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

In this method, a new element is inserted at the specified position in the 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, a new node with given element is created. If the insert position is 1, then the new node is made to head. Otherwise, traverse to the node that is previous to the insert position and check if it is null or not. In case of null, the specified position does not exist. In other case, update the links. The below figure describes the process, if the insert node is other than the head node.

The function push_at is created for this purpose. It is a 6-step process.

void push_at(struct Node** head_ref, int newElement, int position) {     
  
  //1. allocate node to new element
  struct Node *newNode, *temp;
  newNode = (struct Node*)malloc(sizeof(struct Node)); 
  newNode->data = newElement;
  newNode->next = NULL;
  newNode->prev = NULL;

  //2. check if the position is > 0
  if(position < 1) {
    printf("\nposition should be >= 1.");
  } else if (position == 1) {
    
    //3. if the position is 1, make new node as head
    newNode->next = *head_ref;
    (*head_ref)->prev = newNode;
    *head_ref = newNode;
  } else {
    
    //4. Else, make a temp node and traverse to the 
    //   node previous to the position
    temp = *head_ref;
    for(int i = 1; i < position-1; i++) {
      if(temp != NULL) {
        temp = temp->next;
      }
    }
 
    //5. If the previous node is not null, adjust 
    //   the links
    if(temp != NULL) {
      newNode->next = temp->next;
      newNode->prev = temp;
      temp->next = newNode; 
      if(newNode->next != NULL)
        newNode->next->prev = newNode;  
    } else {
      
      //6. When the previous node is null
      printf("\nThe previous node is null.");
    }       
  }
}

The below is a complete program that uses above discussed concept to insert new node at a given position in the 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;
  } else {
    temp = *head_ref;
    while(temp->next != NULL) {
      temp = temp->next;
    }    
    temp->next = newNode;
    newNode->prev = temp;
  }
}

//Inserts a new element at the given position
void push_at(struct Node** head_ref, int newElement, int position) {     
  struct Node *newNode, *temp;
  newNode = (struct Node*)malloc(sizeof(struct Node)); 
  newNode->data = newElement;
  newNode->next = NULL;
  newNode->prev = NULL;
  if(position < 1) {
    printf("\nposition should be >= 1.");
  } else if (position == 1) {
    newNode->next = *head_ref;
    (*head_ref)->prev = newNode;
    *head_ref = newNode;
  } else {
    temp = *head_ref;
    for(int i = 1; i < position-1; i++) {
      if(temp != NULL) {
        temp = temp->next;
      }
    }
    if(temp != NULL) {
      newNode->next = temp->next;
      newNode->prev = temp;
      temp->next = newNode; 
      if(newNode->next != NULL)
        newNode->next->prev = newNode;   
    } else {
      printf("\nThe previous node is null.");
    }       
  }
}   

//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 (temp != NULL) {
      printf("%i ",temp->data);
      temp = temp->next;  
    }
    printf("\n");
  } else {
    printf("The list is empty.\n");
  }   
}

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

  //Add three elements in 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