Priority Queue Introduction and Concept in DSA Javascript - Build from Scratch

Priority Queue Introduction and Concept

📖 Scenario: Imagine you are managing a hospital emergency room. Patients arrive with different levels of urgency. You want to help the most urgent patients first. This is like a priority queue where each patient has a priority number.

🎯 Goal: You will create a simple priority queue using a JavaScript array. You will add patients with their urgency levels, then remove the most urgent patient first.

📋 What You'll Learn

Create an array called patients to hold patient objects with name and priority.

Create a variable called newPatient to hold a new patient object.

Write a function called addPatient that adds newPatient to patients and sorts by priority (lowest number means highest urgency).

Write a function called treatPatient that removes and returns the patient with the highest priority (lowest number).

Print the state of patients after adding and treating patients.

💡 Why This Matters

🌍 Real World

Priority queues are used in hospitals, task schedulers, and anywhere urgent tasks must be handled first.

💼 Career

Understanding priority queues helps in software development roles involving scheduling, resource management, and real-time systems.

1

Create the initial patient list

Create an array called patients with these exact patient objects: {name: 'Alice', priority: 4}, {name: 'Bob', priority: 2}, and {name: 'Charlie', priority: 5}.

DSA Javascript

// Create the patients array with the given patient objects
// Your code here

Hint

Use square brackets [] to create an array. Each patient is an object with name and priority.

2

Add a new patient to the queue

Create a variable called newPatient and set it to the object {name: 'Diana', priority: 1}.

DSA Javascript

const patients = [
  {name: 'Alice', priority: 4},
  {name: 'Bob', priority: 2},
  {name: 'Charlie', priority: 5}
];

// Create the newPatient object
// Your code here

Hint

Use const newPatient = {name: 'Diana', priority: 1}; to create the new patient object.

3

Add and sort patients by priority

Write a function called addPatient that takes no arguments, adds newPatient to patients, and sorts patients so the patient with the lowest priority number comes first.

DSA Javascript

const patients = [
  {name: 'Alice', priority: 4},
  {name: 'Bob', priority: 2},
  {name: 'Charlie', priority: 5}
];

const newPatient = {name: 'Diana', priority: 1};

// Write the addPatient function below
// Your code here

Hint

Use patients.push(newPatient) to add, then patients.sort((a, b) => a.priority - b.priority) to sort by priority.

4

Treat the most urgent patient and print the queue

Write a function called treatPatient that removes and returns the patient with the highest priority (lowest number) from patients. Then call addPatient(), call treatPatient(), and print the remaining patients array.

DSA Javascript

const patients = [
  {name: 'Alice', priority: 4},
  {name: 'Bob', priority: 2},
  {name: 'Charlie', priority: 5}
];

const newPatient = {name: 'Diana', priority: 1};

function addPatient() {
  patients.push(newPatient);
  patients.sort((a, b) => a.priority - b.priority);
}

// Write the treatPatient function below
// Call addPatient(), treatPatient(), and print patients
// Your code here

Hint

Use patients.shift() to remove the first patient (highest priority). Then print patients with console.log(patients).

Practice

1. What is the main difference between a Priority Queue and a regular queue?

easy

A. Elements are always dequeued in the order they were enqueued

B. Elements are dequeued based on priority, not just order of arrival

C. It only stores numbers, not other data types

D. It uses a stack internally to manage elements

Solution

1. Step 1: Understand queue behavior

   A regular queue removes elements in the order they arrive (FIFO).

2. Step 2: Understand priority queue behavior

   A priority queue removes elements based on their priority, not just arrival order.

3. Final Answer:

   Elements are dequeued based on priority, not just order of arrival -> Option B

4. Quick Check:

   Priority Queue = priority-based removal [OK]

Hint: Priority queue removes highest priority first, not oldest [OK]

Common Mistakes:

• Thinking priority queue works like a normal queue
• Assuming priority queue only stores numbers
• Confusing priority queue with stack

2. Which of the following is the correct way to declare a priority queue class in JavaScript?

easy

A. let PriorityQueue = () => { this.items = []; }

B. function PriorityQueue() { this.items = {}; }

C. class PriorityQueue { constructor() { this.items = []; } }

D. var PriorityQueue = new Array();

Solution

1. Step 1: Check class syntax

   class PriorityQueue { constructor() { this.items = []; } } uses correct ES6 class syntax with constructor initializing an array.

