The program does not give the desired output . Wrong implementation of FIFO ?

This is a FIFO program using linked list . The program does not give the desired output but generates a long loop which stops after sometime and there is a message that the program has stopped working. What is the problem ?

#include <iostream>
using namespace std;

struct node {
      int data;
      struct node* previous; // This pointer keeps track of开发者_运维知识库 the address of the previous node
};

struct queue {
      node* first;
      node* last;
};

node* dataNode_P_A; 

bool loop = true;

struct node* enterData();
struct node* enter_N_Data();
void displayQueue();

int main() {
    struct node* dataNode= enterData();

    while( loop ) {
        cout << "Want to enqueue ? Press y/n : ";
        char ans;
        cin >> ans;
        if( ans == 'y' )  {
          struct node* dataNode_N = enter_N_Data();
        } else {
          break;
        }
    }

  displayQueue();
}

 struct node* enterData() {
    cout << "Enter the number : ";
    dataNode_P_A = new node;  // Now dataNode points to a chunk allocated to node
    cin >> dataNode_P_A->data;
    dataNode_P_A->previous = NULL; // this is set to NULL because no one follows till now   
    queue* q = new queue; 
    q->first = dataNode_P_A; // this pointer points to the first element
    return dataNode_P_A;
}

struct node* enter_N_Data() {
    cout << endl << "Enter the number : ";
    node* dataNode = new node;
    cin >> dataNode->data;
    dataNode->previous = dataNode_P_A; 
    queue* q = new queue;
    q->last = dataNode; // this pointer points to the last element
    return dataNode;
}

void displayQueue() {
    while( dataNode_P_A != NULL ) {
        cout << dataNode_P_A->data  << endl;
        dataNode_P_A++;
    }
}

---

You are constructing queues and then abandoning them.

You fail to update dataNode_P_A, so that you are not constructing a list so much as a tassel.

You invoke dataNode_P_A++ when you clearly don't know what it means.

You have written a long, complicated piece of code without testing it along the way.

You should start over, and go step by step.

---

Where to begin... First off the queue data structure isn't particularly used for anything. But that's not the root of your problem. That lies here:

 void displayQueue() { 
   while( dataNode_P_A != NULL ) { 
       cout << dataNode_P_A->data  << endl; 
       dataNode_P_A++; 
   } 
}

When iterating through a linked list, you move to the next element by navigating to ->previous:

 void displayQueue() { 
   while( dataNode_P_A != NULL ) { 
       cout << dataNode_P_A->data  << endl; 
       dataNode_P_A = dataNode_P_A->previous; 
   } 
}

Having said that, you're doing some other things that are questionable - like modifying your global list (dataNode_P_A). That's not a problem in your example, but it can be a problem if you ever want to do anything to the list other than display it.

Here's another version of displayQueue that doesn't have that problem:

 void displayQueue() { 
   node *entry = dataNode_P_A;
   while( entry != NULL ) { 
       cout << entry->data  << endl; 
       entry = entry->previous; 
   } 
}

---

You should edit your enter_N_Data() function like:

node* temp; // global as others in your program

struct node* enter_N_Data() {
cout << endl << "Enter the number : ";
node* dataNode = new node;
cin >> dataNode->data;
temp = new node;
temp = dataNode_P_A;
dataNode_P_A = dataNode;   // update dataNode_P_A

dataNode->previous = temp; 

queue* q = new queue;
q->last = dataNode; // this pointer points to the last element
return dataNode;
}

and keep everything same while following the suggestions by @ Larry Osterman and @ Beta.