Facing problem with linked list queue

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hello !! i am facing little problem here in my code basically i am implementing linked list using queue technique + using templates but the problem in that is when i implement float and string types it shows me perfect answer but it is not working for "int" type i am writing enqueue and dequeue functions here for clearance !!

What I have tried:

#include <iostream>
#include <string>
using namespace std;

template <typename Type>
struct Node
{
	Type info;
	Node<Type> *Next;
};
template <typename Type>
void Queue<Type>::Enqueue(Type x)
{
	Node<Type> *temp;
	temp = new Node<Type>;
	temp->info = x;
	temp->Next = NULL;
	if (front == NULL)
	{
		front = temp;
	}
	else
	{
		rear->Next = temp;
	}

	rear = temp;

}

template <typename Type>
void Queue<Type>::Dequeue()
{
	Node<Type> *temp;
	temp = new Node<Type>;

	if (front == NULL)
	{
		cout << " Queue is Empty. " << endl;
	}
	else
	{
		temp = front;
		front = front->Next;
		cout << endl << temp->info << " was dequeued." << endl;
		delete temp;
	}
}

int main()
{

	cout << "============== Int Queue ==============" << endl;
	Queue<int>intqueue;
	intqueue.Enqueue(20);
	intqueue.Enqueue(30);
	intqueue.Enqueue(40);
	intqueue.Print();
	intqueue.Dequeue();
	intqueue.Print();}

Posted 2-Jan-17 5:51am

mayashah

Updated 3-Jan-17 23:33pm

KarstenK

v2

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Comments

Richard MacCutchan 2-Jan-17 12:35pm

Any clue what the problem is?

[no name] 2-Jan-17 18:12pm

Time to get out the debugger isn't it?

mayashah 2-Jan-17 21:29pm

basically output doesn't seem to match what is desired !!

mayashah 2-Jan-17 21:30pm

it would launch in following manner as we all like to see !!!
20
30
40
//dequeue function called 20 removed !!!
30
40 !!!

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Jochen Arndt · Answer 1 · 2017-01-03T22:21:00

I will answer by adding comments to your code:

template <typename Type>
void Queue<Type>::Dequeue()
{
    Node<Type> *temp;
    // The instance created here is never deleted: Memory leak.
    // Just remove this line.
    temp = new Node<Type>;

    if (front == NULL)
    {
        cout << " Queue is Empty. " << endl;
    }
    else
    {
        // You are removing the front here.
        // So 20 from your example session is removed.
        // If you want to remove the last item you should use rear instead:
        //  temp = rear;
        //  rear = rear->Prev;
        //  rear->Next = NULL;
        temp = front;
        front = front->Next;
        cout << endl << temp->info << " was dequeued." << endl;
        delete temp;
    }
}

nv3 · Answer 2 · 2017-01-03T23:33:00

If you would use the debugger you would quickly see what's wrong. You should really get acquainted to use it.

What I can see in your code is that you don't update front and rear consequently. For example: What does rear point to in an empty queue? Probably NULL, I assume. So what happens in Enqueue when you are inserting the first element? You are using the read pointer (which is null) and do:

C++

rear->Next = temp;

which will produce undefined results. Similarly in Dequeue: What happens when you remove the last element? In that case front and rear should be set to NULL. I don't see that in your code. So again you are provoking undefined behavior.

Facing problem with linked list queue

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