🗃️ Variables and Data Types

Welcome back! In the first lesson, we learned how to create and run a basic C# program. Now, let’s explore how programs remember and work with information. We’ll learn about variables (like labeled boxes for storing data), data types (what kind of data goes in the box), and simple expressions (doing calculations).

Lesson: Storing Your First Pieces of Data

What are Variables?

Imagine you’re writing a program and need to keep track of a player’s score or someone’s name. You need a place to store this information while the program is running. That’s what variables are for! A variable is simply a named spot in the computer’s memory where you can store a piece of data.

To use a variable in C#, you first have to declare it. This tells the computer two things:

1. Data Type: What kind of data will this variable hold? (e.g., a whole number, a number with decimals, some text).
2. Name (Identifier): What name will you use to refer to this variable later?

Here’s how you declare variables for common types:

int score;       // Declares a variable named 'score' to hold whole numbers (integers).
string playerName; // Declares a variable named 'playerName' to hold text (strings).
double price;     // Declares a variable named 'price' to hold numbers with decimals.
bool isActive;    // Declares a variable named 'isActive' to hold true/false values.

• int: Short for integer, used for whole numbers like -5, 0, 100.
• string: Used for sequences of text characters, like "Hello" or "Alice".
• double: Used for numbers that might have a decimal point, like 19.99 or -3.5.
• bool: Short for Boolean, used for values that are either true or false.

Naming Your Variables: It’s a common practice in C# to name variables using camelCase. This means the first word starts with a lowercase letter, and any following words start with an uppercase letter (e.g., playerScore, userName, itemPrice). Choose names that clearly describe what the variable holds!

Giving Variables a Value (Assignment)

Declaring a variable just creates the named box. To put something inside it, you use the assignment operator, which is the equals sign (=):

int score;
score = 100; // Assigns the value 100 to the 'score' variable.

string playerName = "Bob"; // You can declare and assign in one step!

double price = 9.95;

isActive = true;

// You can change the value later!
score = 150; // Now the 'score' variable holds 150
playerName = "Alice"; // Now 'playerName' holds "Alice"

Console.WriteLine("The current score is: " + score); // Prints 150

The assignment works from right to left: the value on the right side of the = is put into the variable on the left side. If the variable already had a value, the old value is replaced.

Important: You usually need to assign a value to a variable before you can use it in calculations or print it.

Simple Calculations: Expressions and Operators

Variables aren’t very useful if you can’t do things with them! We use expressions to perform calculations. An expression combines values (like 100), variables (like score), and operators (like + or -) to produce a new value.

Here are the basic arithmetic operators:

• + (Addition)
• - (Subtraction)
• * (Multiplication)
• / (Division)

Let’s see them in action:

int score = 100;
int bonus = 25;
int totalScore = score + bonus; // totalScore will become 125

int apples = 10;
int eaten = 3;
int applesLeft = apples - eaten; // applesLeft will become 7

int pricePerItem = 5;
int quantity = 4;
int totalCost = pricePerItem * quantity; // totalCost will become 20

double totalDistance = 100.0;
double timeTaken = 2.5;
double averageSpeed = totalDistance / timeTaken; // averageSpeed will become 40.0

Getting Input and Showing Output

Programs often need to interact with the user. We can display messages using Console.WriteLine() and read text input using Console.ReadLine().

Console.WriteLine("What is your name?"); // Ask the user a question
string userName = Console.ReadLine(); // Read the text the user types

Console.WriteLine("Hello, " + userName + "!"); // Greet the user

Notice how we used + to combine the text "Hello, " with the value stored in the userName variable and the text "!" before printing.

Changing Types (Type Conversion)

Sometimes you get input as text (a string), but you need it as a number (int or double) to do calculations. You need to convert the type.

