Structures & Unions in C

Until now, you've learned how to store a single value using variables and multiple values of the same type using arrays. But in real-world applications, we often need to store different types of information about the same entity.

For example, consider a student in KauraX. A student has:

• Name (String)
• Roll Number (Integer)
• CGPA (Float)
• Attendance (Integer)

These values have different data types but belong to the same student. Instead of creating separate variables, C provides Structures and Unions.

Structures and Unions are user-defined data types that help organize related data efficiently.

🎯 Learning Objectives

After completing this lesson, you will be able to:

• Understand Structures.
• Declare and initialize structures.
• Access structure members.
• Create arrays of structures.
• Pass structures to functions.
• Understand nested structures.
• Learn Unions.
• Differentiate between Structures and Unions.
• Apply structures in real-world programs.

📖 What is a Structure?

A Structure is a user-defined data type that groups variables of different data types under a single name.

Example:

A student record may contain:

• Name
• USN
• Age
• CGPA

Instead of four separate variables, we use one structure.

Syntax

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struct StructureName
{
    datatype member1;
    datatype member2;
    datatype member3;
};

Example

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

Declaring Structure Variables

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struct Student s1;

Multiple variables

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struct Student s1, s2, s3;

Accessing Members

Use the dot (.) operator.

Example

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s1.rollNo = 101;

s1.cgpa = 9.2;

Complete Example

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#include <stdio.h>

struct Student
{
    char name[30];
    int roll;
    float cgpa;
};

int main()
{
    struct Student s1 =
    {
        "Pritee",
        101,
        9.45
    };

    printf("Name : %s\n", s1.name);
    printf("Roll : %d\n", s1.roll);
    printf("CGPA : %.2f\n", s1.cgpa);

    return 0;
}

Output

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Name : Pritee
Roll : 101
CGPA : 9.45

Reading Structure Data

cCopy
#include <stdio.h>

struct Student
{
    char name[30];
    int roll;
    float cgpa;
};

int main()
{
    struct Student s;

    printf("Enter Name: ");
    scanf("%s", s.name);

    printf("Enter Roll: ");
    scanf("%d", &s.roll);

    printf("Enter CGPA: ");
    scanf("%f", &s.cgpa);

    printf("\nStudent Details\n");
    printf("%s\n", s.name);
    printf("%d\n", s.roll);
    printf("%.2f\n", s.cgpa);

    return 0;
}

Array of Structures

We can store multiple records.

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struct Student students[3];

Example

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#include <stdio.h>

struct Student
{
    char name[20];
    int marks;
};

int main()
{
    struct Student s[2] =
    {
        {"Aman",85},
        {"Rahul",92}
    };

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

    return 0;
}

Output

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Aman 85
Rahul 92

Nested Structures

A structure can contain another structure.

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#include <stdio.h>

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

struct Student
{
    char name[30];
    struct Address address;
};

int main()
{
    struct Student s =
    {
        "Pritee",
        {"Bengaluru",560064}
    };

    printf("%s\n", s.name);
    printf("%s\n", s.address.city);

    return 0;
}

Passing Structure to a Function

cCopy
#include <stdio.h>

struct Student
{
    char name[30];
    int marks;
};

void display(struct Student s)
{
    printf("%s\n", s.name);
    printf("%d\n", s.marks);
}

int main()
{
    struct Student s =
    {
        "Pritee",
        95
    };

    display(s);

    return 0;
}

Pointer to Structure

Pointers can point to structures.

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struct Student s;

struct Student *ptr = &s;

Access members using

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ptr->name

instead of

cCopy
(*ptr).name

Example

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#include <stdio.h>

struct Student
{
    char name[30];
};

int main()
{
    struct Student s =
    {
        "KauraX"
    };

    struct Student *ptr = &s;

    printf("%s", ptr->name);

    return 0;
}

What is a Union?

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

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

Syntax

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union Data
{
    int number;
    float marks;
    char grade;
};

Union Example

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#include <stdio.h>

union Data
{
    int number;
    float marks;
};

int main()
{
    union Data d;

    d.number = 50;

    printf("%d\n", d.number);

    d.marks = 92.5;

    printf("%.2f\n", d.marks);

    return 0;
}

Structure vs Union

Memory Representation

Structure

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Student

+--------+
| Name   |
+--------+
| Roll   |
+--------+
| CGPA   |
+--------+

Each member has separate memory.

Union

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Data

+-------------+
| Shared Area |
+-------------+

All members use the same memory.

Real-World Example

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#include <stdio.h>

struct Course
{
    char title[40];
    int lessons;
    float rating;
};

int main()
{
    struct Course c =
    {
        "C Programming",
        25,
        4.9
    };

    printf("Course : %s\n", c.title);
    printf("Lessons : %d\n", c.lessons);
    printf("Rating : %.1f\n", c.rating);

    return 0;
}

Output

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Course : C Programming
Lessons : 25
Rating : 4.9

Common Beginner Mistakes

Forgetting struct

Wrong

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Student s;

Correct

cCopy
struct Student s;

(Unless typedef is used.)

Using Dot Instead of Arrow

Wrong

cCopy
ptr.name

Correct

cCopy
ptr->name

Assuming Union Stores All Values

Wrong assumption:

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d.number = 10;

d.marks = 25.5;

The second assignment overwrites the first because both members share the same memory.

Best Practices

• Use meaningful structure names.
• Initialize structures before use.
• Prefer structures over unions unless memory optimization is necessary.
• Use pointers for large structures to improve performance.
• Keep related data together.

🧠 Interview Questions

1. What is a structure?
2. What is a union?
3. Difference between structure and union.
4. What is a nested structure?
5. How do you pass a structure to a function?
6. What is a pointer to a structure?
7. What is the arrow (->) operator?

✍ Practice Questions

1. Store student details using a structure.
2. Store employee information using structures.
3. Create an array of five students.
4. Pass a structure to a function.
5. Use a pointer to access structure members.
6. Create a nested structure.
7. Compare structures and unions with an example.

📚 Lesson Summary

In this lesson, you learned:

• Structures
• Declaring and initializing structures
• Accessing members
• Arrays of structures
• Nested structures
• Pointers to structures
• Unions
• Structure vs Union
• Best practices

Structures are used to model real-world entities by grouping related data of different types, while unions provide memory-efficient storage when only one member is needed at a time.

🚀 What's Next?

In the next lesson, Dynamic Memory Allocation (DMA), you'll learn:

• Stack vs Heap Memory
• malloc()
• calloc()
• realloc()
• free()
• Memory Leaks
• Dangling Pointers
• Practical Examples
• Interview Questions

Dynamic Memory Allocation allows programs to request and release memory at runtime, making them flexible and efficient.

Happy Coding with KauraX 💙