What is a Variable?

A variable is a named location in memory that stores a value. Think of it as a labeled box — you give the box a name, specify what type of thing it can hold, and then put a value in it.

// Variable is: type + name + value
int age = 20;
//  ↑    ↑    ↑
// type name value

In the above example, int is the data type (integer), age is the variable name, and 20 is the value stored in it.

Declaration and Initialization

Declaration – Reserving Memory

Declaration tells the compiler: "Set aside memory for a variable with this name and this type."

int age;        // Declared, not yet initialized
float price;    // Memory reserved for a float
char grade;     // Memory reserved for a character

Initialization – Giving a Value

int age = 20;          // Declare + Initialize together
float price = 99.50;  // Float value
char grade = 'A';     // Character (single quotes!)

// Multiple declarations
int a, b, c;          // Declare multiple at once
int x = 1, y = 2, z = 3; // Declare + init multiple

Data Types in C

Data types tell the compiler what kind of data a variable will hold, and how much memory to allocate.

1. int – Integer Numbers

Stores whole numbers (no decimal). Size: usually 4 bytes.

int marks = 95;
int temperature = -5;   // Negative integers OK
int year = 2025;

2. float – Decimal Numbers (Single Precision)

Stores decimal numbers. Size: 4 bytes. Precision: ~6-7 decimal digits.

float price = 99.99f;   // 'f' suffix for float
float pi = 3.14159f;

3. double – Decimal Numbers (Double Precision)

More precise than float. Size: 8 bytes. Precision: ~15-16 decimal digits. Use double when you need high accuracy.

double pi = 3.14159265358979;  // More precise
double salary = 55000.75;

4. char – Single Character

Stores a single character. Size: 1 byte. Character is enclosed in single quotes.

char grade = 'A';
char initial = 'G';
char newline = '\n';  // Escape character

5. Type Modifiers (short, long, unsigned, signed)

short int s = 100;        // 2 bytes
long int  l = 1000000L;  // 4 or 8 bytes
long long int ll = 9999999999LL; // 8 bytes
unsigned int u = 50000; // Only positive values

Sizes & Ranges Table

Use sizeof() to check the actual size on your system:

#include <stdio.h>
int main() {
    printf("int:    %zu bytes\n", sizeof(int));
    printf("float:  %zu bytes\n", sizeof(float));
    printf("double: %zu bytes\n", sizeof(double));
    printf("char:   %zu bytes\n", sizeof(char));
    return 0;
}

Complete Programs

Program 1: Student Information

#include <stdio.h>
int main() {
    int    rollNo = 101;
    float  percentage = 89.5f;
    char   grade = 'A';
    double fees = 45000.75;

    printf("Roll No:    %d\n",    rollNo);
    printf("Percentage: %.1f%%\n", percentage);
    printf("Grade:      %c\n",    grade);
    printf("Fees:       %.2lf\n", fees);
    return 0;
}

Variable Naming Rules

• Must start with a letter (a-z, A-Z) or underscore (_)
• Can contain letters, digits (0-9), and underscores
• Cannot start with a digit: 2age ❌
• Cannot use C keywords: int, float, for, etc. ❌
• Case-sensitive: age and Age are different variables
• No spaces allowed: my age ❌

Summary

• A variable is a named memory location that holds a value
• Always specify a data type when declaring a variable in C
• Main types: int (whole numbers), float/double (decimals), char (characters)
• Use sizeof() to check memory size of any type
• Format specifiers: %d (int), %f (float), %lf (double), %c (char)
• Always initialize variables to avoid garbage values