2. Step 2: Check other options

   function PriorityQueue() { this.items = {}; } uses function but initializes an object, not array; let PriorityQueue = () => { this.items = []; } uses arrow function incorrectly for class; var PriorityQueue = new Array(); assigns array directly, not a class.

3. Final Answer:

   class PriorityQueue { constructor() { this.items = []; } } -> Option C

4. Quick Check:

   Correct class syntax = class PriorityQueue { constructor() { this.items = []; } } [OK]

Hint: Use ES6 class with constructor for priority queue [OK]

Common Mistakes:

• Using arrow functions as classes
• Initializing items as object instead of array
• Assigning array directly without class

3. Consider this JavaScript code using a simple priority queue implementation:

class PriorityQueue {
  constructor() {
    this.items = [];
  }
  enqueue(element, priority) {
    this.items.push({element, priority});
    this.items.sort((a, b) => a.priority - b.priority);
  }
  dequeue() {
    return this.items.shift().element;
  }
}

const pq = new PriorityQueue();
pq.enqueue('A', 2);
pq.enqueue('B', 1);
pq.enqueue('C', 3);
console.log(pq.dequeue());

What will be printed?

medium

A. 'B'

B. 'A'

C. 'C'

D. undefined

Solution

1. Step 1: Analyze enqueue operations

   Elements are added with priorities: A(2), B(1), C(3). After each enqueue, array is sorted by priority ascending.

2. Step 2: Analyze dequeue operation

   Dequeue removes first element (lowest priority number). After sorting, order is B(1), A(2), C(3). So dequeue returns 'B'.

3. Final Answer:

   'B' -> Option A

4. Quick Check:

   Lowest priority dequeued first = 'B' [OK]

Hint: Lowest priority number dequeued first [OK]

Common Mistakes:

• Assuming FIFO order instead of priority
• Confusing priority sorting order
• Forgetting to return element, not object

4. The following code is intended to implement a priority queue, but it has a bug:

class PriorityQueue {
  constructor() {
    this.items = [];
  }
  enqueue(element, priority) {
    this.items.push({element, priority});
  }
  dequeue() {
    return this.items.shift().element;
  }
}

What is the main problem with this code?

medium

A. It stores elements as objects instead of arrays

B. It uses shift() instead of pop() to remove elements

C. It does not initialize items array in constructor

D. It does not sort items by priority before dequeue

Solution

1. Step 1: Check enqueue method

   Enqueue adds elements but does not sort by priority.

2. Step 2: Check dequeue method

   Dequeue removes first element, but without sorting, priority order is not guaranteed.

3. Final Answer:

   It does not sort items by priority before dequeue -> Option D

4. Quick Check:

   Missing sorting causes wrong dequeue order [OK]

Hint: Sort items by priority before dequeue [OK]

Common Mistakes:

• Assuming shift() removes highest priority
• Ignoring sorting requirement
• Confusing object vs array storage

5. You want to implement a priority queue that supports fast insertion and removal of the highest priority element. Which data structure is best suited for this, and why?

hard

A. Binary heap, because it provides efficient insert and remove operations

B. Linked list, because it maintains insertion order

C. Stack, because it removes the last inserted element quickly

D. Array, because it allows easy sorting after each insertion

Solution

1. Step 1: Understand operation needs

   Priority queue needs fast insertion and removal of highest priority element.

2. Step 2: Compare data structures

   Array requires sorting after insertion (slow). Linked list maintains order but slow for priority. Stack removes last inserted only. Binary heap supports insert and remove in O(log n) time efficiently.

3. Final Answer:

   Binary heap, because it provides efficient insert and remove operations -> Option A

4. Quick Check:

   Binary heap = efficient priority queue [OK]

Hint: Use binary heap for efficient priority queue operations [OK]

Common Mistakes:

• Choosing array and ignoring sorting cost
• Confusing stack with priority queue
• Ignoring time complexity of operations