Module 8: Structures and Unions in C

Learning Objectives

By the end of this module, you will be able to:

• Understand what structures and unions are.
• Create and use structures to store related data.
• Work with nested structures.
• Create arrays of structures.
• Access structure members using pointers.
• Understand the difference between structures and unions.
• Use typedef to simplify data type declarations.
• Define constants using enum.
• Build practical applications such as student, employee, and library management systems.

Introduction

So far, you've worked with basic data types such as:

• int
• float
• char
• double

These data types store only one type of value.

But imagine you need to store information about a student:

• Roll Number
• Name
• Age
• Percentage

Using separate variables becomes difficult.

cCopy
1int roll;
2char name[50];
3int age;
4float percentage;

What if there are 500 students?

This is where structures become useful.

Structures allow you to group multiple variables of different data types under a single name.

What is a Structure?

A structure is a user-defined data type that groups related variables of different data types.

Example:

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1Student
2
3Roll Number
4Name
5Age
6Percentage

Instead of creating four separate variables repeatedly, we create one structure.

Structure Syntax

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1struct StructureName
2{
3    data_type member1;
4    data_type member2;
5    data_type member3;
6};

Example

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1struct Student
2{
3    int roll;
4    char name[50];
5    float marks;
6};

Creating Structure Variables

cCopy
1struct Student student1;

Multiple variables

cCopy
1struct Student student1, student2, student3;

Accessing Structure Members

Members are accessed using the dot (.) operator.

Example

cCopy
1#include <stdio.h>
2
3struct Student
4{
5    int roll;
6    char name[30];
7    float marks;
8};
9
10int main()
11{
12    struct Student student;
13
14    student.roll = 101;
15    student.marks = 92.5;
16
17    printf("%d\n", student.roll);
18    printf("%.2f", student.marks);
19
20    return 0;
21}

Output

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1101
292.50

Reading Structure Data

cCopy
1#include <stdio.h>
2
3struct Student
4{
5    int roll;
6    char name[30];
7    float marks;
8};
9
10int main()
11{
12    struct Student student;
13
14    printf("Enter Roll Number: ");
15    scanf("%d", &student.roll);
16
17    printf("Enter Name: ");
18    scanf("%29s", student.name);
19
20    printf("Enter Marks: ");
21    scanf("%f", &student.marks);
22
23    printf("\nStudent Details\n");
24    printf("Roll: %d\n", student.roll);
25    printf("Name: %s\n", student.name);
26    printf("Marks: %.2f\n", student.marks);
27
28    return 0;
29}

Nested Structures

A structure can contain another structure as one of its members.

Example

cCopy
1#include <stdio.h>
2
3struct Address
4{
5    char city[30];
6    char state[30];
7};
8
9struct Student
10{
11    int roll;
12    char name[30];
13    struct Address address;
14};
15
16int main()
17{
18    struct Student student = {101, "Ankit", {"Delhi", "Delhi"}};
19
20    printf("Name: %s\n", student.name);
21    printf("City: %s\n", student.address.city);
22
23    return 0;
24}

Output

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1Name: Ankit
2City: Delhi

Array of Structures

Arrays of structures are used to store multiple records.

Example

cCopy
1struct Student students[100];

Program

cCopy
1#include <stdio.h>
2
3struct Student
4{
5    int roll;
6    char name[30];
7};
8
9int main()
10{
11    struct Student students[2];
12
13    for(int i = 0; i < 2; i++)
14    {
15        printf("Enter Roll and Name: ");
16        scanf("%d %29s", &students[i].roll, students[i].name);
17    }
18
19    printf("\nStudent List\n");
20
21    for(int i = 0; i < 2; i++)
22    {
23        printf("%d %s\n", students[i].roll, students[i].name);
24    }
25
26    return 0;
27}

Pointer to Structure

Pointers can point to structures.

Declaration

cCopy
1struct Student *ptr;

Accessing Members

There are two ways.

Using dereferencing

cCopy
1(*ptr).roll

Using the arrow operator

cCopy
1ptr->roll

The arrow operator (->) is easier to read and commonly used.

Example

cCopy
1#include <stdio.h>
2
3struct Student
4{
5    int roll;
6    char name[20];
7};
8
9int main()
10{
11    struct Student student = {101, "Ankit"};
12
13    struct Student *ptr = &student;
14
15    printf("%d\n", ptr->roll);
16    printf("%s\n", ptr->name);
17
18    return 0;
19}

Output

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1101
2Ankit

What is a Union?

A union is a user-defined data type similar to a structure.

The key difference is that all members of a union share the same memory location.

Only one member can store a meaningful value at a time.

Union Syntax

cCopy
1union Data
2{
3    int number;
4    float price;
5    char letter;
6};

Structure vs Union

Union Example

cCopy
1#include <stdio.h>
2
3union Data
4{
5    int number;
6    float price;
7};
8
9int main()
10{
11    union Data data;
12
13    data.number = 100;
14    printf("%d\n", data.number);
15
16    data.price = 45.5f;
17    printf("%.2f\n", data.price);
18
19    return 0;
20}

Assigning a new value to one union member overwrites the previous member because they share the same memory.

typedef

The typedef keyword creates an alias (alternate name) for an existing data type.

