Why Queue Exists and What Problems It Solves in DSA C++ - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an element to the end of the queue.

DSA C++

std::queue<int> q;
q.push([1]);

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

A5

Bempty()

Cfront()

Dpop()

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() instead of push()

Trying to access front() inside push()

The push function adds an element to the back of the queue. Here, 5 is added.

2fill in blank

medium

Complete the code to remove the front element from the queue.

DSA C++

std::queue<int> q;
q.push(10);
q.push(20);
q.[1]();

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

Apush

Bfront

Cpop

Dempty

Attempts:

3 left

💡 Hint

Common Mistakes

Using push() instead of pop()

Trying to access front() to remove

The pop() function removes the front element from the queue.

3fill in blank

hard

Fix the error in accessing the front element of the queue.

DSA C++

std::queue<int> q;
q.push(15);
int x = q.[1]();

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

Apop

Bfront

Cpush

Dempty

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() to get the element

Using push() instead of front()

The front() function returns the front element without removing it.

4fill in blank

hard

Fill both blanks to check if the queue is empty and then add an element.

DSA C++

if (q.[1]()) {
    q.[2](100);
}

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

Aempty

Bpush

Cpop

Dfront

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() to add elements

Using front() to check emptiness

empty() checks if the queue has no elements. If true, push() adds 100 to the queue.

5fill in blank

hard

Fill all three blanks to print and remove all elements from the queue.

DSA C++

while (!q.[1]()) {
    std::cout << q.[2]() << ' ';
    q.[3]();
}

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

Aempty

Bfront

Cpop

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using push() inside the loop

Using pop() to access element instead of front()

Loop runs while queue is not empty (empty() returns false). front() accesses the element to print, and pop() removes it.

Practice

1. Why do we use a queue data structure instead of just using an array or list?

easy

A. To sort elements automatically as they are added

B. To store elements in random order for quick access

C. To process elements in the order they arrive, like a real-life line

D. To store only unique elements without duplicates

Solution

1. Step 1: Understand the queue behavior

   A queue processes elements in First-In-First-Out (FIFO) order, meaning the first element added is the first to be removed.

2. Step 2: Compare with other data structures

   Arrays or lists do not enforce this order automatically; they allow random access or different insertion/removal orders.

3. Final Answer:

   To process elements in the order they arrive, like a real-life line -> Option C

4. Quick Check:

   Queue = FIFO order processing [OK]

Hint: Queue means first come, first served order [OK]

Common Mistakes:

• Thinking queue sorts elements automatically
• Confusing queue with stack (LIFO)
• Assuming queue stores only unique items

2. Which of the following is the correct way to add an element to a queue in C++ using the STL queue?

easy

A. queue.enqueue(element);

B. queue.push(element);

C. queue.insert(element);

D. queue.add(element);

Solution

1. Step 1: Recall C++ STL queue methods

   The standard method to add an element to a queue is push().

2. Step 2: Verify other options

   Methods like add(), insert(), or enqueue() do not exist in C++ STL queue.

3. Final Answer:

   queue.push(element); -> Option B

4. Quick Check:

   Use push() to add in C++ queue [OK]

Hint: Remember STL queue uses push() to add [OK]

Common Mistakes:

• Using add() or enqueue() which are not C++ queue methods
• Confusing queue with other containers like vector
• Trying to use insert() which is for other containers

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

#include <iostream>
#include <queue>
using namespace std;

int main() {
    queue<int> q;
    q.push(10);
    q.push(20);
    q.push(30);
    cout << q.front() << endl;
    q.pop();
    cout << q.front() << endl;
    return 0;
}

medium

A. 10
20

B. 30
20

C. 10
30

D. 20
30

Solution

1. Step 1: Trace queue operations

   Elements pushed in order: 10, 20, 30. The front is 10 initially.

2. Step 2: After first pop()

   Pop removes 10, so front becomes 20.

3. Final Answer:

   10\n20 -> Option A

4. Quick Check:

   Queue front after push and pop = 10 then 20 [OK]

Hint: Queue front shows oldest element, pop removes it [OK]

Common Mistakes:

• Assuming pop removes from back instead of front
• Confusing front() with back()
• Printing after pop without updating front

4. Consider this C++ code snippet using queue<int>. It is supposed to print all elements in the queue, but it causes an infinite loop. What is the error?

#include <iostream>
#include <queue>
using namespace std;

int main() {
    queue<int> q;
    q.push(1);
    q.push(2);
    q.push(3);
    while (!q.empty()) {
        cout << q.front() << " ";
        // Missing q.pop() here
    }
    return 0;
}

medium

A. The loop never removes elements, so queue stays non-empty

B. The queue is empty initially, so loop never runs

C. The front() method is used incorrectly

D. The push() method is missing

Solution

1. Step 1: Analyze the loop behavior

   The loop prints front element but never calls pop(), so elements remain in the queue.

2. Step 2: Understand infinite loop cause

   Since queue never empties, q.empty() is always false, causing infinite loop.

3. Final Answer:

   The loop never removes elements, so queue stays non-empty -> Option A

4. Quick Check:

   Missing pop() causes infinite loop [OK]

Hint: Always pop() inside loop to avoid infinite queue processing [OK]

Common Mistakes:

• Forgetting to pop() inside the loop
• Assuming front() removes element
• Thinking queue empties automatically

5. You want to simulate a ticket counter where people arrive and get served in order. Which data structure best solves this problem and why?

hard

A. Stack, because it serves the last person first

B. Array, because it allows random access to people

C. Set, because it stores unique people only

D. Queue, because it serves people in the order they arrive

Solution

1. Step 1: Understand the problem scenario

   People arrive and must be served in the exact order they came, like a real line.

2. Step 2: Match data structure behavior

   A queue follows First-In-First-Out (FIFO), matching the order of arrival and service.

3. Step 3: Eliminate other options

   Stack serves last-in-first-out, array allows random access, set stores unique items without order.

4. Final Answer:

   Queue, because it serves people in the order they arrive -> Option D

5. Quick Check:

   Ticket counter needs FIFO queue [OK]

Hint: FIFO queue models real-life waiting lines perfectly [OK]

Common Mistakes:

• Choosing stack which reverses order
• Thinking array preserves order automatically
• Confusing set with ordered structure