Variables, Data Types & Expressions in C++

1What is a variable?

A variable is a named container in your program that holds a value. Think of it as a labelled box: you put something in, you can change it later, and you can refer to it by name whenever you need its current value.

In C++, every variable has a fixed data type that tells the compiler what kind of data it can store (e.g., integers, decimals, characters). This strict typing helps catch errors early and makes the program more efficient.

Here is a simple declaration and initialisation:

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

The variable name (e.g., age) is used to refer to the stored value. You can change it later by assigning a new value of the same type.

2Built‑in data types in C++

C++ provides several fundamental data types. Each type defines the range of values it can hold and the operations you can perform.

3Declaration and initialisation

Declaration tells the compiler about the variable's name and type. Initialisation gives it an initial value.

You can declare and initialise in one line, or separately:

int age;          // declaration only
age = 18;        // later initialisation

double score = 9.5; // declaration + initialisation

// Multiple variables of the same type:
int a = 1, b = 2, c = 3;

4Using cout with different data types

You can print variables of any type using std::cout (or cout if you have using namespace std;). The output operator << streams the values to the console.

#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;
}

5Expressions and arithmetic operators

An expression is a combination of values, variables, and operators that produces a new value. C++ provides the usual arithmetic operators.

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

6Type conversion

Implicit conversion happens automatically when you assign a value of one type to a variable of another compatible type. For example, assigning an int to a double is safe because the integer can be represented as a decimal.

int x = 5;
double y = x; // implicit conversion: int → double
cout << y << endl; // prints 5.0

Explicit conversion (casting) is when you force the conversion. This is useful to avoid integer division.

int a = 7, b = 2;
double result = (double) a / b; // cast a to double before division
cout << result << endl; // 3.5

7Constants

Sometimes you need a value that never changes, like the mathematical constant π. Use the const keyword to declare a variable that cannot be modified after initialisation.

const double PI = 3.14159;
// PI = 3.14; // error – PI is read‑only

Constants make your code more maintainable: if you need to change the value, you only have to update it in one place.

8Reading input with cin

To get data from the user, use std::cin (or cin). The extraction operator >> reads input and stores it in a variable.

#include <iostream>
using namespace std;

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

9Quiz yourself