Variables

A variable is a named memory location used to store data. The value of a variable can change during program execution.

Syntax

data_type variable_name;

Examples

int age;
float salary;
char grade;

Or with initialization:

int age = 20;
float salary = 35000.50;
char grade = 'A';

Rules for Naming Variables

Must begin with a letter (A–Z, a–z) or underscore (_).
Cannot begin with a digit.
Can contain letters, digits, and underscores.
Cannot contain spaces or special characters.
Cannot use C keywords (e.g., int, float, if, while) as variable names.
Variable names are case-sensitive (Age and age are different).

Valid Variable Names

int marks;
float total_marks;
char studentName;
int _count;

Invalid Variable Names

int 2marks;      // Starts with a digit
float total marks; // Contains a space
char int;        // Keyword used as variable name

Data Types in C

A data type specifies the type of data a variable can store and the amount of memory allocated to it.

1. Basic (Primary) Data Types

Data Type	Description	Typical Size	Example
`int`	Stores integer values	4 bytes	`int age = 25;`
`char`	Stores a single character	1 byte	`char grade = 'A';`
`float`	Stores decimal numbers	4 bytes	`float price = 99.99;`
`double`	Stores large decimal numbers	8 bytes	`double pi = 3.14159265;`
`void`	Represents no value	–	`void display();`

2. Derived Data Types

Arrays
Pointers
Functions

Example

int numbers[5];
int *ptr;

3. User-Defined Data Types

struct
union
enum
typedef

Example

struct Student
{
    int roll;
    char name[30];
};

Example Program

#include &lt;stdio.h>

int main()
{
    int age = 21;
    float salary = 45000.75;
    char grade = 'A';
    double pi = 3.1415926535;

    printf("Age = %d\n", age);
    printf("Salary = %.2f\n", salary);
    printf("Grade = %c\n", grade);
    printf("Value of PI = %.10lf\n", pi);

    return 0;
}

Output

Age = 21
Salary = 45000.75
Grade = A
Value of PI = 3.1415926535

Summary

A variable is a named memory location used to store data.
A data type determines the type of data a variable can hold.
Common data types include int, char, float, double, and void.
C also supports derived and user-defined data types for more complex programming needs.