Priority Queue using Arrays in C Programming

//C program to Demonstrate Priority Queue
#include<stdio.h>
#include<limits.h>

#define MAX 100


// denotes where the last item in priority queue is
// initialized to -1 since no item is in queue
int idx = -1;

// pqVal holds data for each index item
// pqPriority holds priority for each index item
int pqVal[MAX];
int pqPriority[MAX];



int isEmpty ()
{
  return idx == -1;
}

int
isFull ()
{
  return idx == MAX - 1;
}

// enqueue just adds item to the end of the priority queue | O(1)
void enqueue (int data, int priority)
{
  if (!isFull ())
    {

      // Increase the index
      idx++;

      // Insert the element in priority queue
      pqVal[idx] = data;
      pqPriority[idx] = priority;
    }
}

// returns item with highest priority
// NOTE: Max Priority Queue High priority number means higher priority | O(N)
int peek ()
{
  // Note : Max Priority, so assigned min value as initial value
  int maxPriority = INT_MIN;
  int indexPos = -1;

  // Linear search for highest priority
  for (int i = 0; i <= idx; i++)
    {
      // If two items have same priority choose the one with 
      // higher data value 
      if (maxPriority == pqPriority[i] && indexPos > -1
	  && pqVal[indexPos] < pqVal[i])
	{
	  maxPriority = pqPriority[i];
	  indexPos = i;
	}

      // note: using MAX Priority so higher priority number
      // means higher priority
      else if (maxPriority < pqPriority[i])
	{
	  maxPriority = pqPriority[i];
	  indexPos = i;
	}
    }

  // Return index of the element where 
  return indexPos;
}

// This removes the element with highest priority
// from the priority queue | O(N)
void dequeue ()
{
  if (!isEmpty ())
    {
      // Get element with highest priority
      int indexPos = peek ();

      // reduce size of priority queue by first
      // shifting all elements one position left
      // from index where the highest priority item was found
      for (int i = indexPos; i < idx; i++)
	{
	  pqVal[i] = pqVal[i + 1];
	  pqPriority[i] = pqPriority[i + 1];
	}

      // reduce size of priority queue by 1
      idx--;
    }
}

void display ()
{
  for (int i = 0; i <= idx; i++)
    {
      printf ("(%d, %d)\n", pqVal[i], pqPriority[i]);
    }
}

// Driver Code
int main ()
{
  // To enqueue items as per priority
  enqueue (5, 1);
  enqueue (10, 3);
  enqueue (15, 4);
  enqueue (20, 5);
  enqueue (500, 2);

  printf ("Before Dequeue : \n");
  display ();

  // Dequeue the top element
  dequeue ();			// 20 dequeued
  dequeue ();			// 15 dequeued

  printf ("\nAfter Dequeue : \n");
  display ();

  return 0;
}