FOC Unit V: User Defined Data Types

 

User-Defined Data Types (UDDTs)

In C programming, a User-Defined Data Type is a data type created by the programmer to store multiple values of different data types together.
Common UDDTs:

• Structure (struct)
• Union (union)
• typedef
• enum

Structure (struct)

Definition

A structure is a user-defined data type that allows you to combine variables of different data types under a single name.

Why structure?

• To group related data
• Example: student information → roll number, name, marks

Syntax

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

Declaring Structure Variables
struct Student s1, s2;


Accessing Structure Members
Use dot (.) operator.
s1.roll_no = 1;
strcpy(s1.name, "Anisha");
s1.marks = 85.5;

Example
#include <stdio.h>

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

int main() {
    struct Student s1 = {1, "Anisha", 85.5};

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

    return 0;
}

Features of Structure
Stores multiple values of different types
Each member has a separate memory
Total memory = sum of all members
Members are accessed using dot (.) operator
Memory Example
For struct Student:
int roll = 4 bytes
char name[20] = 20 bytes
float marks = 4 bytes
Total = 28 bytes

UNION – User Defined Data Type
Definition:
A union is a user-defined data type in C that allows different data types to share the same memory location
Syntax
union union_name {
    data_type member1;
    data_type member2;
    ....
};

Example:

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

Declaration of Union Variables
union Data d1;

Accessing Union Members
Union members are accessed using the dot (.) operator, same as structure.

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


Example:

#include <stdio.h>

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

int main() {
    union Data d;

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

    d.f = 3.14;
    printf("Float: %.2f\n", d.f);

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

    return 0;
}

Difference Between Structure and Union:


How Structure Elements Are Stored in Memory
In a structure, each data member is stored in a separate memory location.
Example

struct Student {
    int roll_no;   // 4 bytes
    char grade;    // 1 byte
    float marks;   // 4 bytes
};

Array of Structure Elements
An array of structures is used to store multiple records of the same type.
Syntax
struct structure_name array_name[size];

Example
struct Student {
    int roll_no;
    char name[20];
    float marks;
};
struct Student s[3];

Accessing an Array of Structure Elements
Use array index + dot operator.
s[0].roll_no = 1;
s[0].marks = 85.5;

Array of Structures

#include <stdio.h>
struct Student {
    int roll_no;
    float marks;
};
int main() {
    struct Student s[3];
    int i;
    for(i = 0; i < 3; i++) {
        printf("Enter roll no and marks: ");
        scanf("%d %f", &s[i].roll_no, &s[i].marks);
    }
    printf("\nStudent Details:\n");
    for(i = 0; i < 3; i++) {
        printf("Roll No: %d  Marks: %.2f\n",
               s[i].roll_no, s[i].marks);
    }
    return 0;
}

Memory Layout of Array of Structures
Structures are stored in contiguous memory
Each array element occupies structure size
Advantages of Array of Structures
Easy to manage large data
Suitable for database-like applications
Improves code organization