What Is a Variable in C?

In simple terms, a variable is a named storage location in memory. You can imagine it as a small labelled box where you can keep some value.

• The name on the box is the variable name (like age, marks).
• The type of the box tells what kind of data can be stored (integer, decimal, character, etc.).
• The value inside the box is the current data stored in that variable.

int age;      // Declare a variable named 'age' to store an integer
age = 25;     // Store the value 25 inside 'age'

Declaration & Initialization of Variables

In C, when you create a variable, you must tell the compiler: “Intha peru-ku, intha type data store pannaporen”.

Declaration

int count;       // declares an integer variable
float price;     // declares a floating-point variable
char grade;      // declares a character variable

Initialization

Giving a starting value to a variable is called initialization.

int count = 10;
float price = 199.50;
char grade = 'A';

You can also declare multiple variables of the same type:

int a = 10, b = 20, c = 0;
float x = 1.5, y = 2.0;

Rules for Variable Names in C

Variable names (identifiers) must follow some rules:

• Can contain letters (A–Z, a–z), digits (0–9) and underscore _.
• Cannot start with a digit. (e.g., 1age ❌, age1 ✅)
• No spaces or special symbols like @, #, -, etc.
• Case sensitive: age and Age are different.
• Cannot use C keywords as variable names (like int, float, return).

Good Variable Name Examples

• age, totalMarks, unit_price, solarVoltage

Bad Variable Name Examples

• 2value (starts with number)
• total price (space not allowed)
• float (keyword)

Basic Data Types in C

A data type tells the compiler “intha variable-la eppadi data store panna poren?”. Different data types need different amount of memory and have different range of values.

Common Basic Data Types

*Typical sizes on common 32/64-bit systems using GCC/Clang. Use sizeof to check exact size on your system.

#include <stdio.h>

int main() {
    printf("Size of int = %zu bytes\\n", sizeof(int));
    printf("Size of float = %zu bytes\\n", sizeof(float));
    printf("Size of double = %zu bytes\\n", sizeof(double));
    printf("Size of char = %zu bytes\\n", sizeof(char));
    return 0;
}

int, float, double, char – With Simple Examples

int units = 350;
int year = 2025;

char grade = 'B';
char answer = 'Y';

float voltage = 230.5f;
float current = 5.75f;

double distance = 12345.6789;
double pi = 3.1415926535;

Using printf & scanf With Data Types

#include <stdio.h>

int main() {
    int age;
    float temp;
    char grade;

    printf("Enter age: ");
    scanf("%d", &age);

    printf("Enter temperature: ");
    scanf("%f", &temp);

    printf("Enter grade (A/B/C): ");
    scanf(" %c", &grade);   // note the space before %c

    printf("Age = %d, Temp = %.2f, Grade = %c\\n", age, temp, grade);
    return 0;
}

Type Modifiers – short, long, signed, unsigned (Basic idea)

C allows you to modify integer types with keywords like short, long, signed, unsigned.

• short int – uses less memory, smaller range.
• long int – uses more memory, larger range.
• unsigned int – only positive values (and zero), but larger positive range.

short int smallCount;
unsigned int items;
long int bigNumber;

For now, as a beginner, using plain int is enough in most cases. Later, for embedded and optimization, we will see these in detail.

What Is a Constant in C?

A constant is a value that does not change while the program is running. Example: value of π (pi), fixed tax rate, maximum allowed items, etc.

There are three common ways of using constants in C:

• Literal constants (direct values like 10, 3.14, 'A').
• Symbolic constants using #define.
• const keyword with variables.

Literal Constants

int days = 7;          // 7 is a literal integer constant
float pi = 3.14f;      // 3.14f is a float constant
char letter = 'A';     // 'A' is a character constant

Symbolic Constants with #define

#define PI 3.14159
#define MAX_STUDENTS  60

int main() {
    float radius = 5.0f;
    float area = PI * radius * radius;

    printf("Max allowed students = %d\\n", MAX_STUDENTS);
    printf("Area = %.2f\\n", area);
    return 0;
}

Here, PI and MAX_STUDENTS are symbolic names for constant values, replaced by the preprocessor before compilation.

Constants with const Keyword

const int MAX_SPEED = 120;
const float GST_RATE = 0.18f;

If you try to change a const variable later in the code, the compiler will give an error. This protects the value from accidental modification.

Common Beginner Mistakes With Variables & Data Types

• Using a variable without initializing it (random value problem).
• Using wrong format specifier in printf/scanf (e.g., %d for float).
• Overflow: storing a very large number in a small type (e.g., int x = 1000000000; on some systems).
• Forgetting single quotes for char (writing char c = A; instead of 'A').

Quick Checklist Before Compiling

• Did you choose correct data type for your values?
• Are all variables declared before use?
• Did you initialize variables with proper starting values?
• Are format specifiers matching variable types?
• Did you use const or #define for fixed values?

Suggested Featured Image Concept for This Post

For your blog thumbnail, you can design an image like this:

• Three boxes labelled “int”, “float”, “char” with different values inside (e.g., 25, 99.5, ‘A’).
• A separate box with a lock icon labelled “constant (const / #define)”.
• A Tamil technician pointing at the boxes, with a big “C” logo on a laptop screen.
• Top text: “C Variables & Data Types” and subtitle: “Explained in Simple Tamil”.