CODEWITHSUNDEEP
c++doubly-linked-list
Debasis_Buxy
Debasis_Buxy

Reputation: 73

Cannot access adjecent node of linked list

I am trying to implement a doubly linked list in C++ and have run across a problem.

#include <iostream>
#include <string>

struct Node
{
    std::string data;
    Node* prev_link;
    Node* next_link;
    Node(const std::string& data,Node* prev_link=nullptr, Node* next_link=nullptr)
        : data{data},prev_link{prev_link},next_link{next_link} {}// constructor
};

Node* insert(Node* new_node,Node* old_node);// insert node before old node
Node* head(Node* node);// returns a pointer to the head i.e. the left end of the linked list
void print_list(Node* node);//takes the head pointer and executes iterative print
void kill_list(Node* tail_node);// deallocates memory by deleting the list

Node* insert(Node* new_node,Node* old_node)
{
    if(new_node == nullptr) return old_node;
    if(old_node == nullptr) return new_node;
    new_node->next_link = old_node;// p of old node connect to new node
    if(old_node->prev_link) old_node->prev_link->next_link = new_node;//n of old' node connect to new node if old' node exists
    new_node->prev_link = old_node->prev_link;//p of new node connect to old` node
    new_node->next_link = old_node;//n of new node connect to old node
    return new_node;
}

Node* head(Node* node)
{
    while(node->next_link != nullptr) node = node->next_link;
    return node;    
}

void print_list(Node* node)
{
    while(node)
    {
        std::cout << node->data;
        if(node = node->next_link) std::cout << "<->";// if next node is not an end node 
    }

}

void kill_list(Node* tail_node)
{
    Node* temp;
    while (tail_node)
    {
        temp = (tail_node->prev_link)?tail_node->prev_link:tail_node->next_link;
        delete tail_node;
        tail_node = temp;
    }
    std::cout << '\n' <<"List destroyed" << std::endl;
}

int main()
{
    Node* alphabets = new Node("A");
    alphabets = insert(new Node("B"),alphabets);
    alphabets = insert(new Node("C"),alphabets);
    print_list(alphabets);
    std::cout << '\n';
    std::cout << "Head:" << head(alphabets)->data << std::endl;
    std::cout << "Adjacent:" << head(alphabets)->prev_link->data << std::endl;
    kill_list(alphabets);
}

output:

C<->B<->A

Head:A

fish: “./test1” terminated by signal SIGSEGV (Address boundary error)

The head() function returns a pointer to the head node(in this case it's A). The linked list as well as the head node is printed correctly but I cannot access the node adjacent to the head node. Cannot figure out what I am doing wrong. Any help would be appreciated.

Upvotes: 2

Views: 118

Answers (2)

SeventhSon84
SeventhSon84

Reputation: 364

The problem is due to the fact that the prev_link is not set for the head (the prev link is zero for every node), there is an error on the insert function, you never set the prev_link of the old node.

Upvotes: 0

pastaleg
pastaleg

Reputation: 1838

Your error is because there is a null pointer for the neighbor of A. In your insert function, you have this if statement

if(old_node->prev_link) old_node->prev_link->next_link = new_node

However, in the case of A, there is no prev_link but you would still like to assign B. So replacing that with:

old_node->prev_link = new_node;

fixes the issue. However you might want to double check so that this corresponds to your desired logic.

Upvotes: 2

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