Dequeue Operation in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to remove an element from the front of the dequeue.

DSA Javascript

function dequeueFront(queue) {
  if (queue.length === 0) return null;
  return queue.[1]();
}

Drag options to blanks, or click blank then click option'

Apush

Bpop

Cshift

Dunshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() removes from the end, not the front.

Using push() or unshift() adds elements instead of removing.

The shift() method removes the first element from an array, which matches removing from the front of a dequeue.

2fill in blank

medium

Complete the code to remove an element from the rear of the dequeue.

DSA Javascript

function dequeueRear(queue) {
  if (queue.length === 0) return null;
  return queue.[1]();
}

Drag options to blanks, or click blank then click option'

Apop

Bunshift

Cshift

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using shift() removes from the front, not the rear.

Using push() or unshift() adds elements instead of removing.

The pop() method removes the last element from an array, which matches removing from the rear of a dequeue.

3fill in blank

hard

Fix the error in the dequeueFront function to correctly remove from the front.

DSA Javascript

function dequeueFront(queue) {
  if (queue.length === 0) return null;
  return queue.[1]();
}

Drag options to blanks, or click blank then click option'

Aunshift

Bpop

Cpush

Dshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() instead of shift().

Trying to use push() or unshift() which add elements.

shift() correctly removes the first element; pop() removes the last, which is incorrect here.

4fill in blank

hard

Fill both blanks to remove from front and rear correctly.

DSA Javascript

function removeFromDequeue(queue, fromFront) {
  if (queue.length === 0) return null;
  if (fromFront) {
    return queue.[1]();
  } else {
    return queue.[2]();
  }
}

Drag options to blanks, or click blank then click option'

Ashift

Bpop

Cpush

Dunshift

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping shift() and pop() usage.

Using push() or unshift() which add elements.

shift() removes from front, pop() removes from rear, matching the function's logic.

5fill in blank

hard

Fill both blanks to create a function that removes from front or rear and returns the updated queue.

DSA Javascript

function dequeueOperation(queue, removeFront) {
  if (queue.length === 0) return queue;
  if (removeFront) {
    queue.[1]();
  } else {
    queue.[2]();
  }
  return queue;
}

Drag options to blanks, or click blank then click option'

Ashift

Bpop

C;

Attempts:

3 left

💡 Hint

Common Mistakes

Adding a semicolon after return queue causing syntax issues.

Using push() or unshift() instead of shift() or pop().

shift() removes front, pop() removes rear, and returning queue without extra semicolon is correct syntax.

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