Without typedef

cCopy
1struct Student student;

With typedef

cCopy
1typedef struct
2{
3    int roll;
4    char name[30];
5} Student;
6
7Student student;

Advantages

• Shorter code
• Improved readability
• Easier maintenance

enum (Enumeration)

An enumeration is a user-defined data type consisting of named integer constants.

Example

cCopy
1enum Day
2{
3    MONDAY,
4    TUESDAY,
5    WEDNESDAY,
6    THURSDAY,
7    FRIDAY,
8    SATURDAY,
9    SUNDAY
10};

Values assigned automatically

textCopy
1MONDAY = 0
2TUESDAY = 1
3WEDNESDAY = 2
4...

Custom Enum Values

cCopy
1enum Status
2{
3    SUCCESS = 1,
4    FAILED = 0,
5    PENDING = 2
6};

Example

cCopy
1#include <stdio.h>
2
3enum Status
4{
5    SUCCESS = 1,
6    FAILED = 0
7};
8
9int main()
10{
11    enum Status result = SUCCESS;
12
13    printf("%d", result);
14
15    return 0;
16}

Output

textCopy
11

Practice Project 1: Student Database

cCopy
1#include <stdio.h>
2
3struct Student
4{
5    int roll;
6    char name[30];
7    float marks;
8};
9
10int main()
11{
12    struct Student students[3];
13
14    for(int i = 0; i < 3; i++)
15    {
16        printf("Enter Roll, Name and Marks: ");
17        scanf("%d %29s %f",
18              &students[i].roll,
19              students[i].name,
20              &students[i].marks);
21    }
22
23    printf("\nStudent Records\n");
24
25    for(int i = 0; i < 3; i++)
26    {
27        printf("%d %s %.2f\n",
28               students[i].roll,
29               students[i].name,
30               students[i].marks);
31    }
32
33    return 0;
34}

Practice Project 2: Employee Record

cCopy
1#include <stdio.h>
2
3typedef struct
4{
5    int id;
6    char name[30];
7    float salary;
8} Employee;
9
10int main()
11{
12    Employee employee;
13
14    printf("Enter Employee ID: ");
15    scanf("%d", &employee.id);
16
17    printf("Enter Name: ");
18    scanf("%29s", employee.name);
19
20    printf("Enter Salary: ");
21    scanf("%f", &employee.salary);
22
23    printf("\nEmployee Details\n");
24    printf("ID: %d\n", employee.id);
25    printf("Name: %s\n", employee.name);
26    printf("Salary: %.2f\n", employee.salary);
27
28    return 0;
29}

Practice Project 3: Library Management

cCopy
1#include <stdio.h>
2
3typedef struct
4{
5    int bookId;
6    char title[50];
7    char author[50];
8} Book;
9
10int main()
11{
12    Book book;
13
14    printf("Enter Book ID: ");
15    scanf("%d", &book.bookId);
16
17    getchar(); // Clear newline left by scanf
18
19    printf("Enter Title: ");
20    fgets(book.title, sizeof(book.title), stdin);
21
22    printf("Enter Author: ");
23    fgets(book.author, sizeof(book.author), stdin);
24
25    printf("\nLibrary Record\n");
26    printf("Book ID: %d\n", book.bookId);
27    printf("Title: %s", book.title);
28    printf("Author: %s", book.author);
29
30    return 0;
31}

Common Mistakes to Avoid

• Forgetting the semicolon (;) after a structure or union definition.
• Using the dot (.) operator instead of the arrow (->) when working with a pointer to a structure.
• Accessing uninitialized structure members.
• Expecting all union members to hold valid values simultaneously.
• Forgetting to include enough space for character arrays in structures.
• Mixing scanf() and fgets() without handling the leftover newline.

Best Practices

• Use structures to group logically related data.
• Use arrays of structures to manage collections of records.
• Prefer typedef for cleaner and more readable code.
• Use enum instead of hard-coded integer constants to improve readability.
• Use the arrow operator (->) when accessing members through pointers.
• Choose a union only when you need memory optimization and only one member is active at a time.
• Initialize structure variables before use.

Module Summary

In this module, you learned:

• What structures are and how they group related data of different types.
• How to declare, initialize, and access structure members.
• How to create nested structures and arrays of structures.
• How to work with pointers to structures using the -> operator.
• How unions differ from structures and when to use them.
• How typedef simplifies type declarations.
• How enum creates readable named constants.
• How to apply these concepts by building a student database, an employee record system, and a simple library management application.

After completing this module, you'll be ready to learn File Handling in C, including reading and writing files, binary files, file positioning, error handling, and building real-world applications such as student record systems and data storage utilities.