Variables, Data Types & Expressions in C++

What You’ll Learn

By the end of this post, you will:

• Understand what variables and data types are
• Learn how to declare and initialize variables
• Use basic arithmetic operators and expressions
• Learn about type conversion and constants

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What is a Variable?

A variable is a named storage location in your program that holds a value. You can think of it like a labeled container.

In C++, before you use a variable, you must:

1. Declare its type
2. Assign it a value

Example:

int age = 18;
double temperature = 36.5;
char grade = 'A';
bool passed = true;

Each variable has a data type that defines what kind of data it can store.

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Built-in Data Types in C++

Data Type	Description	Example
int	Integer values	int count = 10;
double	Floating-point numbers	double pi = 3.14;
char	Single character	char grade = 'B';
bool	Boolean (true/false)	bool flag = true;
string	Text (needs <string>)	string name = "Ali";

Note:

To use string, you must include the string library:

#include <string>

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Variable Declaration and Initialization

Declaration:

int age;

Initialization:

age = 18;

Or both together:

int age = 18;

Multiple declarations:

int a = 1, b = 2, c = 3;

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Using cout with Different Data Types

#include <iostream>
#include <string>
using namespace std;

int main() {
    int year = 2025;
    double gpa = 3.9;
    string name = "ARJ";
    char grade = 'A';

    cout << "Name: " << name << endl;
    cout << "Year: " << year << endl;
    cout << "GPA: " << gpa << endl;
    cout << "Grade: " << grade << endl;

    return 0;
}

Output:

Name: ARJ
Year: 2025
GPA: 3.9
Grade: A

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Expressions and Arithmetic Operators

An expression is a combination of values and operators that produces a result.

Common Operators:

Operator	Meaning	Example
+	Addition	a + b
-	Subtraction	a - b
*	Multiplication	a * b
/	Division	a / b
%	Modulo (remainder)	a % b

Example:

int a = 10, b = 3;
cout << a + b << endl; // 13
cout << a - b << endl; // 7
cout << a * b << endl; // 30
cout << a / b << endl; // 3 (integer division)
cout << a % b << endl; // 1

Note:

If either operand is a double, division gives a floating-point result:

double result = 10.0 / 3;
cout << result; // 3.33333

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Type Conversion

C++ automatically converts between types in some cases (implicit conversion), but you can also do it manually (explicit conversion).

Implicit:

int x = 5;
double y = x; // x is automatically converted to double

Explicit (Casting):

int a = 7, b = 2;
double result = (double) a / b;
cout << result; // 3.5

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Constants

Use const to create a variable that cannot be changed.

const double PI = 3.14159;

Trying to modify PI later in the code will result in a compiler error.

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Reading Input: cin

You can use cin to take input from the user.

#include <iostream>
using namespace std;

int main() {
    int age;
    cout << "Enter your age: ";
    cin >> age;
    cout << "You entered: " << age << endl;
    return 0;
}

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Mini Exercise

Write a program that:

• Takes two integers from the user
• Prints their sum, product, and remainder

Try it yourself before checking the solution.

Sample Output:

Enter two numbers: 7 3
Sum: 10
Product: 21
Remainder: 1

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Common Mistakes to Avoid

• Forgetting to include #include <iostream> or using namespace std;
• Not initializing variables before using them
• Integer division truncating decimal values
• Using = instead of == in condition checks (covered later)

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Summary

In this post, you learned about:

• Variables and how to declare and initialize them
• C++ built-in data types like int, double, char, and bool
• Arithmetic expressions and operators
• Constants and type casting
• Taking input with cin

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What’s Next?

In the next post, we’ll explore Conditionals and Control Flow — how to make decisions in your code using if, else, and related constructs.