Dequeue Operation in DSA C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a dequeue in data structures?

A dequeue (double-ended queue) is a data structure where elements can be added or removed from both the front and the rear ends.

beginner

Name the two main operations to remove elements in a dequeue.

The two main removal operations are dequeue from front and dequeue from rear.

beginner

What happens if you try to dequeue from an empty dequeue?

Trying to dequeue from an empty dequeue results in an underflow condition, meaning there are no elements to remove.

intermediate

In a circular array implementation of dequeue, how do you update the front index when dequeuing from front?

You move the front index forward by one position using modulo operation: front = (front + 1) % size_of_array.

intermediate

Why is a dequeue useful compared to a simple queue?

A dequeue is more flexible because it allows insertion and deletion at both ends, unlike a simple queue which only allows these operations at one end.

Which operation removes an element from the rear end of a dequeue?

A Dequeue from rear

B Dequeue from front

C Enqueue at front

D Enqueue at rear

Dequeue from rear removes an element from the rear end of the dequeue.

What condition occurs when you try to dequeue from an empty dequeue?

A Overflow

B Segmentation fault

C No operation

D Underflow

Underflow happens when you try to remove an element but the dequeue is empty.

In a circular dequeue, if front = 0 and size = 5, what is the new front after dequeuing from front?

A 4

B 5

C 1

D 0

New front = (0 + 1) % 5 = 1.

Which of these is NOT a valid dequeue operation?

A Remove from middle

B Insert at front

C Insert at rear

D Remove from front

Dequeue operations only allow insertion/removal at front or rear, not from the middle.

What is the main advantage of a dequeue over a simple queue?

A Faster insertion

B Allows insertion and deletion at both ends

C Uses less memory

D Only allows insertion at rear

Dequeue allows insertion and deletion at both front and rear ends, unlike a simple queue.

Explain how the dequeue operation works when removing an element from the front in a circular array implementation.

Think about how the front pointer moves and what happens if the dequeue is empty.

Describe the difference between dequeue operations and simple queue operations.

Focus on where elements can be added or removed.

Practice

1. What is a dequeue in data structures?

easy

A. A double-ended queue allowing insertion and deletion at both ends

B. A queue where elements are inserted only at the front

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

D. A linked list with only one pointer per node

Solution

1. Step 1: Understand the definition of dequeue

   A dequeue is a data structure where elements can be added or removed from both front and rear ends.

2. Step 2: Compare options with definition

   A double-ended queue allowing insertion and deletion at both ends correctly describes a dequeue. Options B, C, and D describe other data structures.

3. Final Answer:

   A double-ended queue allowing insertion and deletion at both ends -> Option A

4. Quick Check:

   Dequeue = double-ended queue [OK]

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

Common Mistakes:

• Confusing dequeue with stack or simple queue
• Thinking insertion is only at one end
• Mixing dequeue with linked list structure

2. Which of the following is the correct syntax to insert an element 10 at the front of a dequeue named dq in C?

easy

A. dq.insertFront(10);

B. insertFront(dq, 10);

C. dq->insertFront(10);

D. insertFront(10, dq);

Solution

1. Step 1: Identify typical C function call syntax

   In C, functions are called with function_name(arguments). Here, insertFront is a function taking dequeue and value.

2. Step 2: Match options with C syntax

   insertFront(dq, 10); matches C style: insertFront(dq, 10);. Options B and C use object syntax (C++ style). insertFront(10, dq); reverses argument order.

3. Final Answer:

   insertFront(dq, 10); -> Option B

4. Quick Check:

   C function call = insertFront(dq, 10) [OK]

Hint: C uses function_name(dequeue, value) syntax [OK]

Common Mistakes:

• Using object method syntax in C
• Swapping argument order
• Using arrow operator without pointer

3. Consider the following C code for dequeue operations:

initialize dequeue dq
insertRear(dq, 5);
insertFront(dq, 3);
insertRear(dq, 7);
deleteFront(dq);
insertFront(dq, 2);
deleteRear(dq);

What is the final state of the dequeue (front to rear)?

medium

A. 2 -> 3 -> null

B. 3 -> 5 -> 7 -> null

C. 2 -> 5 -> null

D. 5 -> 7 -> null

Solution

1. Step 1: Trace each operation step-by-step

   Start empty: [] insertRear 5: [5] insertFront 3: [3,5] insertRear 7: [3,5,7] deleteFront: remove 3 -> [5,7] insertFront 2: [2,5,7] deleteRear: remove 7 -> [2,5]

2. Step 2: Write final dequeue state front to rear

   Final state is 2 at front, then 5, then null.

3. Final Answer:

   2 -> 5 -> null -> Option C

4. Quick Check:

   Trace ops carefully = 2 -> 5 [OK]

Hint: Track each insert/delete step in order [OK]

Common Mistakes:

• Forgetting which end deleteFront/deleteRear affects
• Mixing order of insertFront and insertRear
• Skipping an operation

4. The following code snippet for dequeue operations has a bug. What is the error?

initialize dequeue dq
insertFront(dq, 10);
deleteRear(dq);
deleteRear(dq);

medium

A. Deleting from rear when dequeue is empty causes underflow

B. insertFront syntax is incorrect

C. Cannot deleteRear twice in a row

D. Dequeue must be initialized with size

Solution

1. Step 1: Analyze operations and dequeue state

   Start empty: [] insertFront 10: [10] deleteRear: removes 10 -> [] deleteRear: tries to remove from empty dequeue

2. Step 2: Identify error from empty dequeue deletion

   Deleting from empty dequeue causes underflow error. This is the bug.

3. Final Answer:

   Deleting from rear when dequeue is empty causes underflow -> Option A

4. Quick Check:

   Deleting empty dequeue = underflow error [OK]

Hint: Check if dequeue is empty before delete [OK]

Common Mistakes:

• Ignoring empty dequeue before delete
• Assuming multiple deletes always safe
• Confusing insert syntax with error

5. You have a dequeue initially empty. You perform these operations:

insertRear(dq, 1);
insertRear(dq, 2);
insertFront(dq, 3);
deleteRear(dq);
insertFront(dq, 4);
deleteFront(dq);
insertRear(dq, 5);

What is the final state of the dequeue from front to rear?

hard

A. 4 -> 3 -> 1 -> null

B. 1 -> 5 -> null

C. 4 -> 1 -> 5 -> null

D. 3 -> 1 -> 5 -> null

Solution

1. Step 1: Execute each operation stepwise

   Start: [] insertRear 1: [1] insertRear 2: [1,2] insertFront 3: [3,1,2] deleteRear: remove 2 -> [3,1] insertFront 4: [4,3,1] deleteFront: remove 4 -> [3,1] insertRear 5: [3,1,5]

2. Step 2: Write final dequeue state front to rear

   Final state is 3 at front, then 1, then 5, then null.

3. Final Answer:

   3 -> 1 -> 5 -> null -> Option D

4. Quick Check:

   Trace carefully = 3 -> 1 -> 5 [OK]

Hint: Write state after each operation [OK]

Common Mistakes:

• Deleting wrong end element
• Mixing insertFront and insertRear order
• Skipping delete operations