Variables - Storing Information

What is a Variable?

Imagine you have boxes in your room, and each box has a label. You can put different things in each box:

• A box labeled "age" holds how old you are
• A box labeled "name" holds your name
• A box labeled "score" holds your game score

In C++, variables are these labeled boxes that store information in your computer's memory.

Creating Your First Variable

#include <iostream>

int main() {
    int age;
    age = 13;

    std::cout << "I am " << age << " years old" << std::endl;

    return 0;
}

What's happening here:

1. int age; - Creates a box labeled "age" that can hold whole numbers
2. age = 13; - Puts the number 13 into that box
3. std::cout << age - Looks inside the box and prints what's there

Output:

I am 13 years old

Declaring and Initializing Together

You can create a box and put something in it at the same time:

#include <iostream>

int main() {
    int age = 13;

    std::cout << "I am " << age << " years old" << std::endl;

    return 0;
}

This does the same thing, just faster!

Types of Variables (Data Types)

Just like in real life, different types of information need different types of boxes. C++ has several types:

1. int - Whole Numbers

Use int when you need whole numbers (no decimals).

#include <iostream>

int main() {
    int lives = 3;
    int score = 1500;
    int level = 5;

    std::cout << "Lives: " << lives << std::endl;
    std::cout << "Score: " << score << std::endl;
    std::cout << "Level: " << level << std::endl;

    return 0;
}

Output:

Lives: 3
Score: 1500
Level: 5

Range: An int can hold numbers from about -2 billion to +2 billion.

2. double - Decimal Numbers

Use double when you need numbers with decimal points.

#include <iostream>

int main() {
    double price = 19.99;
    double temperature = 98.6;
    double pi = 3.14159;

    std::cout << "Price: $" << price << std::endl;
    std::cout << "Temperature: " << temperature << "°F" << std::endl;
    std::cout << "Pi: " << pi << std::endl;

    return 0;
}

Output:

Price: $19.99
Temperature: 98.6°F
Pi: 3.14159

3. char - Single Characters

Use char for a single letter, digit, or symbol. Use single quotes ' '.

#include <iostream>

int main() {
    char grade = 'A';
    char initial = 'J';
    char symbol = '@';

    std::cout << "Grade: " << grade << std::endl;
    std::cout << "Initial: " << initial << std::endl;
    std::cout << "Symbol: " << symbol << std::endl;

    return 0;
}

Output:

Grade: A
Initial: J
Symbol: @

Important:

• Single character: 'A' (single quotes)
• Text/string: "Alex" (double quotes)

4. bool - True or False

Use bool for yes/no or true/false values.

#include <iostream>

int main() {
    bool isRaining = true;
    bool isSunny = false;
    bool hasTicket = true;

    std::cout << "Is it raining? " << isRaining << std::endl;
    std::cout << "Is it sunny? " << isSunny << std::endl;
    std::cout << "Has ticket? " << hasTicket << std::endl;

    return 0;
}

Output:

Is it raining? 1
Is it sunny? 0
Has ticket? 1

Note: true displays as 1, false displays as 0.

5. string - Text

Use string for words and sentences. Remember to include <string>!

#include <iostream>
#include <string>

int main() {
    std::string name = "Alex";
    std::string favoriteGame = "Minecraft";
    std::string city = "New York";

    std::cout << "Name: " << name << std::endl;
    std::cout << "Favorite Game: " << favoriteGame << std::endl;
    std::cout << "City: " << city << std::endl;

    return 0;
}

Output:

Name: Alex
Favorite Game: Minecraft
City: New York

Variable Names - The Rules

You can name variables almost anything, but there are rules:

✅ Good Names

int age;
int playerScore;
int number_of_lives;
int total2;
int x;

❌ Bad Names (Will Cause Errors)

int 2fast;
int my-score;
int player score;
int int;
int return;

Rules for Naming:

1. Must start with a letter or underscore (_)
2. Can contain letters, numbers, and underscores
3. No spaces allowed
4. Can't use special C++ words (like int, return, if, etc.)
5. Case matters! age and Age are different variables

Naming Styles

camelCase (popular in C++):

int playerScore;
int numberOfLives;

snake_case:

int player_score;
int number_of_lives;

PascalCase (usually for classes, which we'll learn later):

int PlayerScore;
int NumberOfLives;

Pick one style and stick with it! Consistency makes code easier to read.

Descriptive Names are Better

int s;
int score;

Which is clearer? Use names that describe what the variable holds!

Changing Variable Values

Variables can change (that's why they're called "variables"!)

#include <iostream>

int main() {
    int health = 100;
    std::cout << "Starting health: " << health << std::endl;

    health = 75;
    std::cout << "After taking damage: " << health << std::endl;

    health = 90;
    std::cout << "After healing: " << health << std::endl;

    return 0;
}

Output:

Starting health: 100
After taking damage: 75
After healing: 90

Constants - Values That Never Change

Sometimes you want a value that can never be changed. Use const:

#include <iostream>

int main() {
    const int MAX_LIVES = 5;
    const double PI = 3.14159;
    const int SPEED_OF_LIGHT = 299792458;

    std::cout << "Maximum lives: " << MAX_LIVES << std::endl;
    std::cout << "Value of Pi: " << PI << std::endl;
    std::cout << "Speed of light: " << SPEED_OF_LIGHT << " m/s" << std::endl;

    return 0;
}

Output:

Maximum lives: 5
Value of Pi: 3.14159
Speed of light: 299792458 m/s

Why use constants?

