 16;

if (userAge >= 18)
{
    Console.WriteLine("You are eligible to vote.");
}
else
{
    Console.WriteLine("Sorry, you are not yet eligible to vote."); // This line runs
}

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

Combining Conditions: && (AND) and || (OR)

Sometimes you need to check multiple conditions at once.

• && (AND): Both sides must be true for the whole thing to be true.
  • age >= 18 && hasLicense == true (Must be 18+ AND have a license)
• || (OR): At least one side must be true for the whole thing to be true.
  • isWeekend == true || isHoliday == true (It’s okay if it’s the weekend OR a holiday)

bool hasTicket = true;
bool isMember = false;

// Example using AND (&&)
if (hasTicket && isMember)
{
    Console.WriteLine("Welcome, valued member with a ticket!");
}
else
{
    Console.WriteLine("Ticket and membership required for this area."); // This runs
}

// Example using OR (||)
if (hasTicket || isMember)
{
    Console.WriteLine("You can enter the main area."); // This runs
}
else
{
    Console.WriteLine("You need either a ticket or membership to enter.");
}

These logical operators let you build more complex decision-making rules.

Tutorial: Simple Number Checker

Let’s write a program that asks the user for a number and tells them if it’s positive, negative, or zero using if-else.

Objective: Practice using comparison operators and if-else if-else statements.

Prerequisites: A C# console project.

Step 1: Set up Program.cs

using System;

public class Program
{
    public static void Main(string[] args)
    {
        Console.Write("Enter a whole number: ");
        string input = Console.ReadLine();

        int number;
        bool success = int.TryParse(input, out number);

        if (success)
        {
            // Decision logic goes here
        }
        else
        {
            Console.WriteLine("That wasn't a valid whole number.");
        }
    }
}

Explanation: We get input and use int.TryParse to safely convert it to a number. The if(success) block runs only if the conversion worked.

Step 2: Add if-else if-else Logic

Inside the if (success) block, add the conditions to check the number.

if (success)
{
   // Check if the number is greater than 0
   if (number > 0)
   {
         Console.WriteLine("The number is positive.");
   }
   // If not positive, check if it's less than 0
   else if (number < 0)
   {
         Console.WriteLine("The number is negative.");
   }
   // If it's not positive and not negative, it must be 0
   else
   {
         Console.WriteLine("The number is zero.");
   }
}
else
{
   Console.WriteLine("That wasn't a valid whole number.");
}

Explanation: We first check number > 0. If that’s false, we then check number < 0. If that’s also false, the final else block runs, meaning the number must be 0.

Step 3: Build and Run

1. Save Program.cs.
2. Build (dotnet build) and run (dotnet run).
3. Test by entering a positive number (e.g., 10), a negative number (e.g., -5), zero (0), and invalid text (e.g., hello).

Example Interaction:

Enter a whole number: -7
The number is negative.

Enter a whole number: 0
The number is zero.

Exercise: Simple Access Check

Create a program that simulates a basic access check based on age.

Project Goal: Ask the user for their age and print a specific message based on whether they are under 13, between 13 and 17 (inclusive), or 18 or older.

Requirements:

1. Create a new console project (e.g., AgeCheck).
2. Prompt the user to enter their age.
3. Use int.TryParse to safely get the age as an integer.
4. If the input is invalid, print an error message.
5. If the input is valid, use if-else if-else statements:
   • If age is less than 13, print “Child access only.”
   • If age is greater than or equal to 13 AND less than 18, print “Teenager access granted.”
   • If age is greater than or equal to 18, print “Adult access granted.”

Hints:

• Remember int.TryParse(input, out age).
• For the teenager check, you’ll need the && (AND) operator: age >= 13 && age < 18.

Steps:

1. Write the code in Program.cs.
2. Save, build, and run.
3. Test with ages like 10, 15, 25, and invalid input.

Conclusion

In this lesson, we learned the basics of controlling program flow. We saw how Boolean logic (true/false) and comparison operators (==, >, < etc.) allow us to form conditions. We then used these conditions with if and if-else statements to make our programs execute different code paths based on whether those conditions are met. This ability to make decisions is a fundamental part of programming!