Structures and Unions in C

 A Structure is a collection of dissimilar data type of  particular entity.

For example, An entity Student may have its name (string), roll number (int), marks (float). To store such type of information structure can be used.

The struct keyword is used to define the structure in C programming language.

Syntax: struct structure_name{
              datatype member1;
              datatype member2;
            ........
           ........
           };

• Declaring Structure

One can declare each of the member of structure inside curly braces.

Syntax: struct mystructure{

              int     myname;

              char  myletter;
           };

• Defining Structure

To use structure in our program, we have to define its instance. We can do that by creating variables of the structures type

We can define using 2 methods:

1. Structure variable declaration with structure template 

struct structure_name{
              datatype member1;
              datatype member2;
            ........
           ........
           }variable1,variable2;

2. Structure variable declaration after structure template

(structure declared beforehand)

struct structure_name  variable1,variable2,....;

• Accessing Structure

We can access structure members using (.) dot operator.

Syntax:

structure_name.member1;

structure_name.member2;

• Initializing Structure 

Structure member cannot be initialized with declaration.
The reason is when memory is allocated to datatype declared. Memory allocated only when variable is created.

There are 2 ways to initialize structure:
1)  . (dot operator)  is used to access the structure members using structure variable.
2)  -> (arrow operator) is used to access structure members using structure pointer variable.

Array within structure

An array can be declared inside a structure as a member when we need to store multiple members of the

same type.

Syntax: struct strucure_name{

             Datatype array_Name[arraysize];

              .............

             };

Example: 

#include <stdio.h> #include<string.h> int main() { struct Book { int pages; float price; char name[20]; }; struct Book b1={150,98.50, "C program"}; struct Book b2; { b2.pages=208; b2.price=185.00; strcpy(b2.name,"hai"); } struct Book b3; printf("Enter the pages, price and name of book no 3\n"); scanf("%d %f %s", &b3.pages, &b3.price, &b3.name); printf("The pages, price and name of 3 books are\n"); printf("%d %f %s\n", b1.pages, b1.price, b1.name); printf("%d %f %s\n", b2.pages, b2.price, b2.name); printf("%d %f %s\n",b3.pages, b3.price, b3.name); return 0; }

Array of structures

An array whose elements are of type structure is called array of structure. It is generally useful when we need multiple structure variables in our program.

Example:

#include <stdio.h> #include<stdio.h> int main() { struct Book { int pages; float price; char name [20]; }; struct Book b[3]; printf("Enter the pages, price, and name of 3 books\n"); int i; for(i=0;i<3;i++) scanf("%d %f %s", &b[i].pages,&b[i].price,&b[i].name); printf("The pages, price and name of 3 books are\n"); for(i=0;i<3;i++) printf("%d %f %s\n",b[i].pages,b[i].price,b[i].name); return 0; }

Anonymous Structures

Structures without a name, usually used for a single occurrence.

Nested Structures

Structures inside structures.

example:

struct University { char name[50]; struct Department { char deptName[30]; int numProfessors; } dept; };

Pointers in Structures

The way we can have a pointer pointing to an int, or a pointer pointing to a char, similarly we can have pointer pointing to a struct, such pointers are known as structure pointer.

A structure pointer is defined as the pointer which points to the address of the memory block that stores a structure known as the structure pointer. Complex data structures like linked lists, trees, graphs, etc. are created with the help of structure pointers. The structure pointer tells the address of a structure in memory by pointing its variable to the structure variable.

struct University #include <stdio.h> #include <string.h> struct Student { int roll_no; char name[30]; char branch[40]; int batch; }; int main() { struct Student s, *ptr; ptr = &s; // Taking inputs printf("Enter the Roll Number of Student\n"); scanf("%d", &ptr->roll_no); printf("Enter Name of Student\n"); scanf("%s", &ptr->name); printf("Enter Branch of Student\n"); scanf("%s", &ptr->branch); printf("Enter batch of Student\n"); scanf("%d", &ptr->batch); printf("\nStudent details are: \n"); printf("Roll No: %d\n", ptr->roll_no); printf("Name: %s\n", ptr->name); printf("Branch: %s\n", ptr->branch); printf("Batch: %d\n", ptr->batch); return 0; }

