Queue operations (enqueue, dequeue) in Data Structures Theory - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a queue in data structures?

A queue is a collection where elements are added at one end and removed from the other, following the First-In-First-Out (FIFO) principle.

beginner

What does the enqueue operation do in a queue?

Enqueue adds an element to the back (end) of the queue.

beginner

What does the dequeue operation do in a queue?

Dequeue removes and returns the element at the front (start) of the queue.

beginner

Why is the queue called FIFO?

Because the first element added is the first one to be removed, like a line where the first person in line is served first.

intermediate

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

It usually causes an error or returns a special value indicating the queue is empty, because there is nothing to remove.

Which end of the queue does enqueue add an element to?

A Back (end)

B Front (start)

C Middle

D Random position

Enqueue always adds elements to the back of the queue.

What principle does a queue follow?

A Last-In-First-Out (LIFO)

B Random order

C First-In-First-Out (FIFO)

D Priority order

Queues follow FIFO, meaning the first element added is the first removed.

What does dequeue do?

A Checks if the queue is empty

B Removes the last element

C Adds an element to the queue

D Removes the front element

Dequeue removes the element at the front of the queue.

If a queue is empty, what happens when you try to dequeue?

A Adds a new element

B Returns an error or special value

C Removes a random element

D Nothing happens

Dequeue on an empty queue usually causes an error or returns a special value indicating emptiness.

Which real-life example best represents a queue?

A People waiting in line at a store

B A stack of plates

C Books on a shelf

D Randomly shuffled cards

People waiting in line is a classic example of a queue following FIFO.

Explain the enqueue and dequeue operations in a queue with a real-life example.

Think about how people join and leave a line.

What happens if you try to dequeue from an empty queue and why is it important to handle this case?

Consider what happens when you try to take something from an empty container.

Practice

1. What does the enqueue operation do in a queue?

easy

A. Adds an item to the rear of the queue

B. Removes an item from the front of the queue

C. Checks if the queue is empty

D. Returns the size of the queue

Solution

1. Step 1: Understand enqueue operation

   Enqueue means to add an element to the queue.

2. Step 2: Identify where enqueue adds

   In a queue, new items are added at the rear (end).

3. Final Answer:

   Adds an item to the rear of the queue -> Option A

4. Quick Check:

   Enqueue = Add rear [OK]

Hint: Enqueue always adds at the back of the queue [OK]

Common Mistakes:

• Confusing enqueue with dequeue
• Thinking enqueue removes items
• Assuming enqueue adds at the front

2. Which of the following is the correct syntax to remove an element from a queue in most programming languages?

easy

A. queue.dequeue()

B. queue.enqueue()

C. queue.pop()

D. queue.push()

Solution

1. Step 1: Identify the operation to remove from queue

   Removing an element from a queue is called dequeue.

2. Step 2: Match correct method name

   The standard method to remove is dequeue(), not pop() or push().

3. Final Answer:

   queue.dequeue() -> Option A

4. Quick Check:

   Remove from front = dequeue() [OK]

Hint: Dequeue removes from front, enqueue adds at rear [OK]

Common Mistakes:

• Using pop() which is for stacks
• Confusing enqueue and dequeue methods
• Using push() which adds to stack or queue

3. Consider a queue initially empty. After these operations:
enqueue(5), enqueue(10), dequeue(), enqueue(15), dequeue()
What is the next element to be dequeued?

medium

A. 5

B. 10

C. Queue is empty

D. 15

Solution

1. Step 1: Track queue after each operation

   Start empty: []
   enqueue(5): [5]
   enqueue(10): [5, 10]
   dequeue(): removes 5, queue is [10]
   enqueue(15): [10, 15]
   dequeue(): removes 10, queue is [15]

2. Step 2: Identify next element to dequeue

   Next element at front is 15.

3. Final Answer:

   15 -> Option D

4. Quick Check:

   Remaining front element = 15 [OK]

Hint: Follow FIFO order step-by-step [OK]

Common Mistakes:

• Forgetting to remove elements on dequeue
• Mixing order of enqueue and dequeue
• Assuming queue empties after two dequeues

4. What is wrong with this queue operation sequence?

queue = []
queue.dequeue()

medium

A. Nothing is wrong; it works fine

B. Dequeue on empty queue causes error

C. Queue should be a stack for dequeue

D. Enqueue is missing before dequeue

Solution

1. Step 1: Understand initial queue state

   The queue is empty: [].

2. Step 2: Analyze dequeue on empty queue

   Trying to dequeue when empty causes underflow or error because no element exists to remove.

3. Final Answer:

   Dequeue on empty queue causes error -> Option B

4. Quick Check:

   Empty dequeue = error [OK]

Hint: Never dequeue from an empty queue [OK]

Common Mistakes:

• Assuming dequeue returns null or zero
• Confusing queue with stack behavior
• Ignoring empty state checks

5. You have a queue with elements [2, 4, 6, 8]. You perform these operations:
dequeue(), enqueue(10), dequeue(), enqueue(12), dequeue()
What is the state of the queue after these operations?

hard

A. [4, 6, 8, 10, 12]

B. [6, 8, 10, 12]

C. [8, 10, 12]

D. [10, 12]

Solution

1. Step 1: Track queue after each operation

   Start: [2, 4, 6, 8]
   dequeue(): removes 2 -> [4, 6, 8]
   enqueue(10): add 10 -> [4, 6, 8, 10]
   dequeue(): removes 4 -> [6, 8, 10]
   enqueue(12): add 12 -> [6, 8, 10, 12]
   dequeue(): removes 6 -> [8, 10, 12]

2. Step 2: Confirm final queue state

   The queue now contains [8, 10, 12].

3. Final Answer:

   [8, 10, 12] -> Option C

4. Quick Check:

   Final queue after operations = [8, 10, 12] [OK]

Hint: Remove front, add rear stepwise [OK]

Common Mistakes:

• Not removing correct elements on dequeue
• Adding new elements at front instead of rear
• Losing track of queue order