C Programming - Beginner to Brilliance / Basics of C Programming

Variables & Data Types in C

📦 What is a Variable?

A variable is a named storage location in memory that holds a value which your program can read or change during its execution.

🏠 Real-life Analogy:

Imagine you have labeled containers in your kitchen:

  • One labeled “Sugar” with 2kg inside

  • One labeled “Salt” with 1kg

In C:

  • The label is the variable name (e.g., sugar)

  • The container is the memory space

  • The content is the value (e.g., 2)

✏️ Declaring a Variable

To use a variable in C, you need to declare it like this:

data_type variable_name;

You can also initialize (assign a value) at the same time:

int age = 25;

🧾 Examples with Explanation:

int marks = 85;        // 'marks' is an integer variable storing 85
float weight = 67.8;   // 'weight' is a float variable storing 67.8
char grade = 'A';      // 'grade' is a character variable storing 'A'

Each line:

  • Creates a box in memory

  • Labels it with the variable name

  • Stores a value inside it

🧬 What is a Data Type?

A data type tells the computer what kind of data is being stored in a variable — like whether it’s:

  • A whole number?

  • A decimal?

  • A character?

Just like we use different containers for water, grains, or milk, C uses different data types to store different kinds of information.

🧾 Format Specifiers

In C, format specifiers are used in functions like printf() and scanf() to tell the compiler what type of data you're working with.

📌 Common Format Specifiers:

Specifier

Data Type

Example Value

Example printf()

%d

int

10

printf("Value = %d", 10);

%f

float/double

3.14

printf("Pi = %f", 3.14);

%.2f

float with 2 decimals

3.1415

printf("Pi = %.2f", 3.1415);

%c

char

'A'

printf("Char = %c", 'A');

%s

string

"Hello"

printf("Text = %s", "Hello");

%u

unsigned int

15

printf("Unsigned = %u", 15);

%ld

long int

100000L

printf("Long = %ld", 100000L);

%lf

double

3.1415

printf("Double = %lf", 3.1415);

%x / %X

Hexadecimal

255

printf("Hex = %x", 255); → ff

%p

pointer address

&x

printf("Address = %p", &x);

💡 Example with multiple specifiers:

int age = 25;
float height = 5.9;
char grade = 'A';
printf("Age: %d, Height: %.1f, Grade: %c", age, height, grade);
Age: 25, Height: 5.9, Grade: A

⚠️ Notes:

  • Use correct specifier for correct data type — mismatches can cause wrong output or crash.

  • %f is used for both float and double in printf().

  • Always match order of arguments with the format string.

🎓 Types of Data Types in C

🔹 Basic (Primary) Data Types

These are the most commonly used types.

Data Type

Description

Size (Typical*)

Format Specifier

int

Integer (whole number)

2 or 4 bytes

%d

float

Decimal (single precision)

4 bytes

%f

double

Decimal (double precision)

8 bytes

%lf

char

Single character

1 byte

%c

void

No value

0 bytes

—

📌 Note: Size may vary based on the system (32-bit vs 64-bit)

🔹 Derived Data Types

These are made using basic types:

  • Arrays

  • Pointers

  • Functions

  • Structures

🔹 Enumeration (enum)

An enum is a user-defined data type in C that consists of named integer constants. It is used to assign names to a set of integral values, making code easier to read and maintain.

🧠 Syntax:

enum enum_name {
    VALUE1,
    VALUE2,
    VALUE3,
    ...
}

By default, VALUE1 = 0, VALUE2 = 1, and so on. You can also manually assign values.

💡 Example 1 – Default Values:

#include <stdio.h>

enum Weekday {
    MONDAY,   // 0
    TUESDAY,  // 1
    WEDNESDAY,// 2
    THURSDAY, // 3
    FRIDAY    // 4
};

int main() {
    enum Weekday today = WEDNESDAY;
    printf("Day number: %d\\n", today);  // Output: 2
    return 0;
}

💡 Example 2 – Custom Values:

enum Status {
    SUCCESS = 1,
    FAILURE = 0,
    PENDING = -1
};

You can now use SUCCESS, FAILURE, PENDING as named constants in your program.

🔄 Behind the Scenes:

An enum is essentially an int, so you can:

  • Assign enum values to int

  • Use in switch statements

  • Compare using logical operators

📥 Memory Size of Data Types

You can check memory size using the sizeof() function:

#include <stdio.h>

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

🖨️ Sample Output (on 64-bit system):

Size of int: 4 bytes
Size of float: 4 bytes
Size of double: 8 bytes
Size of char: 1 byte

📋 Integer Types (Variants of int)

C allows various integer types depending on memory and range:

Type

Range (approximate)

Size

short int

-32,768 to 32,767

2 bytes

unsigned int

0 to 4,294,967,295

4 bytes

long int

-2.1 billion to 2.1 billion

4/8 bytes

long long int

-9 quintillion to +9 quintillion

8 bytes

🧪 Example:

unsigned int age = 30;  // Cannot store negative values

🔡 Character Type (char)

  • A char stores a single character in single quotes like 'A'.

  • It is internally stored as an integer (ASCII value).

Example:

char grade = 'A';       // 'A' = 65 in ASCII
printf("%d", grade);    // Output: 65

🧮 Floating-Point Types (float, double)

Used for storing decimal values.

float pi = 3.14;
double price = 99.999999;

Type

Precision

Size

float

6 digits

4 bytes

double

15 digits

8 bytes

🧪 Mixing Data Types

If you mix types in operations, type promotion happens automatically:

int a = 5;
float b = 2.0;

float result = a + b;  // a is promoted to float

⚠️ Common Mistakes

Mistake

Correction

Declaring without data type

❌ num; → ✅ int num;

Wrong type assignment

❌ char ch = "A"; → ✅ char ch = 'A';

Using a variable without declaring it

❌ x = 10; before int x;