1. Structure in C

Definition

A structure is a user-defined data type that allows us to store different types of data under a single name.

For example, a student’s record contains:

  • Roll Number (int)
  • Name (char array)
  • Marks (float)

Since these are different data types, we use a structure.

Syntax

struct Student
{
    int roll;
    char name[30];
    float marks;
};
  • struct → Keyword used to define a structure.
  • Student → Structure name.
  • roll, name, marks → Members of the structure.

Example Program

#include <stdio.h>

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

int main()
{
    struct Student s1;

    s1.roll = 101;
    strcpy(s1.name, "Rahul");
    s1.marks = 89.5;

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

    return 0;
}

Note: To use strcpy(), include the header file:

#include <string.h>

Output

Roll Number : 101
Name        : Rahul
Marks       : 89.50

Memory Representation of Structure

Structure Student

+----------------+
| Roll = 101     |
+----------------+
| Name = Rahul   |
+----------------+
| Marks = 89.5   |
+----------------+

Each member gets its own memory.

Advantages of Structure

  • Stores different data types together.
  • Represents real-world entities (Student, Employee, Book, etc.).
  • Makes programs organized and easier to manage.
  • Multiple structure variables can be created.

2. Union in C

Definition

A union is also a user-defined data type, but all members share the same memory location.

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

Syntax

union Data
{
    int i;
    float f;
    char ch;
};

Example Program

#include <stdio.h>

union Data
{
    int i;
    float f;
    char ch;
};

int main()
{
    union Data d;

    d.i = 100;
    printf("Integer = %d\n", d.i);

    d.f = 12.5;
    printf("Float = %.1f\n", d.f);

    d.ch = 'A';
    printf("Character = %c\n", d.ch);

    return 0;
}

Output

Integer = 100
Float = 12.5
Character = A

Important: After assigning d.ch = 'A';, the previous values of d.i and d.f are overwritten because all members share the same memory.

Memory Representation of Union

Union Data

+----------------+
| Shared Memory  |
| int            |
| float          |
| char           |
+----------------+

Only one value is stored at a time.

Difference Between Structure and Union

StructureUnion
Each member has separate memory.All members share the same memory.
All members can store values simultaneously.Only one member can store a valid value at a time.
Memory size is the sum of all members (plus possible padding).Memory size is equal to the size of the largest member.
More memory is required.Less memory is required.
Used when all data members are needed together.Used when only one member is needed at a time.

Real-Life Example

Structure

A Student Record stores:

  • Roll Number
  • Name
  • Marks

All these details exist together, so Structure is the correct choice.

Union

A Payment Method may be:

  • Credit Card
  • UPI
  • Cash

At any moment, only one payment method is used, so Union is more suitable.

Exam Definition

Structure

A structure is a user-defined data type that groups variables of different data types under a single name. Each member has its own memory location.

Union

A union is a user-defined data type in which all members share the same memory location. Only one member can contain a valid value at a time.

Category: MCA

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