• Top - is a node with the highest priority within the queue.

Priority Queue is a unique variation of the Queue data structure, as it will sort the stored data by default. Data stored within a Priority Queue will be sorted based on their priority, which means the first data to come out is the one with the highest priority, regardless of when they got inserted. Knowing this, we can utilize Priority Queues to solve problems where different priorities exist.

"pqueue" might be used to shorten "Priority Queue" for the duration of this module.

• isEmpty - to check whether the pqueue is empty or not.
• push - to insert new data.
• pop - to remove data with the highest priority.
• top - to get the data with the highest priority/located at the top.

Pqeues could be used to solved the following problems:

1. CPU program scheduling.
2. Implementation of Dijkstra Shortest Path and Prim's Minimum Spanning Tree algorithms.
3. Problems where the data have different priority levels.

A complete implementation of Priority Queue can be accessed here.

In this module, the Min-Priority Queue will be implemented, where nodes with the smaller numbers will be prioritized.

The simplest way to implement such pqueue is by modifying the Singly Linked List data structure, where the top pointer is used to refer to the node with the highest priority.

Sidenote: Implementing a Priority Queue data structure by using Linked List is not the most efficient implementation. One way to achieve higher efficiency is by implementing it with the Heap Tree data structure.

• Node Representation

  Nodes of a pqueue can be represented by a struct named pqueueNode that stores an int variable and a pointer to refer to the next node.

  typedef struct pqueueNode_t {
      int data;
      struct pqueueNode_t *next;
  } PQueueNode;

• Pqueue Structure

  A pqueue has one pointer top to refer the node with the highest priority within its structure.

  typedef struct pqueue_t {
      PQueueNode *_top;
      unsigned _size;
  } PriorityQueue;

• isEmpty

  To check whether a pqueue is empty or not, simply check whether its top refers to NULL or not.

  bool pqueue_isEmpty(PriorityQueue *pqueue) {
      return (pqueue->_top == NULL);
  }

• push

  To do a push, kindly follow these steps:

  • store the current top node within a temporary variable
  • make a new node.
  • if the pqueue is empty, make the new node as the top.
  • if it is not empty, there can be two cases.

  First case, the new node contains smaller data than the one stored at the top (higher priority)

  • make the next of the new node refer to the current top.
  • make the top refer to the new node.

  

  Second case, the new node contains bigger data than the one stored at the top (lower priority)

  • iterate through the queue until the data within new node isn't bigger.
  • or iterate until the end of pqeueu, where next is NULL.
  • make the next of the new node refer to the next of the temp node.
  • make the next of the temp node to the new node.

  

  void pqueue_push(PriorityQueue *pqueue, int value)
  {
      PQueueNode *temp = pqueue->_top;
      PQueueNode *newNode = \
          (PQueueNode*) malloc (sizeof(PQueueNode));
      newNode->data = value;
      newNode->next = NULL;
  
      if (pqueue_isEmpty(pqueue)) {
          pqueue->_top = newNode;
          return;
      }
  
      if (value < pqueue->_top->data) {
          newNode->next = pqueue->_top;
          pqueue->_top = newNode;
      }
      else {
          while ( temp->next != NULL && 
                  temp->next->data < value)
              temp = temp->next;
          newNode->next = temp->next;
          temp->next = newNode;
      }
  }

• pop

  To do the pop, follow these steps:

  • make sure that the pqueue is not empty.
  • make a temp (temporary) node that points to the top node.
  • make the top to point to the current top's next node.
  • remove the temp node.

  void pqueue_pop(PriorityQueue *pqueue)
  {
      if (!pqueue_isEmpty(pqueue)) {
          PQueueNode *temp = pqueue->_top;
          pqueue->_top = pqueue->_top->next;
          free(temp);
      }
  }

• top

  int pqueue_top(PriorityQueue *pqueue) {
      if (!pqueue_isEmpty(pqueue))
          return pqueue->_top->data;
      else return 0;
  }