Structures in C

1. Definition

A structure is a user-defined data type in C that groups related variables of different data types into a single unit. It is used to represent a complex data entity such as a record.

2. Why Use Structures?

• To group related data (e.g., a student’s name, roll number, and marks).
• To create user-defined data types.
• Useful in creating data structures like linked lists, trees, and graphs.

3. Defining a Structure

Syntax:

struct structure_name {
    data_type member1;
    data_type member2;
    ...
};

Example:

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

4. Declaring Structure Variables

1. Directly:

   struct Student s1;
   

2. During Definition:

   struct Student {
       int roll_no;
       char name[50];
       float marks;
   } s1, s2;
   

5. Accessing Structure Members

Use the dot operator (.) to access members of a structure.

Example:

s1.roll_no = 101;
strcpy(s1.name, "Alice");
s1.marks = 95.5;

printf("Roll No: %d\n", s1.roll_no);
printf("Name: %s\n", s1.name);
printf("Marks: %.2f\n", s1.marks);

6. Array of Structures

You can create an array of structures to store multiple records.

Example:

struct Student students[3];

students[0].roll_no = 101;
strcpy(students[0].name, "Alice");
students[0].marks = 95.5;

students[1].roll_no = 102;
strcpy(students[1].name, "Bob");
students[1].marks = 88.0;

for (int i = 0; i < 2; i++) {
    printf("Student %d: %s, %.2f\n", students[i].roll_no, students[i].name, students[i].marks);
}

7. Nested Structures

A structure can have another structure as a member.

Example:

struct Address {
    char city[30];
    int zip;
};

struct Student {
    int roll_no;
    char name[50];
    struct Address addr;
};

struct Student s = {101, "Alice", {"New York", 10001}};

printf("City: %s, ZIP: %d\n", s.addr.city, s.addr.zip);

8. Structure and Functions

Structures can be passed to functions by value or by reference.

Example: Passing by Value

void display(struct Student s) {
    printf("Name: %s\n", s.name);
    printf("Marks: %.2f\n", s.marks);
}

Example: Passing by Reference

void modify(struct Student *s) {
    s->marks += 5; // Using arrow operator for pointers
}

9. Pointer to a Structure

You can use pointers to access and modify structure members.

Example:

struct Student *ptr = &s1;
ptr->marks = 90.0; // Arrow operator (->) for accessing members

10. typedef with Structures

typedef allows creating a shorthand for structure definitions.

Example:

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

Student s1; // No need to write 'struct'

11. Dynamic Memory Allocation for Structures

You can allocate memory for structures dynamically using malloc.

Example:

struct Student *ptr = (struct Student *)malloc(sizeof(struct Student));
ptr->roll_no = 101;
strcpy(ptr->name, "Alice");
ptr->marks = 95.5;
free(ptr); // Freeing allocated memory

12. Difference Between Structure and Array

13. Applications of Structures

1. Database Records: Storing employee, student, or product details.
2. Graphics Programming: Representing points, shapes, or colors.
3. System Programming: Creating hardware-related data structures.