C++ doubly linked list - c++

Question

I got a problem with my doubly linked list. How can i make the input unique ( i don`t want it to be repeated )
for example i can input 1 and then again 1 i will have a list of 1 and 1. I need to forbid this somehow :) so the list can contain only not repeating numbers.
#include <cstdlib>
#include <iostream>
using namespace std;
struct node
{
int data;
node* next;
node* prev;
};
class Node
{
public:
Node();
~Node();
void setKopa();
void printForward();
private:
node* head;
node* tail;
node* n;
};
Node::Node()
{
setKopa();
}
Node::~Node()
{
delete n;
}
void Node::setKopa()
{
int lenght;
do
{
cout << "Input list lenght (how many elements): ";
cin >> lenght;
if(lenght<2)
cout << "Error list has to have atleast 2 elements!" <<endl;
}
while(lenght<2);
int fill;
cout << "Input "<< lenght <<" elements: "<<endl;
for (int i=0; i<lenght; i++)
{
cin>>fill;
n = new node;
n->data = fill;
if (i==0)
{
n->prev = NULL;
head = n;
tail = n;
}
else if (i+1==lenght)
{
n->prev = tail;
tail->next = n;
tail = n;
tail->next = NULL;
}
else
{
n->prev = tail;
tail->next = n;
tail = n;
}
}
}
void Node::printForward()
{
node* temp = head;
while(temp != NULL)
{
cout << temp->data << " ";
temp = temp-> next;
}
cout << endl;
}
int main()
{
Node a;
a.printForward();
system("pause");
return 0;
}

Answer 1

When you read input, go through the list to see if the input is already there.
With that (simple) answer out of the way, I would like to address some other things regarding your code. The first is that you have a memory leak in that you never delete the list. The second is that you don't need the class member variable n, it might as well be a local variable inside the setKopa loop.
Your way of adding new nodes is also, well, weird. It should, in my opinion, be more general instead of using the loop counter to check what to do. What I suggest is that you make a member function to add new nodes, taking the integer data as argument. This way you can call this function to add nodes anywhere, and not just in the setKopa function. In fact, I think the list should not handle that input at all, instead it should be a free-standing function called from main and which calls the addNode function.
Also the node structure doesn't need to be in the global namespace, it could be a private structure in the Node class. And speaking of the Node class, shouldn't it really be called List instead?
So if I may suggest, you might want to do something like this:
#include <iostream>
class List
{
public:
List()
: head(nullptr), tail(nullptr)
{}
~List();
void addNode(const int data);
void printAll() const;
private:
struct node
{
node()
: next(nullptr), prev(nullptr)
{}
node* next;
node* prev;
int data;
};
node* head;
node* tail;
};
List::~List()
{
for (node* next, *cur = head; cur; cur = next)
{
next = cur->next;
delete cur;
}
}
void List::addNode(const int data)
{
node* n = new node;
n->data = data;
if (tail == nullptr)
{
// First node in list
head = tail = n;
}
else
{
n->prev = tail;
tail->next = n;
tail = n;
}
}
void List::printAll() const
{
std::cout << "{ ";
for (node* cur = head; cur != nullptr; cur = cur->next)
std::cout << cur->data << ' ';
std::cout << "}\n";
}
int main()
{
List list;
for (int i = 0; i < 10; ++i)
list.addNode(i);
list.printAll();
}
The above code should print
{ 0 1 2 3 4 5 6 7 8 9 }
Replace the node-adding loop with your own.