C Program to Implement Doubly Linked List

Introduction

A doubly linked list is a dynamic data structure where each element, called a node, contains a data part and two pointers: one pointing to the next node and another pointing to the previous node. The doubly linked list allows traversal in both directions (forward and backward), making it more flexible than a singly linked list.

Example:

• Operations: Insert nodes at the beginning, at the end, and in the middle of the list; delete nodes; traverse the list forward and backward.
• Output: The linked list elements are displayed after each operation.

Problem Statement

Create a C program that:

• Implements a doubly linked list with basic operations such as insertion, deletion, and traversal.
• Allows the user to perform these operations interactively.

Solution Steps

1. Include the Standard Libraries: Use #include <stdio.h> and #include <stdlib.h> for standard input-output functions and dynamic memory allocation.
2. Define the Node Structure: Create a structure for the node containing an integer data part and two pointers (one to the next node and one to the previous node).
3. Implement Doubly Linked List Operations: Implement functions for inserting a node at the beginning, at the end, and after a specific node; deleting a node; and traversing the list forward and backward.
4. Create a Menu-Driven Program: Allow the user to interactively choose and perform operations on the doubly linked list.
5. Display the Doubly Linked List: Display the contents of the doubly linked list after each operation.

C Program to Implement Doubly Linked List

#include <stdio.h> #include <stdlib.h> // Step 2: Define the Node Structure struct Node { int data; struct Node* next; struct Node* prev; }; // Function to insert a node at the beginning void insertAtBeginning(struct Node** head_ref, int new_data) { struct Node* new_node = (struct Node*)malloc(sizeof(struct Node)); new_node->data = new_data; new_node->next = (*head_ref); new_node->prev = NULL; if (*head_ref != NULL) { (*head_ref)->prev = new_node; } (*head_ref) = new_node; } // Function to insert a node at the end void insertAtEnd(struct Node** head_ref, int new_data) { struct Node* new_node = (struct Node*)malloc(sizeof(struct Node)); struct Node* last = *head_ref; new_node->data = new_data; new_node->next = NULL; if (*head_ref == NULL) { new_node->prev = NULL; *head_ref = new_node; return; } while (last->next != NULL) { last = last->next; } last->next = new_node; new_node->prev = last; } // Function to insert a node after a specific node void insertAfter(struct Node* prev_node, int new_data) { if (prev_node == NULL) { printf("The given previous node cannot be NULL.\n"); return; } struct Node* new_node = (struct Node*)malloc(sizeof(struct Node)); new_node->data = new_data; new_node->next = prev_node->next; new_node->prev = prev_node; if (new_node->next != NULL) { new_node->next->prev = new_node; } prev_node->next = new_node; } // Function to delete a node by key void deleteNode(struct Node** head_ref, int key) { struct Node* temp = *head_ref; while (temp != NULL && temp->data != key) { temp = temp->next; } if (temp == NULL) return; if (*head_ref == temp) { *head_ref = temp->next; } if (temp->next != NULL) { temp->next->prev = temp->prev; } if (temp->prev != NULL) { temp->prev->next = temp->next; } free(temp); } // Function to traverse and display the list in forward direction void traverseForward(struct Node* node) { printf("Doubly Linked List (forward): "); while (node != NULL) { printf("%d -> ", node->data); node = node->next; } printf("NULL\n"); } // Function to traverse and display the list in backward direction void traverseBackward(struct Node* node) { if (node == NULL) return; while (node->next != NULL) { node = node->next; } printf("Doubly Linked List (backward): "); while (node != NULL) { printf("%d -> ", node->data); node = node->prev; } printf("NULL\n"); } int main() { struct Node* head = NULL; int choice, data, key; while (1) { // Step 4: Create a Menu-Driven Program printf("\nMenu:\n"); printf("1. Insert at Beginning\n"); printf("2. Insert at End\n"); printf("3. Insert After\n"); printf("4. Delete a Node\n"); printf("5. Display List Forward\n"); printf("6. Display List Backward\n"); printf("7. Exit\n"); printf("Enter your choice: "); scanf("%d", &choice); switch (choice) { case 1: printf("Enter data to insert at beginning: "); scanf("%d", &data); insertAtBeginning(&head, data); break; case 2: printf("Enter data to insert at end: "); scanf("%d", &data); insertAtEnd(&head, data); break; case 3: printf("Enter the data of the node after which to insert: "); scanf("%d", &key); printf("Enter data to insert: "); scanf("%d", &data); struct Node* temp = head; while (temp != NULL && temp->data != key) { temp = temp->next; } if (temp != NULL) { insertAfter(temp, data); } else { printf("Node with data %d not found.\n", key); } break; case 4: printf("Enter data of the node to delete: "); scanf("%d", &key); deleteNode(&head, key); break; case 5: traverseForward(head); break; case 6: traverseBackward(head); break; case 7: exit(0); default: printf("Invalid choice! Please enter a valid option.\n"); } } return 0; // Return 0 to indicate successful execution } 

Explanation

Step 2: Define the Node Structure

• The Node structure contains three members: an integer data to store the node’s data, a pointer next to point to the next node in the list, and a pointer prev to point to the previous node in the list.

Insertion Functions

• insertAtBeginning: Inserts a node at the beginning of the list.
• insertAtEnd: Inserts a node at the end of the list.
• insertAfter: Inserts a node after a specified node.

Deletion Function

• deleteNode: Deletes the first node with the specified key.

Traversal Functions

• traverseForward: Traverses the doubly linked list and prints each node’s data in the forward direction.
• traverseBackward: Traverses the doubly linked list and prints each node’s data in the backward direction.

Step 4: Create a Menu-Driven Program

• The main() function provides a menu to the user for performing different operations on the doubly linked list. The program continues to run until the user chooses to exit.

Output Example

Example Output:

Menu: 1. Insert at Beginning 2. Insert at End 3. Insert After 4. Delete a Node 5. Display List Forward 6. Display List Backward 7. Exit Enter your choice: 1 Enter data to insert at beginning: 10 Menu: 1. Insert at Beginning 2. Insert at End 3. Insert After 4. Delete a Node 5. Display List Forward 6. Display List Backward 7. Exit Enter your choice: 2 Enter data to insert at end: 20 Menu: 1. Insert at Beginning 2. Insert at End 3. Insert After 4. Delete a Node 5. Display List Forward 6. Display List Backward 7. Exit Enter your choice: 5 Doubly Linked List (forward): 10 -> 20 -> NULL 

Conclusion

This C program demonstrates how to implement a doubly linked list with basic operations such as insertion, deletion, and traversal. The menu-driven interface allows users to interactively perform operations on the doubly linked list, making it a useful example for beginners learning data structures in C programming.