In C programming, variables are one of the most fundamental concepts. A variable is a named memory location used to store data that can change during program execution.
Without variables, a program would have no way to store inputs, outputs, or temporary values needed for computations.

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What Is a Variable?

A variable is a container (memory location) that holds a value. The value stored in a variable can be read, modified, or updated throughout the program.

Example:

int age = 20;
age = 25;   // Value updated

Here, age is a variable storing the number 20, and later its value changes to 25.

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Characteristics of Variables

• They are named locations in memory
• Their value can change
• They must be declared before use
• Their data type decides how much memory they occupy and what type of value they can store

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Variable Declaration in C

Before using a variable, you must declare it.

Syntax:

data_type variable_name;

Examples:

int marks;
float price;
char grade;

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Variable Initialization

Initialization means assigning an initial value at the time of declaration.

Example:

int age = 18;
float pi = 3.14;
char gender = 'M';

You can also declare multiple variables at once:

int a = 10, b = 20, c = 30;

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Rules for Naming Variables

These rules are the same as identifiers:

1. Must start with a letter or underscore
2. Cannot start with a digit
3. Cannot contain spaces
4. Cannot use special characters (@, #, %, etc.)
5. Cannot use keywords
6. Case-sensitive

Valid variable names:

total
_sum
marks1
value2025

Invalid:

1value     // starts with digit
student name  // space not allowed
float*data    // special character
return        // keyword

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Types of Variables in C

C categorizes variables based on their scope, lifetime, and storage class.

Below are the major types:

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1. Local Variables

Declared inside a function or block, accessible only within that block.

void main() {
    int x = 10; // local variable
}

They are destroyed once the function ends.

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2. Global Variables

Declared outside all functions.

Accessible throughout the entire program.

int count = 0;  // global variable

void display() {
    printf("%d", count);
}

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3. Static Variables

Retain their value even after the function ends.

void show() {
    static int x = 0;
    x++;
    printf("%d ", x);
}

Calling show() multiple times prints:
1 2 3 4 ...

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4. Automatic Variables (auto)

These are local variables created automatically.
By default, all local variables are auto.

void main() {
    auto int a = 10;
}

Using auto explicitly is rare.

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5. External Variables (extern)

Used to declare a global variable in another file.

extern int total; // variable exists in another file

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Memory Allocation Based on Variable Type

Different variable types occupy different memory sizes:

Data Type	Typical Size
char	1 byte
int	2 or 4 bytes
float	4 bytes
double	8 bytes

Memory size may vary depending on compiler and architecture.

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Example Program Showing All Variable Types

#include <stdio.h>

int globalVar = 50;   // global variable

void test() {
    static int s = 0; // static variable
    s++;
    printf("Static: %d\n", s);
}

int main() {
    int localVar = 10;    // local variable
    float price = 99.5;   // initialized variable

    printf("Local Variable: %d\n", localVar);
    printf("Global Variable: %d\n", globalVar);
    printf("Price: %.2f\n", price);

    test();  
    test();  
    test();  // Observe static variable behavior

    return 0;
}

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Difference Between Variables and Constants

Feature	Variable	Constant
Value	Changeable	Fixed
Defined by	Programmer	const or #define
Memory	Yes	Usually no (for #define)
Example	int age=20	const int AGE=20

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Conclusion

Variables play a crucial role in programming by enabling storage and manipulation of data. Understanding how to declare, initialize, modify, and categorize variables will help you write clean, efficient, and logically structured C programs.

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https://www.w3schools.com/c