Queue Implementation Using Array in DSA Typescript - Build from Scratch

Queue Implementation Using Array

📖 Scenario: Imagine you are managing a line of customers waiting to buy tickets at a movie theater. You want to keep track of who is next in line and who has already been served.

🎯 Goal: You will build a simple queue using an array in TypeScript. You will add customers to the line, remove them when they buy tickets, and see the current state of the line.

📋 What You'll Learn

Create an array to represent the queue

Add a variable to track the maximum size of the queue

Write code to add customers to the queue (enqueue)

Write code to remove customers from the queue (dequeue)

Print the queue after operations to show the current line

💡 Why This Matters

🌍 Real World

Queues are used in many places like lines at stores, print job management, and task scheduling.

💼 Career

Understanding queues helps in software development roles that involve managing ordered tasks or requests.

1

Create the queue array

Create an empty array called queue to hold the customers waiting in line.

DSA Typescript

# Your code here
// Create an empty array called queue

Hint

Use const queue: string[] = []; to create an empty array for strings.

2

Set the maximum queue size

Create a variable called maxSize and set it to 5 to limit how many customers can wait in line.

DSA Typescript

const queue: string[] = [];
// Create a variable maxSize and set it to 5

Hint

Use const maxSize: number = 5; to set the queue limit.

3

Add customers to the queue

Write code to add these customers to the queue array in order: 'Alice', 'Bob', 'Charlie'. Use the push method.

DSA Typescript

const queue: string[] = [];
const maxSize: number = 5;
// Add 'Alice', 'Bob', and 'Charlie' to the queue using push

Hint

Use queue.push('Name'); to add each customer.

4

Remove a customer and print the queue

Remove the first customer from the queue using shift(). Then print the current state of the queue using console.log(queue).

DSA Typescript

const queue: string[] = [];
const maxSize: number = 5;
queue.push('Alice');
queue.push('Bob');
queue.push('Charlie');
// Remove the first customer using shift
// Print the queue

Hint

Use queue.shift(); to remove the first customer and console.log(queue); to print the queue.

Practice

1. What is the main characteristic of a queue data structure implemented using an array?

easy

A. It stores elements in sorted order automatically

B. It follows Last-In-First-Out (LIFO) order

C. It allows random access to elements

D. It follows First-In-First-Out (FIFO) order

Solution

1. Step 1: Understand queue behavior

   A queue adds elements at the rear and removes from the front, so the first element added is the first removed.

2. Step 2: Compare with options

   FIFO means First-In-First-Out, which matches queue behavior. LIFO is for stacks, random access is for arrays, and sorting is unrelated.

3. Final Answer:

   It follows First-In-First-Out (FIFO) order -> Option D

4. Quick Check:

   Queue = FIFO [OK]

Hint: Queue always removes the oldest element first [OK]

Common Mistakes:

• Confusing queue with stack (LIFO)
• Thinking queue allows random access
• Assuming queue sorts elements automatically

2. Which of the following is the correct way to add an element `x` to the rear of a queue implemented as an array named `queue` in TypeScript?

easy

A. queue.unshift(x);

B. queue.push(x);

C. queue.shift(x);

D. queue.pop(x);

Solution

1. Step 1: Identify array methods for queue operations

   To add at the rear of the queue, we use the array method that appends elements at the end, which is push().

2. Step 2: Check each option

   push(x) adds x at the end; pop() removes from end; shift() removes from front; unshift() adds at front.

3. Final Answer:

   queue.push(x); -> Option B

4. Quick Check:

   Enqueue = push() [OK]

Hint: Use push() to add at the end of array [OK]

Common Mistakes:

• Using pop() which removes instead of adds
• Using shift() or unshift() incorrectly for enqueue
• Confusing push() with unshift()

3. What will be the output of the following TypeScript code implementing a queue using an array?

const queue: number[] = [];
queue.push(10);
queue.push(20);
const removed = queue.shift();
queue.push(30);
console.log(queue);

medium

A. [20, 30]

B. [10, 20, 30]

C. [30, 20]

D. [10, 30]

Solution

1. Step 1: Trace queue operations step-by-step

   Initially queue = []. After push(10), queue = [10]. After push(20), queue = [10, 20]. shift() removes first element 10, queue = [20]. Then push(30), queue = [20, 30].

2. Step 2: Determine final queue state

   After all operations, queue contains [20, 30].

3. Final Answer:

   [20, 30] -> Option A

4. Quick Check:

   After shift(), first element removed, queue = [20, 30] [OK]

Hint: shift() removes front, push() adds rear [OK]

Common Mistakes:

• Forgetting shift() removes first element
• Assuming push() adds at front
• Confusing order of elements after operations

4. Identify the error in the following TypeScript code for queue implementation using an array:

const queue: number[] = [];
queue.push(1);
queue.push(2);
const removed = queue.pop();
console.log(queue);

medium

A. push() should be replaced with unshift()

B. queue should be declared as a linked list

C. Using pop() removes from rear, not front

D. No error, code works correctly

Solution

1. Step 1: Understand queue removal operation

   Queue removes elements from the front (FIFO). Using pop() removes from the rear (last element), which breaks FIFO.

2. Step 2: Check code behavior

   pop() removes last element (2), but queue should remove first element (1) using shift().

3. Final Answer:

   Using pop() removes from rear, not front -> Option C

4. Quick Check:

   Dequeue = shift(), not pop() [OK]

Hint: Use shift() to remove front element, not pop() [OK]

Common Mistakes:

• Using pop() instead of shift() for dequeue
• Thinking push() adds at front
• Confusing array methods for queue operations

5. You want to implement a queue using a fixed-size array of length 3 in TypeScript. After enqueueing 3 elements, you dequeue one element and then enqueue another. What problem will you face if you do not use a circular queue approach?

hard

A. You cannot enqueue the new element because the rear index reached the array end

B. The queue will automatically resize to fit new elements

C. Dequeue operation will remove the wrong element

D. The queue will lose all previous elements

Solution

1. Step 1: Understand fixed-size array queue behavior

   With a fixed-size array, rear index moves forward on enqueue. After 3 enqueues, rear is at end.

2. Step 2: Analyze enqueue after dequeue without circular logic

   Dequeue moves front forward but rear stays at end. Without circular wrap, rear cannot move back to start, so enqueue fails.

3. Final Answer:

   You cannot enqueue the new element because the rear index reached the array end -> Option A

4. Quick Check:

   Fixed array + no circular = enqueue blocked at end [OK]

Hint: Fixed array needs circular logic to reuse space [OK]

Common Mistakes:

• Assuming array resizes automatically
• Thinking dequeue affects rear index
• Ignoring circular queue necessity