Variables and Data Types in C++

              

📌 Variables in C++

🔹 What is a Variable?

A variable is a container used to store data in a program.

👉 Example (simple):

int age = 20;

• int → data type

• age → variable name

• 20 → value

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🔹 Why Variables are Needed?

• To store data

• To change data during program execution

• To perform calculations

👉 Example:

int a = 5;
int b = 10;
int sum = a + b;

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🔹 Rules for Naming Variables

1. Must start with a letter or underscore (_)

2. Cannot start with a number ❌

3. No spaces allowed

4. Cannot use C++ keywords

✅ Valid:

int marks;
int _total;
int age1;

❌ Invalid:

int 1age;
int total marks;
int int;

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🔹 Types of Variables (Based on Scope)

1️⃣ Local Variable

• Declared inside a function

• Used only inside that function

void show() {
    int x = 10;
}

2️⃣ Global Variable

• Declared outside all functions

• Can be used anywhere

int x = 10;

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📌 Data Types in C++

🔹 What is a Data Type?

A data type tells the compiler:

• What type of data a variable will store

• How much memory is needed

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🔹 Main Types of Data Types in C++

1️⃣ Basic (Primitive) Data Types

Data Type	Meaning	Example
int	Integer numbers	int a = 10;
float	Decimal numbers	float b = 3.14;
double	Large decimals	double x = 10.5678;
char	Single character	char ch = 'A';
bool	True / False	bool isPass = true;

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🔹 int

Stores whole numbers

int marks = 95;

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🔹 float

Stores decimal numbers (less precision)

float pi = 3.14;

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🔹 double

Stores large decimal numbers (more precision)

double value = 123.456789;

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🔹 char

Stores single character (in single quotes)

char grade = 'A';

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🔹 bool

Stores only true or false

bool result = true;

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2️⃣ Derived Data Types

🔹 Array

Stores multiple values of same type

int arr[3] = {10, 20, 30};

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🔹 Pointer

Stores address of another variable

int x = 10;
int *p = &x;

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🔹 Function

Block of code that performs a task

void show() {
    cout << "Hello";
}

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3️⃣ User-Defined Data Types

🔹 Structure (struct)

Used to store different data types together

struct Student {
    int roll;
    char grade;
};

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🔹 Union

Stores different data types but uses same memory

union Data {
    int a;
    float b;
};

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🔹 Enum

Used to define constant values

enum Day {Mon, Tue, Wed};

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🔹 Size of Data Types (Important for Exams)

Data Type	Size
int	4 bytes
float	4 bytes
double	8 bytes
char	1 byte
bool	1 byte

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📌 Example Program (Very Simple)

#include <iostream>
using namespace std;

int main() {
    int age = 20;
    float height = 5.6;
    char grade = 'A';
    bool pass = true;

    cout << age << endl;
    cout << height << endl;
    cout << grade << endl;
    cout << pass << endl;

    return 0;
}

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📌 Difference Between Variable and Data Type

Variable	Data Type
Stores data	Defines type of data
Has a name	Has a size
Example: age	Example: int

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📌 Short Exam Answer (2–3 Lines)

Variables are containers used to store data in a program.
Data types specify the type of data a variable can store and the memory required.

          

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