Dequeue Operation in DSA Javascript - Build from Scratch

Dequeue Operation

📖 Scenario: Imagine you are managing a line of customers waiting for service. Sometimes customers join the line at the front or the back, and sometimes customers leave from the front or the back. This line is called a deque (double-ended queue).

🎯 Goal: You will create a deque using a JavaScript array and perform operations to add and remove customers from both ends. Finally, you will print the current state of the deque.

📋 What You'll Learn

Create a deque array with initial customers

Add a customer to the front of the deque

Add a customer to the back of the deque

Remove a customer from the front of the deque

Remove a customer from the back of the deque

Print the final state of the deque

💡 Why This Matters

🌍 Real World

Deques are useful in real life for managing lines where people can join or leave from both ends, like waiting lines or task scheduling.

💼 Career

Understanding deque operations helps in software development tasks like designing efficient data structures, managing buffers, and implementing algorithms that require double-ended access.

1

Create the initial deque

Create an array called deque with these exact customers in order: 'Alice', 'Bob', 'Charlie'

DSA Javascript

// Create the deque array with 'Alice', 'Bob', 'Charlie'
// Your code here

Hint

Use square brackets [] to create an array and separate names with commas.

2

Add customers to front and back

Add 'Diana' to the front of deque using unshift() and add 'Evan' to the back of deque using push()

DSA Javascript

const deque = ['Alice', 'Bob', 'Charlie'];
// Add 'Diana' to the front and 'Evan' to the back
// Your code here

Hint

Use unshift() to add at the front and push() to add at the back.

3

Remove customers from front and back

Remove one customer from the front of deque using shift() and one customer from the back of deque using pop()

DSA Javascript

const deque = ['Alice', 'Bob', 'Charlie'];
deque.unshift('Diana');
deque.push('Evan');
// Remove one customer from front and one from back
// Your code here

Hint

Use shift() to remove from the front and pop() to remove from the back.

4

Print the final deque

Print the deque array to show the current customers in order

DSA Javascript

const deque = ['Alice', 'Bob', 'Charlie'];
deque.unshift('Diana');
deque.push('Evan');
deque.shift();
deque.pop();
// Print the deque array
// Your code here

Hint

Use console.log(deque) to print the array.

Practice

1. What is a dequeue in data structures?

easy

A. A stack that allows only last-in, first-out operations

B. A queue where elements can only be added at the front

C. A list that only allows insertion at the middle

D. A double-ended queue where elements can be added or removed from both ends

Solution

1. Step 1: Understand the definition of dequeue

   A dequeue is a data structure that allows insertion and deletion from both front and rear ends.

2. Step 2: Compare options with definition

   A double-ended queue where elements can be added or removed from both ends correctly describes a dequeue. Options A, B, and D describe other structures or incorrect behaviors.

3. Final Answer:

   A double-ended queue where elements can be added or removed from both ends -> Option D

4. Quick Check:

   Dequeue = Double-ended queue [OK]

Hint: Dequeue means double-ended queue, both ends allowed [OK]

Common Mistakes:

• Confusing dequeue with stack or normal queue
• Thinking elements can only be added at one end
• Assuming dequeue is a type of list

2. Which of the following is the correct syntax to add an element 5 at the front of a dequeue named dq in JavaScript using an array?

easy

A. dq.pushFront(5);

B. dq.unshift(5);

C. dq.addFront(5);

D. dq.insertFront(5);

Solution

1. Step 1: Recall JavaScript array methods for front insertion

   JavaScript arrays use unshift() to add elements at the front.

2. Step 2: Match method with options

   Only dq.unshift(5); uses unshift(5), which is correct. Other options are invalid methods.

3. Final Answer:

   dq.unshift(5); -> Option B

4. Quick Check:

   Array front insert = unshift() [OK]

Hint: Use unshift() to add at front in JS arrays [OK]

Common Mistakes:

• Using pushFront which is not a JS array method
• Confusing push (adds at end) with front insertion
• Assuming addFront or insertFront exist in JS arrays

3. What will be the output of the following code?

let dq = [];
dq.push(10);
dq.unshift(20);
dq.push(30);
dq.shift();
console.log(dq);

medium

A. [10, 30]

B. [20, 10, 30]

C. [20, 30]

D. [10, 20, 30]

Solution

1. Step 1: Trace each operation on the dequeue array

   Start with empty dq = []. push(10) adds 10 at end: [10]. unshift(20) adds 20 at front: [20, 10]. push(30) adds 30 at end: [20, 10, 30]. shift() removes front element (20): [10, 30].

2. Step 2: Determine final state of dq

   After all operations, dq is [10, 30].

3. Final Answer:

   [10, 30] -> Option A

4. Quick Check:

   After shift(), front removed, dq = [10, 30] [OK]

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

Common Mistakes:

• Confusing shift() with pop()
• Mixing order of unshift and push
• Forgetting shift() removes front element

4. Identify the error in the following dequeue operation code:

let dq = [];
dq.push(1);
dq.popFront();
console.log(dq);

medium

A. popFront() is not a valid JavaScript array method

B. push() cannot be used on empty arrays

C. popFront() should be pop() to remove last element

D. dq should be initialized as a Dequeue object, not array

Solution

1. Step 1: Check JavaScript array methods used

   JavaScript arrays do not have a method named popFront(). The correct method to remove front element is shift().

2. Step 2: Identify the error

   Using popFront() causes a runtime error as it is undefined.

3. Final Answer:

   popFront() is not a valid JavaScript array method -> Option A

4. Quick Check:

   popFront() invalid in JS arrays [OK]

Hint: Use shift() to remove front, pop() for end [OK]

Common Mistakes:

• Assuming popFront() exists like pop()
• Confusing pop() with front removal
• Not knowing shift() removes front element

5. You have a dequeue implemented as a JavaScript array dq with elements [5, 10, 15, 20]. You want to remove the element 5 from the front and add 25 at the rear, then remove the element 20 from the rear and add 30 at the front. What will be the final state of dq?

hard

A. [30, 5, 10, 15, 25]

B. [10, 15, 25, 30]

C. [30, 10, 15, 25]

D. [10, 15, 20, 25, 30]

Solution

1. Step 1: Remove front element

   Start dq = [5, 10, 15, 20]. Remove front element 5 using shift(): dq = [10, 15, 20]. Rear is now 20.

2. Step 2: Remove rear, add rear, add front

   Remove rear element 20 using pop(): dq = [10, 15]. Add 25 at rear using push(): dq = [10, 15, 25]. Add 30 at front using unshift(): dq = [30, 10, 15, 25].

3. Final Answer:

   [30, 10, 15, 25] -> Option C

4. Quick Check:

   Front removed 5, rear removed 20, added 30 front, 25 rear [OK]

Hint: Use shift/pop for removal, unshift/push for addition [OK]

Common Mistakes:

• Removing wrong elements from front or rear
• Mixing order of operations
• Using pop() instead of shift() for front removal