Console.Write("Enter your age: ");
string ageString = Console.ReadLine(); // Reads age as text

int age = Convert.ToInt32(ageString); // Converts the text to a whole number

int ageNextYear = age + 1;
Console.WriteLine("Next year you will be " + ageNextYear);

• Convert.ToInt32() tries to change a string into an int.
• Convert.ToDouble() tries to change a string into a double.

Caution: If the user types something that isn’t a valid number (like “abc”), these Convert methods will cause an error! We’ll learn how to handle errors later.

Tutorial: Simple Age Calculator

Let’s build a small program that asks for your name and birth year, then calculates your approximate age.

Objective: Create a program that uses variables, input, output, type conversion, and a simple calculation.

Prerequisites: A C# console project set up (like HelloWorld from the previous lesson, or create a new one: dotnet new console -o AgeCalculator, then cd AgeCalculator).

Step 1: Set up Program.cs

Open Program.cs in VS Code.

using System;

public class Program
{
    public static void Main(string[] args)
    {
        // We'll add our code here
    }
}

Step 2: Declare Variables

We need variables for the name, birth year (as text and number), current year, and age.

    // Inside Main...
    string name;              // For user's name
    string yearOfBirthString; // For birth year input (text)
    int yearOfBirth;          // For birth year (number)
    int currentYear;          // For the current year
    int age;                  // For the calculated age

Step 3: Get the User’s Name

Ask for the name and read it.

    // ... after variable declarations
    Console.Write("Please enter your name: "); // Ask without starting a new line
    name = Console.ReadLine(); // Read the typed name

Step 4: Get the User’s Birth Year

Ask for the birth year and read it as text.

    // ... after getting name
    Console.Write("Please enter your year of birth: ");
    yearOfBirthString = Console.ReadLine(); // Read the year as text

Step 5: Convert Birth Year to a Number

Change the text input into an integer.

    // ... after getting birth year string
    yearOfBirth = Convert.ToInt32(yearOfBirthString); // Convert text to number

Step 6: Get the Current Year

C# can tell us the current year.

    // ... after converting birth year
    currentYear = DateTime.Now.Year; // Gets the current year (e.g., 2025)

Step 7: Calculate the Age

Subtract the birth year from the current year.

    // ... after getting current year
    age = currentYear - yearOfBirth; // Simple subtraction

Step 8: Display the Result

Show a message to the user.

    // ... after calculating age
    Console.WriteLine("Hey " + name + ", you are approximately " + age + " years old this year.");
}

Step 9: Build and Run

1. Save Program.cs.
2. In your terminal (in the project folder), run dotnet build.
3. Then run dotnet run.
4. Try entering your name and birth year!

Example Interaction:

Please enter your name: Charlie
Please enter your year of birth: 2005
Hey Charlie, you are approximately 20 years old this year.

(The age will depend on the current year when you run it.)

Exercise: Rectangle Area Calculator

Let’s practice! Create a program that calculates the area of a rectangle.

Project Goal: Ask the user for the width and height of a rectangle and display its area.

Requirements:

1. Create a new console project (e.g., RectangleArea).
2. Ask the user to enter the width.
3. Read the width as text (string).
4. Convert the width text to a number (double is good here to allow decimals).
5. Ask the user to enter the height.
6. Read the height as text (string).
7. Convert the height text to a number (double).
8. Calculate the area: area = width * height.
9. Display the width, height, and calculated area.

Hints:

• Use Console.Write() for prompts.
• Use Console.ReadLine() convert text to a decimal number.
• Use Console.WriteLine() to show the results (e.g., Console.WriteLine("Area: " + area);).

Steps:

1. Set up the project.
2. Write the code in Program.cs.
3. Save, build (dotnet build), and run (dotnet run).
4. Test with different numbers.

Conclusion

In this lesson, we learned how to use variables (int, string, double, bool) to store different kinds of data. We saw how to assign values using =, perform simple calculations with +, -, *, /, get input from the user with Console.ReadLine(), display output with Console.WriteLine(), and convert between text and numbers using Convert. These are essential building blocks for almost any program!