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

In this method, a new element is inserted at the specified position in the circular doubly linked list. For example - if the given list is 10->20->30 and a new element 100 is added at position 2, the list becomes 10->100->20->30.

First, create two nodes: 1. newNode with given element and 2. temp to traverse through the list. After that count the number of elements in the list to check whether the insertion position is valid or not (It must lie in the range of [1, n + 1], where n is number of elements in the list). If the insertion position is valid and equal to 1 then make the newNode as head and adjust links accordingly. If the insertion position is valid and greater than 1 then traverse to the specified position and insert the newNode and adjust links accordingly.

Circular Doubly Linked List - Add Node At Start

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

void push_at(struct Node** head_ref, int newElement, int position) {  
  
  //1. allocate node to new element and 
  //   create a temp node to traverse the list
  struct Node *newNode = (struct Node*)malloc(sizeof(struct Node)); 
  newNode->data = newElement;
  newNode->next = NULL;
  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 insertion position is valid
  if(position < 1 || position > (NoOfElements+1)) {
    printf("\nInavalid position.");
  } else if (position == 1) {
  
  //4. if the position is 1, make next of the
  //   new node as head and new node as head
    if(*head_ref == NULL) {
      *head_ref = newNode;
      (*head_ref)->next = *head_ref;
      (*head_ref)->prev = *head_ref;
    } else {
      while(temp->next != *head_ref) {
        temp = temp->next;
      }
      temp->next = newNode;
      newNode->prev = temp;
      newNode->next = *head_ref;
      (*head_ref)->prev = newNode;
      *head_ref = newNode;
    }
  } else {

   //5. Else, traverse to the node previous to
   //   the given position, make newNode next 
   //   as temp next and temp next as newNode.
    temp = *head_ref;
    for(int i = 1; i < position-1; i++) 
      temp = temp->next;
    newNode->next = temp->next;
    newNode->next->prev = newNode;
    newNode->prev = temp;
    temp->next = newNode;  
  }
}

The below is a complete program that uses above discussed concept to insert a new node at 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;
  }
}

//Inserts a new element at the given position 
void push_at(struct Node** head_ref, int newElement, int position) {  
  struct Node *newNode = (struct Node*)malloc(sizeof(struct Node)); 
  newNode->data = newElement;
  newNode->next = NULL;
  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+1)) {
    printf("\nInavalid position.");
  } else if (position == 1) {
    if(*head_ref == NULL) {
      *head_ref = newNode;
      (*head_ref)->next = *head_ref;
      (*head_ref)->prev = *head_ref;
    } else {
      while(temp->next != *head_ref) {
        temp = temp->next;
      }
      temp->next = newNode;
      newNode->prev = temp;
      newNode->next = *head_ref;
      (*head_ref)->prev = newNode;
      *head_ref = newNode;
    }
  } else {

    temp = *head_ref;
    for(int i = 1; i < position-1; i++) 
      temp = temp->next;
    newNode->next = temp->next;
    newNode->next->prev = newNode;
    newNode->prev = temp;
    temp->next = newNode;  
  }
}  

//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);

  //Insert an element at position 2
  push_at(&MyList, 100, 2);
  PrintList(MyList);  

  //Insert an element at position 1
  push_at(&MyList, 200, 1);
  PrintList(MyList); 

  return 0; 
}

The above code will give the following output:

The list contains: 10 20 30 
The list contains: 10 100 20 30 
The list contains: 200 10 100 20 30