Structure in C (English Medium)

Structure

Definition

Structures are user-defined data types in the C language that allow us to group different types of data (such as int, float, char, etc.) under a single name.
Each individual data element inside a structure is called a member.

OR

A structure is a user-defined data type in which different types of variables are grouped together under one name.

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Declaring a Structure

Syntax:

struct structure_name {

    data_type variable1;

    data_type variable2;

    data_type variable3;

};

Example:

struct Student {

    int roll;

    char name[20];

    float marks;

};

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Memory Allocation

A structure is a user-defined data type in which different data types are grouped together.
The memory of a structure is equal to the sum of the memory of all its members.

 

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Declaring a Structure Variable

After defining a structure, its variable can be created in several ways.

Method 1: Variable outside the structure

struct Student s1;

Method 2: Variable declared along with structure

struct Student {

    int roll;

    char name[20];

    float marks;

} s1, s2;

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Initialization of Structure

We can assign values to a structure variable during initialization.

Example:

struct Student s1 = {101, "Amit", 89.5};

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Accessing Structure Members

To access member values, the dot operator ( . ) is used.

Example:

printf("%d", s1.roll);    // Prints roll value

printf("%s", s1.name);    // Prints name

printf("%f", s1.marks);   // Prints marks

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Advantages of Structure

·        Stores different types of data together.

·        Helps manage data in a logical and organized way.

·        Useful in large programs for real-world data (students, employees, products).

·        Makes code readable and structured.

·        Can be easily passed to functions.

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Disadvantages of Structure

·        Uses more memory because each member has separate memory.

·        Cannot contain functions/methods (C limitation).

·        Data security is low.

·        Multiple structure variables consume large memory.

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Types of Structure

1.     Simple Structure

2.     Array of Structures

3.     Nested Structure

4.     Structure with Functions

5.     Pointer to Structure

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1. Simple Structure

#include <stdio.h>

struct Student {

    int roll;

    float marks;

};

int main() {

    struct Student s;

    s.roll = 10;

    s.marks = 89.5;

    printf("Roll: %d\n", s.roll);

    printf("Marks: %.2f", s.marks);

    return 0;

}

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2. Array of Structures

#include <stdio.h>

struct Student {

    int roll;

    float marks;

};

int main() {

    struct Student s[3];   // Array of 3 students

    for(int i = 0; i < 3; i++) {

        printf("Enter roll and marks: ");

        scanf("%d %f", &s[i].roll, &s[i].marks);

    }

    printf("\n--- Student Data ---\n");

    for(int i = 0; i < 3; i++) {

        printf("Roll: %d  Marks: %.2f\n", s[i].roll, s[i].marks);

    }

    return 0;

}

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3. Nested Structure

#include <stdio.h>

struct Address {

    char city[20];

    int pin;

};

struct Student {

    char name[20];

    struct Address addr;  // Nested structure

};

int main() {

    struct Student s = {"Amit", {"Lucknow", 226001}};

    printf("Name: %s\n", s.name);

    printf("City: %s\n", s.addr.city);

    printf("PIN: %d", s.addr.pin);

    return 0;

}

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4. Structure with Functions

#include <stdio.h>

struct Student {

    int roll;

    float marks;

};

void display(struct Student s) {

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

}

int main() {

    struct Student s1 = {15, 92.3};

    display(s1);

    return 0;

}

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5. Pointer to Structure

#include <stdio.h>

struct Student {

    int roll;

    float marks;

};

int main() {

    struct Student s = {20, 88.6};

    struct Student *ptr;

    ptr = &s;

    printf("Roll: %d\n", ptr->roll);

    printf("Marks: %.2f", ptr->marks);

    return 0;

}

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Union

Definition

A union is a user-defined data type in which multiple variables are grouped together, but all members share the same memory location.
Therefore, only one member’s value remains valid at a time.

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Declaring a Union

Syntax:

union union_name {

    data_type member1;

    data_type member2;

    data_type member3;

};

Example:

union Data {

    int i;

    float f;

    char ch;

};

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Declaring a Union Variable

Method 1: Outside the union

union Data d1;

Method 2: Along with the union

union Data {

    int i;

    float f;

    char ch;

} d1, d2;

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Initialization of Union

union Data d = {10};

// OR assign later

d.f = 22.5;

Note:
Only the last assigned value remains valid.

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Example Program

#include <stdio.h>

union Data {

    int i;

    float f;

    char ch;

};

int main() {

    union Data d;

    d.i = 10;

    printf("Integer: %d\n", d.i);

    d.f = 20.5;

    printf("Float: %.2f\n", d.f);

    d.ch = 'A';

    printf("Character: %c\n", d.ch);

    return 0;

}

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Types of Union in C

1.     Simple Union

2.     Array of Union

3.     Union inside Structure

4.     Structure inside Union

1. Simple Union

union Data {

    int i;

    float f;

};

2. Array of Union

union Data {

    int i;

    float f;

};

union Data d[5];

3. Union inside Structure

struct Student {

    char name[20];

    union Marks {

        int theory;

        int practical;

    } m;

};

4. Structure inside Union

union Info {

    int id;

    struct Address {

        char city[20];

        int pin;

    } addr;

};

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Advantages of Union

·        Uses very little memory because all members share the same memory.

·        Useful when only one value is needed at a time.

·        Good for embedded systems and low-memory devices.

·        Helps create memory-efficient programs.

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Disadvantages of Union

·        Only one member’s data is valid at a time.

·        Assigning value to one member erases the previous value.

·        Debugging becomes difficult.

·        Higher chances of errors in complex programs.

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Difference Between Structure and Union

Structure	Union
All members have separate memory.	All members share the same memory.
All values can be stored at the same time.	Only one value is valid at a time.
More memory usage.	Very low memory usage.
Data does not overwrite.	Data can overwrite.
Size = sum of sizes of all members.	Size = size of the largest member.