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.