Priority Queue using array in C++

Priority Queue is an extension of the Queue data structure where each element has a particular priority associated with it. It is based on the priority value, the elements from the queue are deleted.

Operations on Priority Queue:

• enqueue(): This function is used to insert new data into the queue.
• dequeue(): This function removes the element with the highest priority from the queue.
• peek()/top(): This function is used to get the highest priority element in the queue without removing it from the queue.

Approach: The idea is to create a structure to store the value and priority of the element and then create an array of that structure to store elements. Below are the functionalities that are to be implemented:

• enqueue(): It is used to insert the element at the end of the queue.
• peek():
  • Traverse across the priority queue and find the element with the highest priority and return its index.
  • In the case of multiple elements with the same priority, find the element with the highest value having the highest priority.
• dequeue():
  • Find the index with the highest priority using the peek() function let’s call that position as ind, and then shift the position of all the elements after the position ind one position to the left.
  • Decrease the size by one.

Below is the implementation of the above approach:

C++

// C++ program for the above approach

#include <bits/stdc++.h>
using namespace std;

// Structure for the elements in the
// priority queue
struct item {
    int value;
    int priority;
};

// Store the element of a priority queue
item pr[100000];

// Pointer to the last index
int size = -1;

// Function to insert a new element
// into priority queue
void enqueue(int value, int priority)
{
    // Increase the size
    size++;

    // Insert the element
    pr[size].value = value;
    pr[size].priority = priority;
}

// Function to check the top element
int peek()
{
    int highestPriority = INT_MIN;
    int ind = -1;

    // Check for the element with
    // highest priority
    for (int i = 0; i <= size; i++) {

        // If priority is same choose
        // the element with the
        // highest value
        if (highestPriority
                == pr[i].priority
            && ind > -1
            && pr[ind].value
                   < pr[i].value) {
            highestPriority = pr[i].priority;
            ind = i;
        }
        else if (highestPriority
                 < pr[i].priority) {
            highestPriority = pr[i].priority;
            ind = i;
        }
    }

    // Return position of the element
    return ind;
}

// Function to remove the element with
// the highest priority
void dequeue()
{
    // Find the position of the element
    // with highest priority
    int ind = peek();

    // Shift the element one index before
    // from the position of the element
    // with highest priority is found
    for (int i = ind; i < size; i++) {
        pr[i] = pr[i + 1];
    }

    // Decrease the size of the
    // priority queue by one
    size--;
}

// Driver Code
int main()
{
    // Function Call to insert elements
    // as per the priority
    enqueue(10, 2);
    enqueue(14, 4);
    enqueue(16, 4);
    enqueue(12, 3);

    // Stores the top element
    // at the moment
    int ind = peek();

    cout << pr[ind].value << endl;

    // Dequeue the top element
    dequeue();

    // Check the top element
    ind = peek();
    cout << pr[ind].value << endl;

      // Dequeue the top element
    dequeue();

      // Check the top element
    ind = peek();
    cout << pr[ind].value << endl;

    return 0;
}

Output

16
14
12

Complexity Analysis:

• enqueue(): O(1)
• peek(): O(N)
• dequeue: O(N)

Application of Priority Queue:

• For Scheduling Algorithms the CPU has to process certain tasks having priorities. The process of having higher priority gets executed first.
• In a time-sharing computer system, the process of waiting for the CPU time gets loaded in the priority queue.
• A Sorting-priority queue is used to sort heaps.