Structures

🏗️ C Structures: Building Your Own Data Types

The Story Begins: Meet the Box Factory 📦

Imagine you have a magical box factory. Each box can hold many different things together — a toy car, a label with its name, and a price tag. Instead of carrying three separate items around, you put them all in ONE box.

That’s exactly what a Structure does in C! It lets you bundle different pieces of information into ONE custom package.

🌟 Structures Overview

What is a Structure?

A structure is like a recipe for a custom box. You decide what goes inside:

// This is like designing a box
struct Student {
    char name[50];
    int age;
    float grade;
};

Why do we need structures?

Think about a student. They have:

• A name (text)
• An age (number)
• A grade (decimal number)

Without structures, you’d need THREE separate variables. With structures? Just ONE variable holds everything!

Real Life Example:

• Your school ID card = a structure (has name, photo, class, roll number)
• A contact in your phone = a structure (has name, phone number, email)

📋 Structure Declaration

How to Create Your Custom Box Design

There are 3 ways to declare a structure:

Method 1: Declare first, use later

struct Book {
    char title[100];
    int pages;
    float price;
};

// Later, create a book:
struct Book myBook;

Method 2: Declare and create together

struct Car {
    char brand[30];
    int year;
} myCar, yourCar;

Method 3: Use typedef (give it a shorter name)

typedef struct {
    char name[50];
    int age;
} Person;

// Now just say:
Person friend1;

Think of it like this:

• struct Book = Your box design blueprint
• myBook = An actual box made from that blueprint

🔑 Structure Members and Access

Reaching Inside the Box

Each item inside a structure is called a member. To get or set a member, use the dot operator (.).

struct Pet {
    char name[30];
    int age;
    char type[20];
};

struct Pet dog;

// Put values IN the box:
strcpy(dog.name, "Buddy");
dog.age = 3;
strcpy(dog.type, "Golden Retriever");

// Take values OUT of the box:
printf("My pet is %s\n", dog.name);
printf("Age: %d years\n", dog.age);

The dot (.) is like opening the box and pointing to what you want!

graph TD
    A["dog"] --> B[".name = Buddy"]
    A --> C[".age = 3"]
    A --> D[".type = Golden Retriever"]

🎁 Structure Initialization

Filling Your Box Right Away

You can put items in your box at the moment you create it:

Method 1: In order (most common)

struct Point {
    int x;
    int y;
};

struct Point p1 = {10, 20};
// x = 10, y = 20

Method 2: By name (C99 and later)

struct Point p2 = {
    .y = 50,
    .x = 30
};
// Order doesn't matter when you use names!

Method 3: Partial initialization

struct Point p3 = {5};
// x = 5, y = 0 (unset = 0)

Pro tip: If you don’t initialize, members have garbage (random) values. Always initialize!

🎭 Nested Structures

Boxes Inside Boxes!

Sometimes you need a box that contains another box. That’s called nesting.

// First box: Date
struct Date {
    int day;
    int month;
    int year;
};

// Second box: Contains a Date box inside!
struct Employee {
    char name[50];
    int id;
    struct Date birthday;  // A box inside!
    struct Date joinDate;  // Another box inside!
};

How to access nested members? Use TWO dots!

struct Employee emp;

strcpy(emp.name, "Alice");
emp.id = 101;
emp.birthday.day = 15;
emp.birthday.month = 6;
emp.birthday.year = 1995;

graph TD
    E["Employee"] --> N[".name"]
    E --> I[".id"]
    E --> B[".birthday"]
    B --> BD[".day"]
    B --> BM[".month"]
    B --> BY[".year"]

📚 Array of Structures

A Shelf Full of Boxes

What if you have 30 students? Create an array of structures!

struct Student {
    char name[50];
    int rollNo;
    float marks;
};

// Array of 30 student boxes:
struct Student class[30];

// Access first student:
strcpy(class[0].name, "Tom");
class[0].rollNo = 1;
class[0].marks = 85.5;

// Access fifth student:
class[4].rollNo = 5;

Think of it like:

• class = A shelf
• class[0] = First box on shelf
• class[0].name = Name inside first box

Loop through all students:

for(int i = 0; i < 30; i++) {
    printf("Student: %s\n", class[i].name);
}

🚀 Structures and Functions

Passing Boxes Around

Structures can be sent to functions and returned from them!

Passing structure to function:

void printStudent(struct Student s) {
    printf("Name: %s\n", s.name);
    printf("Roll: %d\n", s.rollNo);
}

// Call it:
printStudent(class[0]);

Returning structure from function:

struct Point createPoint(int a, int b) {
    struct Point p;
    p.x = a;
    p.y = b;
    return p;
}

// Use it:
struct Point newPoint = createPoint(5, 10);

Passing by pointer (more efficient for big structures):

void updateAge(struct Student *s) {
    s->age = 20;  // Arrow -> for pointers!
}

// Call it:
updateAge(&class[0]);

Remember:

• Use . with structure variables
• Use -> with structure pointers

🔗 Self-Referential Structures

Boxes That Know About Other Boxes

A self-referential structure contains a pointer to itself. This is the magic behind linked lists, trees, and graphs!

struct Node {
    int data;
    struct Node *next;  // Points to another Node!
};

Why is this powerful?

You can create chains of boxes where each box points to the next:

struct Node first, second, third;

first.data = 10;
second.data = 20;
third.data = 30;

first.next = &second;   // First → Second
second.next = &third;   // Second → Third
third.next = NULL;      // Third → Nothing (end)

graph LR
    A["first<br>data=10"] -->|next| B["second<br>data=20"]
    B -->|next| C["third<br>data=30"]
    C -->|next| D["NULL"]

Real world example:

• A train! Each coach (structure) connects to the next coach.
• A chain of paper clips!

Important: You can’t put a full structure inside itself (infinite size!), but you CAN put a pointer to another structure of the same type.

🎯 Quick Summary

🌈 You Did It!

Congratulations! You now understand C Structures — your first step to creating powerful, organized programs. Structures are the foundation for:

• Databases
• Games (characters, items)
• Operating systems
• Almost every real program!

Remember: A structure is just a custom box that holds related things together. Simple as that! 📦✨