Variables and Data Types

Up to now, our programs have only displayed pre-determined (hardcoded) text. But useful programs need to be able to store and process data — usernames, game scores, room temperatures, and so on. For that, we need variables.

What Is a Variable?

Think of a variable as a labeled storage box. Each box:

• Has a name (the label on the outside) — so you know which box contains what
• Has a type (the size and shape of the box) — determines what can be stored inside
• Has contents (the stored value) — the data inside the box

┌─────────────┐    ┌─────────────┐    ┌─────────────┐
│   age       │    │   name      │    │   height    │
│  ┌───────┐  │    │  ┌───────┐  │    │  ┌───────┐  │
│  │  17   │  │    │  │"Budi" │  │    │  │ 170.5 │  │
│  └───────┘  │    │  └───────┘  │    │  └───────┘  │
│  type: int  │    │ type:string │    │ type:double │
└─────────────┘    └─────────────┘    └─────────────┘

Declaring Variables

To create a variable in C++, you need to specify the data type and variable name:

data_type variable_name;

Examples:

int age;           // Create a box called "age" for whole numbers
double height;     // Create a box called "height" for decimal numbers
std::string name;  // Create a box called "name" for text

This is called a declaration — you’re telling the compiler: “I need a box to store this kind of data.”

Initializing Variables

After creating the box, you need to fill it with a value. This is called initialization:

int age = 17;                    // Declaration + initialization at once
double height = 170.5;
std::string name = "Budi";

You can also separate the declaration and initialization:

int age;        // Declare first
age = 17;       // Fill it in later

But generally, it’s better to initialize right at declaration to avoid bugs.

int x;                      // Not initialized — DANGEROUS
std::cout << x << std::endl; // Could display a random number!

Basic Data Types

C++ has several basic data types. Here are the most commonly used:

int — Integers

For storing whole numbers with no decimals: …-3, -2, -1, 0, 1, 2, 3…

int age = 17;
int temperature = -5;
int score = 100;
int population = 270000000;  // 270 million

double — Decimal Numbers

For storing numbers with a decimal point:

double height = 170.5;
double pi = 3.14159;
double temperature = -12.8;
double price = 25000.0;    // Even if it's a whole number, use .0 for double type

char — Single Character

For storing one character. Its value is enclosed in single quotes (not double):

char letter = 'A';
char digit = '7';
char symbol = '@';
char space = ' ';

Important: char uses single quotes ('A'), while string uses double quotes ("A"). They are different!

std::string — Text/Character Sequence

For storing text consisting of zero or more characters:

#include <string>  // Don't forget to include this!

std::string name = "Rina Sari";
std::string city = "Jakarta";
std::string empty = "";        // An empty string is valid too
std::string greeting = "Hello, world!";

bool — Boolean (True/False)

For storing truth values — can only be true or false:

bool hasGraduated = true;
bool isRaining = false;
bool isStudent = true;

bool will be very useful when we learn branching (if/else) in Unit 2.

Data Type Summary Table

Naming Convention (Naming Rules)

Variable names in C++ must follow these rules:

Required rules:

• Can only contain letters, digits, and underscores (_)
• Cannot start with a digit
• Cannot use C++ keywords (int, return, if, etc.)
• Case-sensitive: age, Age, and AGE are different variables!

Conventions (best practices):

• Use descriptive names: age is better than a
• Use camelCase: studentCount, averageScore
• Or snake_case: student_count, average_score
• Pick one style and be consistent!

// Good - clear and descriptive
int studentAge = 15;
double examScore = 87.5;
std::string fullName = "Ahmad Fauzi";

// Bad - confusing
int x = 15;          // What is x?
double n = 87.5;     // What does n stand for?
std::string s = "Ahmad Fauzi";  // s?

Changing Variable Values

Once declared, a variable’s value can be changed at any time (except const, which we’ll cover later):

#include <iostream>

int main() {
    int score = 0;
    std::cout << "Initial score: " << score << std::endl;

    score = 10;  // Change the value
    std::cout << "Score after round 1: " << score << std::endl;

    score = 25;  // Change again
    std::cout << "Score after round 2: " << score << std::endl;

    return 0;
}

Output:

Initial score: 0
Score after round 1: 10
Score after round 2: 25

Notice: when changing the value, you don’t need to write the data type again. score = 10; is enough — you don’t need int score = 10; because the variable was already declared earlier.

Full Program: All Data Types

#include <iostream>
#include <string>

int main() {
    // Declare various data types
    std::string name = "Rina";
    int age = 16;
    double height = 162.5;
    char bloodType = 'O';
    bool inHighSchool = true;

    // Display all data
    std::cout << "=== Personal Data ===" << std::endl;
    std::cout << "Name       : " << name << std::endl;
    std::cout << "Age        : " << age << " years" << std::endl;
    std::cout << "Height     : " << height << " cm" << std::endl;
    std::cout << "Blood Type : " << bloodType << std::endl;
    std::cout << "In High School: " << inHighSchool << std::endl;  // 1 = true

    return 0;
}

Output:

=== Personal Data ===
Name       : Rina
Age        : 16 years
Height     : 162.5 cm
Blood Type : O
In High School: 1

Common Mistakes

1. Mismatched data types:

int age = 17.5;      // 17.5 gets truncated to 17 (the decimal part is lost!)

2. Declaring a variable twice:

int score = 10;
int score = 20;  // ERROR: a variable named 'score' already exists

3. Using a variable before declaring it:

std::cout << age;  // ERROR: 'age' has not been declared
int age = 17;

4. Variable name starting with a digit:

int 2ndPlace = 2;   // ERROR: name cannot start with a digit
int secondPlace = 2; // OK

Exercises

Create a program that declares variables to store personal data, then displays them on screen. Use at least 5 variables with different data types:

#include <iostream>
#include <string>

int main() {
    // Declare the following variables and fill with your own data:
    // - Full name (string)
    // - Age (int)
    // - Height in cm (double)
    // - First name initial (char)
    // - Whether you like programming (bool)

    // Display all data on screen

    return 0;
}

Expected output example:

Name     : Ahmad Fauzi
Age      : 15 years
Height   : 165.3 cm
Initial  : A
Likes programming: 1

Summary

• A variable is a place to store data, with a name, type, and value
• Declaration: type name; — creates a variable
• Initialization: type name = value; — creates and fills a variable
• Basic data types: int (integer), double (decimal), char (character), std::string (text), bool (true/false)
• Variable names should be descriptive and follow naming conventions
• Always initialize variables at declaration

In the next lesson, we’ll learn about input and output — how a program communicates with the user!