C - Delete a node at the given position in the Circular Doubly Linked List

In this method, a node at the specified position in the circular doubly linked list is deleted. For example - if the given list is 10->20->30 and the 2^nd node is deleted, the list becomes 10->20.

First, create two nodes temp and nodeToDelete to traverse through the list and track the node to delete respectively. After that count the number of elements in the list to check whether the specified position is valid or not (It must lie in the range of [1, n], where n is number of elements in the list). If the specified valid position is 1 and head is the only element in the list, then make the head as null. If the specified valid position is 1 and list contains more than one elements, then make next of head as new head and adjust links accordingly. If the specified valid position is greater than 1 then traverse to the node previous to the given position and delete the given node and adjust links accordingly.

Circular Doubly Linked List - Delete Node

The function pop_at is created for this purpose. It is a 5-step process.

void pop_at(struct Node** head_ref, int position) {  
  
  //1. create two nodes - temp and nodeToDelete 
  //   to traverse and track the node to delete  
  struct Node *nodeToDelete = *head_ref;
  struct Node *temp = *head_ref;
  int NoOfElements = 0;

  //2. Find the number of elements in the list
  if(temp != NULL) {
    NoOfElements++;
    temp = temp->next;
  }
  while(temp != *head_ref) {
    NoOfElements++;
    temp = temp->next;
  }

  //3. check if the specified position is valid
  if(position < 1 || position > NoOfElements) {
    printf("\nInavalid position.");
  } else if (position == 1) {
  
  //4. if the position is 1 and head is the only element
  //   in the list, then make it null, else make next
  //   of head as new head and adjust links accordingly
    if((*head_ref)->next == *head_ref) {
      *head_ref = NULL;
    } else {
      while(temp->next != *head_ref)
        temp = temp->next;
      *head_ref = (*head_ref)->next;
      temp->next = *head_ref; 
      (*head_ref)->prev = temp;
      free(nodeToDelete);
    }
  } else {

   //5. Else, traverse to the node previous to
   //   the given position and delete the given 
   //   node and adjust links accordingly 
    temp = *head_ref;
    for(int i = 1; i < position-1; i++) 
      temp = temp->next;
    nodeToDelete = temp->next;
    temp->next = temp->next->next;
    temp->next->prev = temp;
    free(nodeToDelete);  
  }
}

The below is a complete program that uses above discussed concept to delete a node at a given position in the circular doubly linked list.

#include <stdio.h>
#include <stdlib.h>

//node structure
struct Node {
  int data;
  struct Node* next;
  struct Node* prev;
};

//Add new element at the end of the list
void push_back(struct Node** head_ref, int newElement) {  
  struct Node *newNode, *temp;
  newNode = (struct Node*)malloc(sizeof(struct Node)); 
  newNode->data = newElement;  
  newNode->next = NULL;
  newNode->prev = NULL;
  if(*head_ref == NULL) {
    *head_ref = newNode;
     newNode->next = *head_ref;
     newNode->prev = *head_ref;
  } else {
    temp = *head_ref;
    while(temp->next != *head_ref) {
      temp = temp->next;
    }    
    temp->next = newNode;
    newNode->next = *head_ref;
    newNode->prev = temp;
    (*head_ref)->prev = newNode;
  }
}

//Delete an element at the given position
void pop_at(struct Node** head_ref, int position) {  
  struct Node *nodeToDelete = *head_ref;
  struct Node *temp = *head_ref;
  int NoOfElements = 0;

  if(temp != NULL) {
    NoOfElements++;
    temp = temp->next;
  }
  while(temp != *head_ref) {
    NoOfElements++;
    temp = temp->next;
  }

  if(position < 1 || position > NoOfElements) {
    printf("\nInavalid position.");
  } else if (position == 1) {
  
    if((*head_ref)->next == *head_ref) {
      *head_ref = NULL;
    } else {
      while(temp->next != *head_ref)
        temp = temp->next;
      *head_ref = (*head_ref)->next;
      temp->next = *head_ref; 
      (*head_ref)->prev = temp;
      free(nodeToDelete);
    }
  } else {
    temp = *head_ref;
    for(int i = 1; i < position-1; i++) 
      temp = temp->next;
    nodeToDelete = temp->next;
    temp->next = temp->next->next;
    temp->next->prev = temp;
    free(nodeToDelete);  
  }
}  

//display the content of the list
void PrintList(struct Node* head_ref) {
  struct Node* temp = head_ref;
  if(head_ref != NULL) {
    printf("The list contains: ");
    while (1) {
      printf("%i ",temp->data);
      temp = temp->next;
      if(temp == head_ref)
        break;    
    }
    printf("\n");
  } else {
    printf("The list is empty.\n");
  }   
}

// test the code 
int main() {
  struct Node* MyList = NULL;

  //Add three elements at the end of the list.
  push_back(&MyList, 10);
  push_back(&MyList, 20);
  push_back(&MyList, 30);
  PrintList(MyList);

  //Delete an element at position 2
  pop_at(&MyList, 2);
  PrintList(MyList);  

  //Delete an element at position 1
  pop_at(&MyList, 1);
  PrintList(MyList); 

  return 0; 
}

The above code will give the following output:

The list contains: 10 20 30
The list contains: 10 30
The list contains: 30