Dequeue Operation in DSA C++ - 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 C++ std::deque, add some customers, remove customers from both ends, and then print the final state of the deque.

📋 What You'll Learn

Create a std::deque<int> named customers with initial values 10, 20, 30, 40, 50

Create an integer variable remove_count and set it to 2

Remove remove_count customers from the front and from the back of the deque

Print the remaining customers in the deque separated by spaces

💡 Why This Matters

🌍 Real World

Deques are useful in real-world situations like managing lines where people can join or leave from both ends, such as ticket counters or call centers.

💼 Career

Understanding deque operations is important for software roles that involve efficient data processing, scheduling, and real-time systems.

1

Create the initial deque

Create a std::deque<int> called customers with these exact values in order: 10, 20, 30, 40, 50

DSA C++

#include 
#include 

int main() {
    // Create deque customers with 10, 20, 30, 40, 50
    // Your code here
    return 0;
}

Hint

Use std::deque<int> customers = {10, 20, 30, 40, 50}; to create the deque.

2

Add a removal count variable

Create an integer variable called remove_count and set it to 2

DSA C++

#include 
#include 

int main() {
    std::deque customers = {10, 20, 30, 40, 50};
    // Create remove_count and set to 2
    // Your code here
    return 0;
}

Hint

Use int remove_count = 2; to create the variable.

3

Remove customers from front and back

Use a for loop with variable i from 0 to less than remove_count to remove one customer from the front and one from the back of customers in each iteration

DSA C++

#include 
#include 

int main() {
    std::deque customers = {10, 20, 30, 40, 50};
    int remove_count = 2;
    // Remove remove_count customers from front and back
    // Your code here
    return 0;
}

Hint

Use pop_front() and pop_back() inside the loop to remove elements.

4

Print the remaining customers

Use a for loop with variable customer to iterate over customers and print each customer followed by a space

DSA C++

#include 
#include 

int main() {
    std::deque customers = {10, 20, 30, 40, 50};
    int remove_count = 2;
    for (int i = 0; i < remove_count; i++) {
        customers.pop_front();
        customers.pop_back();
    }
    // Print remaining customers separated by space
    // Your code here
    return 0;
}

Hint

Use a range-based for loop to print each customer followed by a space.

Practice

1. What is the main difference between a deque and a regular queue in data structures?

easy

A. Deque allows insertion and deletion from both ends, queue only from front and rear respectively

B. Deque only allows insertion at the rear, queue allows insertion at both ends

C. Deque is always sorted, queue is not

D. Deque uses arrays, queue uses linked lists

Solution

1. Step 1: Understand queue operations

   A regular queue allows insertion at the rear and deletion from the front only.

2. Step 2: Understand deque operations

   A deque (double-ended queue) allows insertion and deletion at both front and rear ends.

3. Final Answer:

   Deque allows insertion and deletion from both ends, queue only from front and rear respectively -> Option A

4. Quick Check:

   Deque = double-ended queue [OK]

Hint: Deque means double-ended, so both ends allowed [OK]

Common Mistakes:

• Thinking deque only inserts at rear
• Confusing deque with sorted data structure
• Assuming deque uses only arrays

2. Which of the following is the correct syntax to insert an element 10 at the front of a std::deque<int> named d in C++?

easy

A. d.push(10);

B. d.push_front(10);

C. d.insert_front(10);

D. d.add_front(10);

Solution

1. Step 1: Recall deque insertion methods

   In C++, std::deque uses push_front() to insert at the front.

2. Step 2: Check method names

   Only push_front(10); is valid syntax for front insertion.

3. Final Answer:

   d.push_front(10); -> Option B

4. Quick Check:

   push_front() inserts at front [OK]

Hint: Use push_front() to add at front in deque [OK]

Common Mistakes:

• Using push() which is not a deque method
• Using insert_front() which doesn't exist
• Confusing with vector methods

3. What will be the output of the following C++ code snippet?

std::deque<int> d = {1, 2, 3};
d.push_front(0);
d.push_back(4);
d.pop_front();
for (int x : d) std::cout << x << " ";

medium

A. 1 2 3 4

B. 0 1 2 3 4

C. 1 2 3

D. 0 1 2 3

Solution

1. Step 1: Trace deque operations

   Initial deque: [1, 2, 3]
   After push_front(0): [0, 1, 2, 3]
   After push_back(4): [0, 1, 2, 3, 4]
   After pop_front(): removes 0, deque becomes [1, 2, 3, 4]

2. Step 2: Print elements

   Elements printed are 1 2 3 4 with spaces.

3. Final Answer:

   1 2 3 4 -> Option A

4. Quick Check:

   pop_front removes first element [OK]

Hint: pop_front removes first inserted element [OK]

Common Mistakes:

• Forgetting pop_front removes 0
• Confusing push_front and push_back order
• Printing without spaces

4. Identify the error in the following C++ code using std::deque:

std::deque<int> d;
d.push_back(5);
d.pop_front();
std::cout << d.front();

medium

A. deque must be initialized with size

B. push_back() syntax is incorrect

C. pop_front() called on empty deque causes undefined behavior

D. front() cannot be used after pop_front()

Solution

1. Step 1: Analyze deque state after operations

   Initially, deque is empty.
   push_back(5) adds 5: deque = [5]
   pop_front() removes 5: deque becomes empty again.

2. Step 2: Check front() usage

   Calling front() on empty deque is undefined behavior and may cause runtime error.

3. Final Answer:

   pop_front() called on empty deque causes undefined behavior -> Option C

4. Quick Check:

   front() on empty deque is invalid [OK]

Hint: Don't pop from empty deque before accessing front [OK]

Common Mistakes:

• Assuming front() returns 5 after pop_front
• Thinking push_back syntax is wrong
• Believing deque needs size at initialization

5. Given a std::deque<int> d = {10, 20, 30, 40, 50}, which sequence of operations will result in the deque containing 10 20 30 after all operations?

hard

A. d.pop_back(); d.pop_front();

B. d.pop_front(); d.pop_back();

C. d.pop_front(); d.pop_front();

D. d.pop_back(); d.pop_back();

Solution

1. Step 1: Start with initial deque

   Deque = [10, 20, 30, 40, 50]

2. Step 2: Apply operations for each option

   d.pop_back(); d.pop_back();: pop_back() removes 50 -> [10, 20, 30, 40], pop_back() removes 40 -> [10, 20, 30]
   d.pop_front(); d.pop_back();: pop_front() removes 10 -> [20, 30, 40, 50], pop_back() removes 50 -> [20, 30, 40]
   d.pop_front(); d.pop_front();: pop_front() twice removes 10 and 20 -> [30, 40, 50]
   d.pop_back(); d.pop_front();: pop_back() removes 50 -> [10, 20, 30, 40], pop_front() removes 10 -> [20, 30, 40]

3. Step 3: Compare results

   Only d.pop_back(); d.pop_back(); results in [10, 20, 30]. The other sequences result in different contents.

4. Final Answer:

   d.pop_back(); d.pop_back(); -> Option D

5. Quick Check:

   Two pop_back() removes the last two elements [OK]

Hint: pop_back twice removes last two elements [OK]

Common Mistakes:

• Removing wrong ends in wrong order
• Assuming pop_back twice keeps 20
• Confusing pop_front and pop_back effects