Self-referential Structures

A structure containing a member that is a pointer to its own type is self referential structure. A self-referential structure is a structure that contains a member variable that is a pointer to a variable of the same structure type. This allows the structure to create linked data structures where elements reference each other .Those structure in which one or more pointer points to the structure of the same type.

struct University { char name[50]; struct Department { char deptName[30]; int numProfessors; } dept; };

Pointers in Structures

The way we can have a pointer pointing to an int, or a pointer pointing to a char, similarly we can have pointer pointing to a struct, such pointers are known as structure pointer.

A structure pointer is defined as the pointer which points to the address of the memory block that stores a structure known as the structure pointer. Complex data structures like linked lists, trees, graphs, etc. are created with the help of structure pointers. The structure pointer tells the address of a structure in memory by pointing its variable to the structure variable.

#include <stdio.h> // Define the self-referential structure struct Node { int data; struct Node *next; // Pointer to the next Node }; int main() { // Declare instances of the structure struct Node node1, node2, node3; // Initialize data for each node node1.data = 1; node2.data = 2; node3.data = 3; // Link the nodes together node1.next = &node2; node2.next = &node3; node3.next = NULL; // Marks the end of the linked list // Traverse the linked list and print the data struct Node *current = &node1; // Start from the first node while (current != NULL) { printf("%d ", current->data); current = current->next; } printf("\n"); return 0; }

Structure Padding in C

Structure padding is a concept in C that adds the one or more empty bytes between the memory addresses to align the data in memory.

Union in C

 The Union is a user-defined data type in C language that can contain elements of the different data types just like structure. But unlike structures, all the members in the C union are stored in the same memory location. Due to this, only one member can store data at the given instance.

Bit Fields in C

In C, bit fields are used to specify the size of individual members within a structure. They allow for more efficient use of memory by packing multiple bit fields into a single storage unit, such as a byte or a word. This is particularly useful when dealing with hardware registers, protocol headers, or other situations where memory efficiency is crucial. 

#include <stdio.h> struct dob{ int date; int month; int year; }; int main() { printf("size of the struct is %ld \n",sizeof(struct dob)); struct dob myDOB={06,11,2001}; printf("my dob is %d-%d-%d",myDOB.date,myDOB.month,myDOB.year); }

typedef in C

The typedef is a keyword used in C programming to provide some meaningful names to the already existing variable in the C program. It behaves similarly as we define the alias for the commands. In short, we can say that this keyword is used to redefine the name of an already existing variable.

Syntax: 

• typedef <existing_name> <alias_name>  
• typedef unsigned int unit;  

Questions:

Can you access a structure member without -> operator using pointer to structure?

Yes, you can access a structure member without using the ‘->‘ operator by dereferencing the pointer using the ‘*‘ operator combined with the dot ‘.‘ operator. For example, ‘(*ptr).member‘.

What are Structures?

 A Structure is a collection of dissimilar data type of  particular entity.

What is Structure padding?

Structure padding is the addition of extra memory to align data in a structure to word boundaries, which can improve the performance of memory access but may also increase the size of the structure.

What is meant by an anonymous structure?

An anonymous structure is a structure definition without a tag or name, often used for nested structures to define complex data models without having to name intermediate structures.

What is are self referential structures?

Self-referential structures are structures that contain at least one member that is a pointer to the structure of its own type. They are commonly used for creating linked lists and trees.

What is the use of .(dot) operator?

The dot ‘.‘ operator is used to access members of a structure when you have a structure variable.

What is the use of ->(arrow) operator?

The arrow ‘->‘ operator is used to access members of a structure through a pointer to the structure.