priority queue sort [C++]

Question

I have an assignment due and it requires me to add a sorting method to a priority-queue that I did for my last assignment.

Well, I did add the sort function, but I'm not sure how to complete my main with it since it seems like main already sorts the queue. I do have an example of what main could look like but I believe the professor wants us to work with what we submitted before so I am stuck on what to do. Please help.

Oh, and I did get a point taken away for my last assignment and all he left for a comment was "Segmentation fault while making calls to enqueue()". I sent him an email what that means, he did not reply back (gave out grades yesterday). I think the error comes from not calling malloc on line 90.

#include <stdlib.h> //NEEDED TO RUN calloc
#include <cstdio> // NEEDED TO RUN perror
#include <iostream>

using namespace std;

// c-style struct
// node used to implement linked list
typedef struct _node
{
    int value;
    struct _node *next;
}
node;

/* Standard Stack LIFO ADT */
class Priority_Queue
{

    /* tracks number of nodes in Priority_Queue */
    int nNodes;
    /* keeps track of first node in Priority_Queue */
    node *head;
    void     sort( void ); //Sort linked list using selection sorting algorithm

    //anything above public is private
    public:

        Priority_Queue( void ); //find a constructor everytime the user             extanjiates Priority_Queue object.

        void    enqueue( int );  //insert node/ enqueue on an integer.
        void    dequeue( void );  //remove node/ dequeue off node from the top.
        bool    empty( void );  //test whether Priority_Queue is empty
        int     size( void );  //return size
        int     top( void );  // access top of the Priority_Queue/ next node.
        int     back( void );
};

/* init head and nNodes */
Priority_Queue::Priority_Queue( void )
{
    head = NULL; //everytime a user enstantiates one of the Priority_Queues
    nNodes = 0; // so we know how many nodes are sitting on our Priority_Queue.
}

/* place new node at top of Priority_Queue */
void Priority_Queue::enqueue( int value ) //enqueue on a value/ user wants     to place a Priority_Queue
{

    if( head == NULL )
    {
        if(( head = (node*)calloc(1, sizeof(node))) == NULL )
        {
            perror( "Could not calloc memory" );
        }
        else
        {
            head->value = value;
            nNodes++; // increase the number of nodes in link list
        }
    }
    else
    {
        node *temp;

        if( ( temp = (node*)malloc( sizeof( node ) ) ) == NULL ) //malloc     because both values for stut will be set immediatly
        {
            perror( "Could not malloc memory" );
        }
        else
        {
            temp->value = value;  //
            node* p = head;

            if ( value<head->value )
            {
                temp -> next = head;
                head = temp;
                temp = NULL;
            }
            while (p->next != NULL && p->next->value<value )
            {
                p = p-> next; // p is pointintint to the next node
            }
            temp->next = p->next;
            p->next = temp;
            temp = NULL;
            nNodes++; // increase the number of nodes in link list
        }
        //malloc( sizeof (node));
    }

}

/* remove the first node from top of Priority_Queue */
void Priority_Queue::dequeue( void )
{
    node *temp;

    if( head != NULL ) // allow users to keep calling dequeue even if it is empty
    {
        temp = head;
        head = head->next; // head is now pointing to 3

        free( temp );
        temp = NULL;

        nNodes--; //decrease node count for the Priority_Queue
    }

}

/* return true if Priority_Queue is empty */
bool Priority_Queue::empty( void )
{
    return ( head == NULL ); //valuated as true or false.
}

/* return number of nodes in Priority_Queue */
int Priority_Queue::size( void )
{
    return nNodes; //PLACING RESPONSIBILITY ON USER TO REMEMBER 28:00 part1
}

/* return value of node at top of Priority_Queue */
int Priority_Queue::top( void )
{
    return head->value; // PLACING RESPONSIBILITY ON USER TO REMEMBER 28:00 part1
}

int Priority_Queue::back( void )
{
    node *p = head;
    while (p->next != NULL)
    {
        p = p->next;
    }
    return p->value; //
}

void Priority_Queue::sort( void )
{
        node *temp;
    bool onward = true; //while loop set to true
    //compare each pair of adjacent elements
    //switches elements if in wrong order
    //Continues operations until elements are sorted.
    while( onward )
    {
        // onward set to false so just in case it is sorted, it will loop through the elements.
        for( temp = head, onward = false; temp->next != NULL; temp = temp->next )
        {
            //Check to see if current element is greater than the next
            if( temp->value > temp->next->value )
            {
                //swapping current elements value
                int a = temp->value;
                temp->value = temp->next->value;
                temp->next->value = a;
                onward = true; //
            }
        }
    }
}

int main( void )
{
    /* instantiate new Priority_Queue */
    Priority_Queue my_Priority_Queue = Priority_Queue();
    int i = 0;
    /* enqueue some values onto Priority_Queue */
    for( i = 0; i < 10; i++ )
        my_Priority_Queue.enqueue( i );
    /* print values stored in Priority_Queue until Priority_Queue is empty */
    while( !my_Priority_Queue.empty() )
    {
        cout << "Value: " << my_Priority_Queue.top() << endl; // Give access to the top of the Priority_Queue.
        my_Priority_Queue.dequeue();    // dequeue the top off so to see a different node at the top in the next line.
        cout << "Node Count: " << my_Priority_Queue.size() << endl << endl;
    }
    my_Priority_Queue.dequeue();

    return 0;
}

Answer 1

A priority queue will normally have a function to get the highest priority element.

To sort all the elements, all you need to do is pop the elements one by one, and they will be in sorted order.

Normally, a priority queue is implemented as a heap, and a sorting algorithm using a heap as a priority queue is called heapsort.

Since you haven't given your priority queue code, using the std::priority_queue as an example, here is how you would get a sorted list of elements:

template<class T, class Container, class Compare>
std::vector<T> get_sorted(std::priority_queue<T, Container, Compare> pq)
{
  std::vector<T> sorted;
  sorted.reserve(pq.size());
  while (!pq.empty())
  {
    sorted.push_back(pq.top());
    pq.pop();
  }
  return sorted;
}