C Programming - C Variables

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C Programming - C Variables

A variable in C is a named storage location in memory used to hold data. Variables play a fundamental role in programming by allowing data manipulation and storage for calculations, logic, and data processing.

Key Points about Variables

1. Declaration: Variables must be declared before use.
2. Naming Rules:
• Can contain letters, digits, and underscores (_).
• Must begin with a letter or underscore.
• Cannot be a keyword.
• Case-sensitive (age and Age are different).
3. Data Type: Every variable must have a type (int, float, etc.).

Syntax of Variable Declaration

datatype variable_name;
datatype variable1, variable2, ... , variablen;

Examples

int age;           // Declares an integer variable
float salary;      // Declares a floating-point variable
char grade;        // Declares a character variable
int x = 10, y = 20; // Declares and initializes multiple variables

Variable Initialization

Variables can be initialized at the time of declaration.

Syntax

datatype variable_name = value;

Examples

int num = 5;
float pi = 3.14;
char letter = 'A';

Types of Variables in C

1. Local Variables:
• Declared inside a function or block.
• Accessible only within the block where they are declared.

Example:

void display() {
    int num = 10;  // Local variable
    printf("%d\n", num);
}

1. Global Variables:
• Declared outside all functions.
• Accessible by all functions in the program.

Example:

int count = 0;  // Global variable

void increment() {
    count++;
}

1. Static Variables:
• Retain their value between function calls.
• Default value is 0.

Example:

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

1. Automatic Variables:
• Default for local variables.
• Keyword: auto (optional).

Example:

void display() {
    auto int num = 5;  // Automatic variable
    printf("%d\n", num);
}

1. External Variables:
• Declared using extern and defined elsewhere.
• Used to share variables across multiple files.

Example:

extern int globalVar;

Scope of Variables

1. Block Scope:
• Variable accessible only within the block {} where it is defined.
2. Function Scope:
• Variables declared inside a function are accessible only within the function.
3. File Scope:
• Global variables are accessible throughout the file.
4. Program Scope:
• extern variables accessible across multiple files.

Lifetime of Variables

1. Automatic:
• Exist only during the execution of the block where they are declared.
2. Static:
• Retain their value between function calls.
3. Dynamic:
• Allocated and deallocated explicitly using malloc/free.

Example: Variables in Action

#include <stdio.h>
 
// Global variable
int globalVar = 10;

void demo() {
    // Local variable
    int localVar = 20;

    // Static variable
    static int staticVar = 0;

    staticVar++;
    printf("Local: %d, Static: %d, Global: %d\n", localVar, staticVar, globalVar);
}

int main() {
    demo();
    demo();
    return 0;
}

Output

Local: 20, Static: 1, Global: 10
Local: 20, Static: 2, Global: 10

Summary