Doubly Linked List - cannot in delete the first Node - c++

Question

struct Node
{
int data;
Node *next;
Node *prev;
};
class DoublyLinkedList
{
ofstream cout3;
Node *head;
public:
DoublyLinkedList()
{
head = NULL;
cout3.open("task3.out");
}
void insert(int num)
{
Node *temp = new Node;
//To insert if there are no elements
if(head == NULL)
{
temp->prev = NULL;
temp->data = num;
temp->next = NULL;
head = temp;
}
//To insert if there are elements
else
{
temp->prev = NULL;
temp->data = num;
temp->next = head;
head->prev = temp;
head = temp;
}
cout3<<"inserted "<<num<<endl;
}
void dele(int num)
{
Node *temp = head;
int found_num = 0;
while(temp != NULL)
{
if(temp->data == num)
{
found_num = 1;
break;
}
else
temp = temp->next;
}
if(found_num == 0)
cout3<<"cannot delete "<<num<<endl;
//To delete first element
else if (temp->prev == NULL)
{
head = temp->next;
(temp->next)->prev == NULL;
delete temp;
cout3<<"deleted "<<num<<endl;
}
//To delete last element
else if (temp->next == NULL)
{
(temp->prev)->next = NULL;
cout3<<"deleted "<<num<<endl;
delete temp;
}
//To delete any other element
else
{
(temp->prev)->next = temp->next;
(temp->next)->prev = temp->prev;
cout3<<"deleted "<<num<<endl;
delete temp;
}
}
void search(int num)
{
Node *temp = head;
int found_num = 0;
while(temp != NULL)
{
if(temp->data == num)
{
found_num = 1;
break;
}
else
temp = temp->next;
}
if(found_num == 0)
cout3<<"not found "<<num<<endl;
else
cout3<<"found "<<num<<endl;
}
void display()
{
Node *temp = head;
while(temp != NULL)
{
cout3<<temp->data<<" ";
temp = temp->next;
}
cout3<<endl;
}
};
My implementation of Doubly Linked List.
I only insert at the beginning and delete the first occurrence of the number.
However if I want to delete the first element then it prints "deleted number" but when i display the number is still there.
Problem seems to be in my delete function but I cannot find what it is

Answer 1

See this line: (temp->next)->prev == NULL;
You wrote == instead of = , this seems to be the problem.
You dont show how you print the value but im guessing you move backward untill null value before you start..

Answer 2

Just expend the code to test it, it will give out the warning. Fix it, then the program will function as expected.
$ g++ test.cpp
test.cpp:66:30: warning: equality comparison result unused
[-Wunused-comparison]
(temp->next)->prev == NULL;
~~~~~~~~~~~~~~~~~~~^~~~~~~
test.cpp:66:30: note: use '=' to turn this equality comparison into an
assignment
(temp->next)->prev == NULL;
^~
=
1 warning generated.
test.cpp
#include <iostream>
#include <fstream>
struct Node
{
int data;
Node *next;
Node *prev;
};
class DoublyLinkedList
{
std::ofstream cout3;
Node *head;
public:
DoublyLinkedList()
{
head = NULL;
cout3.open("task3.out");
}
void insert(int num)
{
Node *temp = new Node;
//To insert if there are no elements
if(head == NULL)
{
temp->prev = NULL;
temp->data = num;
temp->next = NULL;
head = temp;
}
//To insert if there are elements
else
{
temp->prev = NULL;
temp->data = num;
temp->next = head;
head->prev = temp;
head = temp;
}
cout3<<"inserted "<<num<<std::endl;
}
void dele(int num)
{
Node *temp = head;
int found_num = 0;
while(temp != NULL)
{
if(temp->data == num)
{
found_num = 1;
break;
}
else
temp = temp->next;
}
if(found_num == 0)
cout3<<"cannot delete "<<num<<std::endl;
//To delete first element
else if (temp->prev == NULL)
{
head = temp->next;
(temp->next)->prev == NULL;
delete temp;
cout3<<"deleted "<<num<<std::endl;
}
//To delete last element
else if (temp->next == NULL)
{
(temp->prev)->next = NULL;
cout3<<"deleted "<<num<<std::endl;
delete temp;
}
//To delete any other element
else
{
(temp->prev)->next = temp->next;
(temp->next)->prev = temp->prev;
cout3<<"deleted "<<num<<std::endl;
delete temp;
}
}
void search(int num)
{
Node *temp = head;
int found_num = 0;
while(temp != NULL)
{
if(temp->data == num)
{
found_num = 1;
break;
}
else
temp = temp->next;
}
if(found_num == 0)
cout3<<"not found "<<num<<std::endl;
else
cout3<<"found "<<num<<std::endl;
}
void display()
{
Node *temp = head;
while(temp != NULL)
{
cout3<<temp->data<<" ";
temp = temp->next;
}
cout3<<std::endl;
}
};
int main()
{
DoublyLinkedList list;
list.insert(3);
list.insert(4);
list.insert(5);
list.display();
list.dele(3);
list.display();
}

Answer 3

In below link there is a guide to free the memory. Delete function not work mostly you can free method.
https://www.geeksforgeeks.org/write-a-function-to-delete-a-linked-list/