Unit 13: Structures and Unions

1. Introduction to Structures

A Structure is a user-defined data type in C that allows you to combine different data types under a single name. Unlike arrays, which store multiple elements of the same type, a structure can group variables of different types (int, float, char, etc.) that are logically related.

Example: To store information about a "Student," you might need an integer for Roll No, a string for Name, and a float for Marks. A structure allows you to group these into one entity.

2. Structure Declaration, Definition, and Initialization

Declaration

The struct keyword is used to declare a structure. This creates a new data type template but does not allocate memory.

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

Definition (Variable Creation)

Once declared, you must define variables of that type to allocate memory.

struct Student s1, s2;

Initialization

Structures can be initialized at the time of definition using curly braces.

struct Student s1 = {101, "Rahul", 85.5};

3. Accessing Structure Members

To access individual members of a structure variable, we use the Dot Operator (.).

s1.rollNo = 102;
printf("Name: %s", s1.name);
    

4. Accessing Structures in Functions

Structures can be passed to functions just like standard variables. There are two main ways:

• Pass by Value: A complete copy of the structure is passed. Changes inside the function do not affect the original.
• Pass by Reference: The address of the structure is passed using pointers. This is more memory-efficient for large structures.

5. Structures and Pointers

When using a pointer to a structure, we use the Arrow Operator (->) instead of the dot operator to access members.

struct Student *ptr;
ptr = &s1;
printf("Roll No: %d", ptr->rollNo); // Equivalent to (*ptr).rollNo
    

6. Array of Structures and Nested Structures

Array of Structures

Used to store records for multiple entities of the same category.

struct Student classBCA[50]; // Stores info for 50 students

Nested Structures

A structure can contain another structure as its member.

struct Date {
    int day, month, year;
};
struct Student {
    int rollNo;
    struct Date dob; // Nested structure
};
    

7. Self-referential Structures and Unions

Self-referential Structures

These are structures that contain a pointer to a structure of the same type. They are the foundation of complex data structures like Linked Lists and Trees.

struct Node {
    int data;
    struct Node *next; // Points to the next node
};
    

Unions

A Union is similar to a structure, but all its members share the same memory location.

8. Exam Focus Enhancements