Weird Enqueue Function

[Raman009]

Hi there ,
I have written a enqueue function for a priority queue . This doesn't have any compilation error but my program ends abruptly.
Here's the code :

void Queue::enqueue(int item , int prior)
{
    node *ptr = new node();
    ptr->info = item ;
    ptr->priority = prior ;

    if(front == NULL)  // queue is empty
    {
        ptr->next = NULL ;
        front = ptr ;
    }
    else     //queue not empty
    {
        node *loc , *preloc  ;
        while(loc->priority <= prior)
        {
            preloc = loc ;
            loc = loc->next ;
        }
        preloc->next = ptr ;
        ptr->next = loc ;
    }
}

If you have any suggestions , let me know ,,thanks

[Peter87]

loc is used without being initialized.

[Raman009]

void Queue::enqueue(int item , int prior)
{
    node *ptr = new node();
    ptr->info = item ;
    ptr->priority = prior ;

    if(front == NULL)  // queue is empty
    {
        ptr->next = NULL ;
        front = ptr ;
    }
    else     //queue not empty
    {
        node *loc , *preloc  ;
        loc = front ;
        while(loc->priority <= prior)
        {
            preloc = loc ;
            loc = loc->next ;
        }
        preloc->next = ptr ;
        ptr->next = loc ;
    }
}

Still program stops abruptly ...

[coder777]

When it comes to loop think of the 'extrems'.

What happens when

the first element is loc->priority > prior i.e. no loop at all
no element is loc->priority > prior i.e. how does the loop come to an end

[Raman009]

I learned using debugger . and it gives a segmentation fault error.
Well , In my program the number with largest priority is kept in the front and lower priorities after that and so on..
I initially entered 2,9(item,prior)..it gets stored within the queue.
Then I entered 23,2(item,prior)..it gives a segmentation fault error on the line 21 in my previous post.
*preloc = cannot access memory at address at 0x2 why ?

Initially I thought there might be some null pointer problem but there's none.
Here's the full code of the problem :
Main.cpp

#include <iostream>
#include "Queue.h"
#include <cstdlib>
using namespace std;

int main()
{
    Queue myqueue ;
    int ans , item ,pr;
    cout<<"\n\n Queue Operations\n";
    while(true)
    {
        cout<<"\n1. Enqueue";
        cout<<"\n2. Dequeue";
        cout<<"\n3. Peep";
        cout<<"\n4. IsEmpty";
        cout<<"\n5. Traverse";
        cout<<"\n6. Dispose";
        cout<<"\n7. Exit";

        myqueue.showVar();
        cout <<endl;
        myqueue.traverse();
        cout <<endl;

        cout<<"Enter any operation ";
        cin>>ans;

        switch(ans)
        {
        case 1 :
            cout<<"Enter an item and its priority ";
            cin>>item>>pr;
            myqueue.enqueue(item,pr);
            break ;

        case 2 :
            cout<<"\nThe element is "<< myqueue.dequeue();
            break ;

        case 3 :
            cout<<"\nThe topmost element is "<<myqueue.peep();
            break ;

        case 4 :
            cout<<myqueue.isEmpty();
            break ;

        case 5 :
            myqueue.traverse();
            break ;

        case 6 :
            myqueue.dispose();
            break ;

        case 7 :
            exit(0);
            break;

        default:
            cout<<"\nInvalid input ";
            exit(0);
        }
    }
    return 0;
}

Queue.h

#ifndef QUEUE_H
#define QUEUE_H
#define arrsize 10

typedef struct nodeType  {
    int info ;
    int priority ;
    struct nodeType *next ;
} node ;

class Queue
{
    private:
       node *front ;

    public:
        Queue();
        void traverse();
        bool isEmpty();
        void enqueue(int,int);
        int dequeue();
        int peep();
        void showVar();
        void dispose();
};

#endif // QUEUE_H 

Queue.cpp

#include "queue.h"
#include <iostream>
#include <cstdlib>

int Queue::dequeue()
{
    node *ptr = front ;
    int item ;
    if(isEmpty())
    {
        std::cout <<"\nQueue Empty!!";
        exit(0);
    }
    else
    {
        item = ptr -> info ;
        front = front -> next;
        delete ptr ;
    }
    return item ;
}

int Queue::peep()
{
    node *ptr = front;
    if(isEmpty())
    {
        std::cout << "Queue empty"<<std::endl;
        exit(0) ;
    }
    else
    {
        return ptr->info;
    }
}

void Queue::enqueue(const int item ,const int prior)
{
    node *ptr = new node();
    ptr->info = item ;
    ptr->priority = prior ;

    if(front == NULL)  // queue is empty
    {
        ptr->next = NULL ;
        front = ptr ;
    }
    else     //queue not empty
    {
        node *loc , *preloc;
        loc = front ;
        if(ptr->priority >= front->priority)
        {
            ptr->next = front ;
            front = ptr;
        }
        else if(ptr->priority < front->priority)
        {
            while(ptr->priority < loc->priority && loc != NULL) 
            {
                preloc = loc ;
                loc = loc->next ;
            }
            preloc->next = ptr ;
            ptr->next = loc ;
        }
    }
}

void Queue::showVar()
{
    std::cout<<"\nFront: "<<front;
}

void Queue::traverse()
{
    node *ptr = front ;
    while(ptr != NULL)
    {
        std::cout << ptr -> info << ":" << ptr->priority <<" ";
        ptr = ptr -> next ;
    }
}

bool Queue::isEmpty()
{
    return (front == NULL);
}

Queue::Queue()
{
    front = NULL ;
}

void Queue::dispose()
{
    while( front != NULL)
    {
        node *ptr = front ;
        front = front ->next ;
        delete ptr ;
    }
    front = NULL ;
}

Any kind of suggestions are welcome..thanks

[Peter87]

You did not initialize preloc. If the new element has smaller priority than any other element the loop body will never run and preloc remains uninitialized.

[Raman009]

Error found :)
The line in enqueue

while(ptr->priority < loc->priority && loc != NULL)

should be written like

while(loc != NULL && ptr->priority < loc->priority)

The compiler executes lines from left to right .
Thus when it executes this line it first checks whether the loc is NULL or not and then checks it's priority. Thus it saves from SEGMENTATION FAULT error.

Is there any better way to write this part of code ?