enqueue and dequeue pointer

I'm trying to create a menu style enqueue and dequeue with pointers. I have this so far I want to use the left and right pointers to lets say dequeue a number. I also thought about using two temps. Any suggestions?

Code:

---


#include<iostream>
using namespace std;

struct qNode{
  int num;
  qNode *next;
  qNode *left;
  qNode *right;
};


int main(){
  qNode *temp = NULL;
  qNode *front = NULL;
  qNode *back = NULL;

  temp = new qNode;
 
  temp->num = 7;
 
  cout<<temp->num<<endl;
  cout<<endl;
  //  cout<<temp<<endl;
 
  front = temp;
  back = front;
 
  // cout<<"front: "<<front<<endl;
  // cout<<"back: "<<back<<endl;

  temp = new qNode;
  temp->num = 9;
 
  back->next = temp;
  back = temp;
 
  cout<<front->next->num<<endl;

  temp = front;
  while(temp){
    cout<<temp->num<<endl;
    temp = temp->next;
  }

  front = front->next;
  delete temp;

  return 0;

}

---

I don't see why you would need left and right, to be honest; if it's an honest-to-goodness queue, you're only go to go through the thing one way.

I'm guessing this is your enqueue:

Code:

---

  back->next = temp;
  back = temp;

---

After this, you need to set back->next to NULL, otherwise your "walk through the queue" loop becomes infinite (unless you happen to find a NULL pointer on your random walk through core). (That is why you asked the question, isn't it?)

Also, I'm guessing this is the dequeue:

Code:

---

  front = front->next;
  delete temp;

---

You would need to set temp = front first before this. (As it stands in your program, temp would have to be NULL to get here after the while loop; I can never remember whether delete NULL is syntactically valid or not, but regardless, I would be willing to bet it isn't what you want.)

Are there any simple Linked lists "examples" out there that you can enqueue, dequeue and print. I'm getting confused with my pointers and I wanted to see how you can actually implement this Linked list with a Menu. I think it would help me understand it better. I've searched the internet but I can't find anything that's easy for me to understand. I'd like to implement my linked list like I sorta have it in my first post.

ok so i figured some more stuff out. I can't seem to get my dequeue working though. I need to also figure out how to show the queue.

Code:

---

#include<iostream>
using namespace std;

class qNode{
public:
  int num;
  qNode *next;
};

int main(){
  int num;
  int selection;
  void menu();
  int enqueue(int);
  void dequeue();
  qNode *temp = NULL;
  qNode *front = NULL;
  qNode *back = NULL;
 
  do{
  cout<<"1. Enqueue"<<endl;
  cout<<"2. Dequeue"<<endl;
  cout<<"3. Print"<<endl;
  cout<<"4. Exit"<<endl;
  cout<<"Enter a selection: ";
  cin>>selection;
 
  if(selection == 1){
    cout<<"Enter a number: ";
    cin>>num;
    temp = new qNode;
    temp->num=num;
    if(front == NULL){
      front = temp;
      back = front;
      cout<<num<<endl;;
    }
    else{
      back->next = temp;
      back = temp;
    }
   
  }
 
  if(selection == 2){
    temp = front;
    front = front->next;
    delete temp;
   
  }

}while(selection != 4);
    return 0;

}

---

Maybe a hint: add

Code:

---

  printf("%p %p %p\n", front->next, back, back->next);

---

at the bottom of your do-while loop and watch it go through some enqueue-dequeue cycles. What happens when you do three enqueues and three dequeues?

And implement choice 3, to print the list. That way, you can again, observe what's going on with the list. I don't see anything obviously wrong, but then I only spent a few seconds on it.

I may come back to it.

Edit: What do you set "next" to?

--
Mats

Quote:

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Originally Posted by tabstop

Maybe a hint: add

Code:

---

  printf("%p %p %p\n", front->next, back, back->next);

---

at the bottom of your do-while loop and watch it go through some enqueue-dequeue cycles. What happens when you do three enqueues and three dequeues?

---

In the C++ forum, it behooves you to suggest cout instead of printf. :)

Code:

---

  if(selection == 1){
    cout<<"Enter a number: ";
    cin>>num;
    temp = new qNode;
    temp->num=num;
    if(front == NULL){
      front = temp;
      back = front;
      cout<<num<<endl;;

---

What about setting temp->next to NULL? And why the double semicolon? ;) [edit] matsp's edit may have beaten me, we cannot know! :) [/edit]

Printing the queue should be easy. Something as simple as

Code:

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for(cur = front; cur; cur = cur->next) {
    cout << cur->data << endl;
}

---

should cut it.

Quote:

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Originally Posted by dwks

In the C++ forum, it behooves you to suggest cout instead of printf. :)

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I'm over here sitting in the corner now ;) Today I have learned: just because the title bar says "C Board" doesn't mean I'm in the C board.

Quote:

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Originally Posted by dwks

What about setting temp->next to NULL? And why the double semicolon? ;) [edit] matsp's edit may have beaten me, we cannot know! :) [/edit]

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If we say it three times, it must be true.

haha... Thanks for the help guys. Yeah I left an extra semicolon there....