Variables in C++

In C++, variables are used to store data that your program can manipulate. They act as named containers that hold values which may change (or not) during program execution. Understanding variables is fundamental to learning any programming language.

What is a Variable?

A variable is a named memory location used to store a value of a specific data type. You can think of it as a labeled box where you can put and retrieve values.

Syntax:

variable_syntax.cpp

#include <iostream>
 
int main() {
    int age = 25;
    std::cout << "Age: " << age << std::endl;
    return 0;
}

In this example:

int is the data type
age is the variable name
25 is the value assigned

Declaring Variables

You declare a variable using the following format:

<data_type> <variable_name> = <initial_value>;

Examples:

variable_examples.cpp

#include <iostream>
 
int marks = 90;
float temperature = 36.6;
char grade = 'A';

Rules for Naming Variables

Understanding naming rules ensures your code is both valid and readable.

✅ Allowed:

Letters (a–z, A–Z)
Digits (0–9), but not as the first character
Underscore (_)

❌ Not Allowed:

Spaces
Special characters (@, #, $, etc.)
Starting with a digit

Best Practices

Rule	Example
Start with a letter	`score`, `height`
Use descriptive names	`totalMarks`, `age`
Avoid reserved keywords	`int`, `return` ❌
Use camelCase or snake_case for readability	`userName` or `user_name`

Variable Declaration Without Initialization

You can also declare a variable without assigning a value initially:

variable_declare_only.cpp

#include <iostream>
 
int main() {
    int count; // declared, not initialized
    count = 10; // value assigned later
    std::cout << count << std::endl;
    return 0;
}

Note: Using an uninitialized variable without assigning a value leads to undefined behavior.

Multiple Declarations in One Line

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

multi_variable.cpp

int x = 5, y = 10, z = 15;

Constants vs Variables

A variable can change during execution. A constant cannot.

constant_variable.cpp

const int maxScore = 100;  // value cannot be changed
int score = 90;            // value can change

Practice: Yes or No?

Let’s reinforce the rules with a few yes/no questions.

Q1. Can a variable name start with a number? ❌ No

Q2. Is float a valid variable name? ❌ No – it’s a reserved keyword

Q3. Can you declare a variable without initializing it? ✅ Yes

Q4. Can a variable name contain an underscore (_)? ✅ Yes

Q5. Is int 5age = 30; valid? ❌ No – variable name cannot start with a digit

Practice Code: Area of a Circle

circle_area.cpp

#include <iostream>
 
int main() {
    float radius = 4.5;
    float area = 3.14 * radius * radius;
    std::cout << "Area: " << area << std::endl;
    return 0;
}

Practice Code: Swapping Two Numbers

swap_numbers.cpp

#include <iostream>
 
int main() {
    int a = 5, b = 10;
    int temp = a;
    a = b;
    b = temp;
    std::cout << "a: " << a << ", b: " << b << std::endl;
    return 0;
}

Summary

Concept	Description
Variable	Named container to hold a value
Declaration	`type name;` or `type name = value;`
Initialization (optional)	Assign a value at the time of declaration
Naming rules	Start with a letter or `_`, no symbols or spaces
Multiple declaration	Allowed if of the same type
Constants (`const`)	Used when values must not change

Variables are the foundation of all logical operations in C++. Mastering them will enable you to manage memory, perform calculations, and build interactive programs effectively.

Wide Character Constants