C Structures and Unions

In real-life problems, we often need to store different kinds of information together — for example, a student's name (string), roll number (integer), and marks (float). C provides a feature called a Structure to group such related data items of different types under one name. Similarly, Unions are another feature that help save memory when only one of the members is used at a time.

Structure in C

A Structure is a user-defined data type that allows you to combine variables of different types. It’s useful for representing complex data like students, employees, or products.

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

Example:

#include <stdio.h>
struct Student {
    int roll;
    char name[30];
    float marks;
};

int main() {
    struct Student s1 = {101, "Ravi", 89.5};
    printf("Roll: %d\\n", s1.roll);
    printf("Name: %s\\n", s1.name);
    printf("Marks: %.2f\\n", s1.marks);

    return 0;
}

Accessing Structure Members

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

s1.roll = 102;
strcpy(s1.name, "Asha");
s1.marks = 92.3;

printf("%s scored %.1f marks", s1.name, s1.marks);

---

Array of Structures

We can create multiple records of the same type using arrays of structures.

struct Student s[3] = {
	{1, "Ravi", 88.5},
	{2, "Meena", 91.0},
	{3, "Ajay", 79.5}
	};

	for(int i = 0; i < 3; i++)
	printf("%d %s %.1f\\n", s[i].roll, s[i].name, s[i].marks);

---

Structure within Structure (Nested Structure)

struct Date {
    int day, month, year;
};

struct Student {
    int roll;
    char name[30];
    struct Date dob; // nested structure
};

struct Student s1 = {101, "Ravi", {12, 5, 2004}};
printf("DOB: %d-%d-%d", s1.dob.day, s1.dob.month, s1.dob.year);

---

Passing Structures to Functions

void display(struct Student s) {
    printf("%d %s %.1f\\n", s.roll, s.name, s.marks);
}
int main() {
    struct Student s1 = {101, "Ravi", 88.5};
    display(s1);
}

Note: You can pass structures by value or by reference (using pointers).

---

Pointers to Structures

struct Student s1 = {101, "Ravi", 88.5};
struct Student *ptr = &s1;

printf("%s", ptr->name);   // use arrow operator (->)

Union in C

A Union is similar to a structure, but it shares the same memory location for all members. Only one member can contain a value at a time — making it memory-efficient.

union Data {
    int i;
    float f;
    char str[20];
};

int main() {
    union Data d1;

    d1.i = 10;
    printf("i = %d\\n", d1.i);

    d1.f = 22.5;
    printf("f = %.1f\\n", d1.f);

    strcpy(d1.str, "Hello");
    printf("str = %s\\n", d1.str);

    return 0;
}

Difference Between Structure and Union

Summary

• Structures group variables of different types together.
• Use . and -> operators to access members.
• Unions save memory by sharing space between members.
• Structures are used for data records; Unions are for type-flexible data storage.