C Data Structures - Doubly Linked List Other Related Topics

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



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

First, the specified position must be greater than equal to 1. If the specified position is 1 and head is not null, then make the head next as head and delete the previous head. Else, traverse to the node that is previous to the specified position. If the specified node and previous to the specified node are not null then adjust the link. In other case, the specified node will be already null. The below figure describes the process, if the deletion node is other than the head node.

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. check if the position is > 0
  if(position < 1) {
    printf("\nposition should be >= 1.");
  } else if (position == 1 && *head_ref != NULL) {
    
    //2. if the position is 1 and head is not null, make
    //   head next as head and delete previous head  
    struct Node* nodeToDelete = *head_ref;
    *head_ref = (*head_ref)->next;
    free(nodeToDelete);
    if(*head_ref != NULL)
      (*head_ref)->prev = NULL;
  } else {

    //3. Else, make a temp node and traverse to the 
    //   node previous to the position
    struct Node *temp;
    temp = *head_ref;
    for(int i = 1; i < position-1; i++) {
      if(temp != NULL) {
        temp = temp->next;
      }
    }
 
    //4. If the previous node and next of the previous  
    //   is not null, adjust links 
    if(temp != NULL && temp->next != NULL) {
      struct Node* nodeToDelete = temp->next;
      temp->next = temp->next->next; 
      if(temp->next->next != NULL)
        temp->next->next->prev = temp->next;   
      free(nodeToDelete); 
    } else {
      
      //5. Else the given node will be empty.
      printf("\nThe node is already null.");
    }       
  }
}      

The below is a complete program that uses above discussed concept to delete a 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;
  }
}

//Delete an element at the given position
void pop_at(struct Node** head_ref, int position) {     
  if(position < 1) {
    printf("\nposition should be >= 1.");
  } else if (position == 1 && *head_ref != NULL) {
    struct Node* nodeToDelete = *head_ref;
    *head_ref = (*head_ref)->next;
    free(nodeToDelete);
    if(*head_ref != NULL)
      (*head_ref)->prev = NULL;
  } else {
    struct Node *temp;
    temp = *head_ref;
    for(int i = 1; i < position-1; i++) {
      if(temp != NULL) {
        temp = temp->next;
      }
    }
    if(temp != NULL && temp->next != NULL) {
      struct Node* nodeToDelete = temp->next;
      temp->next = temp->next->next; 
      if(temp->next->next != NULL)
        temp->next->next->prev = temp->next;   
      free(nodeToDelete); 
    } else {
      printf("\nThe node is already 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 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 

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