• Prevents accidental changes
• Makes code more readable
• Easier to update (change in one place)

Convention: Constant names are usually ALL_CAPS with underscores.

If you try to change a constant, you'll get an error:

const int MAX_LIVES = 5;
MAX_LIVES = 10;

Compiler error: assignment of read-only variable 'MAX_LIVES'

Multiple Variables

You can declare multiple variables of the same type in one line:

#include <iostream>

int main() {
    int x = 10, y = 20, z = 30;

    std::cout << "x = " << x << std::endl;
    std::cout << "y = " << y << std::endl;
    std::cout << "z = " << z << std::endl;

    return 0;
}

Output:

x = 10
y = 20
z = 30

But for beginners, writing them separately is clearer:

int x = 10;
int y = 20;
int z = 30;

Getting User Input

Let's make programs interactive! Use std::cin to get input from the user.

#include <iostream>
#include <string>

int main() {
    std::string name;
    int age;

    std::cout << "What is your name? ";
    std::cin >> name;

    std::cout << "How old are you? ";
    std::cin >> age;

    std::cout << "\nNice to meet you, " << name << "!" << std::endl;
    std::cout << "You are " << age << " years old." << std::endl;

    return 0;
}

Running this program:

What is your name? Alex
How old are you? 13

Nice to meet you, Alex!
You are 13 years old.

Understanding cin:

• std::cout = output (printing TO screen)
• std::cin = input (reading FROM keyboard)
• << = send data TO cout
• >> = get data FROM cin

Reading Full Sentences

There's a problem with std::cin - it stops at spaces!

#include <iostream>
#include <string>

int main() {
    std::string favoriteGame;

    std::cout << "What's your favorite game? ";
    std::cin >> favoriteGame;

    std::cout << "Cool! You like " << favoriteGame << std::endl;

    return 0;
}

If you type "Minecraft Java Edition":

What's your favorite game? Minecraft Java Edition
Cool! You like Minecraft

It only read "Minecraft"! To read the whole line, use getline:

#include <iostream>
#include <string>

int main() {
    std::string favoriteGame;

    std::cout << "What's your favorite game? ";
    std::getline(std::cin, favoriteGame);

    std::cout << "Cool! You like " << favoriteGame << std::endl;

    return 0;
}

Now it works:

What's your favorite game? Minecraft Java Edition
Cool! You like Minecraft Java Edition

Data Type Sizes

Different types use different amounts of computer memory:

Type	Typical Size	Range/Notes
char	1 byte	-128 to 127 or one character
int	4 bytes	-2,147,483,648 to 2,147,483,647
long long	8 bytes	Really huge numbers
float	4 bytes	Decimals (7 digits precision)
double	8 bytes	Decimals (15 digits precision)
bool	1 byte	true or false

For really big numbers, use long long:

#include <iostream>

int main() {
    long long bigNumber = 9223372036854775807;
    std::cout << "Huge number: " << bigNumber << std::endl;
    return 0;
}

Type Conversion

Sometimes you need to convert between types:

#include <iostream>

int main() {
    int wholeNumber = 7;
    double decimal = wholeNumber;

    std::cout << "Whole: " << wholeNumber << std::endl;
    std::cout << "Decimal: " << decimal << std::endl;

    double pi = 3.14159;
    int truncated = pi;

    std::cout << "Pi: " << pi << std::endl;
    std::cout << "Truncated: " << truncated << std::endl;

    return 0;
}

Output:

Whole: 7
Decimal: 7
Pi: 3.14159
Truncated: 3

Warning: Converting double to int removes the decimal part!

Common Mistakes

Mistake 1: Using a Variable Before Declaring It

score = 100;
int score;

Fix: Declare before using:

int score;
score = 100;

Or do both at once:

int score = 100;

Mistake 2: Forgetting Semicolons

int age = 13
std::cout << age;

Fix: Add the semicolon:

int age = 13;
std::cout << age;

Mistake 3: Wrong Type for Data

int price = 19.99;

This will become 19! Use double for decimals:

double price = 19.99;

Mistake 4: Forgetting to Initialize

int score;
std::cout << score;

score has a random value! Always initialize:

int score = 0;
std::cout << score;

Practice Project: Profile Creator

Let's build a program that asks for multiple pieces of information and displays a profile:

#include <iostream>
#include <string>

int main() {
    std::string name;
    int age;
    std::string favoriteColor;
    double height;
    bool likesGaming;

    std::cout << "=== Create Your Profile ===" << std::endl;
    std::cout << "\nWhat's your name? ";
    std::cin >> name;

    std::cout << "How old are you? ";
    std::cin >> age;

    std::cout << "What's your favorite color? ";
    std::cin >> favoriteColor;

    std::cout << "How tall are you in feet? (like 5.5) ";
    std::cin >> height;

    std::cout << "Do you like gaming? (1 for yes, 0 for no) ";
    std::cin >> likesGaming;

    std::cout << "\n=== YOUR PROFILE ===" << std::endl;
    std::cout << "Name: " << name << std::endl;
    std::cout << "Age: " << age << " years old" << std::endl;
    std::cout << "Favorite Color: " << favoriteColor << std::endl;
    std::cout << "Height: " << height << " feet" << std::endl;
    std::cout << "Likes Gaming: " << likesGaming << std::endl;

    return 0;
}

Challenge: Add more fields like favorite food, number of siblings, or favorite number!

What's Next?

Now you know how to store and use different types of information! Next up:

• Math Operations - Making your computer calculate things
• Operators - Comparing values and making decisions
• Conditions - Making your programs smart with if/else

The fun is just beginning! 🚀