Queue Concept and FIFO Principle in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an element to the queue.

DSA C++

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

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

Aempty

Bpop

Cfront

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() instead of push() to add elements.

Trying to access front() to add elements.

The push() function adds an element to the back of the queue.

2fill in blank

medium

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

DSA C++

std::queue<int> q;
q.push(5);
q.push(7);
q.[1]();

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

Apop

Bpush

Cempty

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using push() instead of pop() to remove elements.

Trying to use front() to remove elements.

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(3);
int x = q.[1]();

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

Afront

Bpush

Cpop

Dempty

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() which removes the element instead of accessing it.

Using push() which adds elements.

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 print a message.

DSA C++

if (q.[1]()) {
    std::cout << "Queue is [2]" << std::endl;
}

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

Aempty

Bfull

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using size() instead of empty() to check emptiness.

Printing 'full' instead of 'empty' when queue is empty.

The empty() function checks if the queue has no elements. If true, print 'empty'.

5fill in blank

hard

Fill all three blanks to create a queue, add elements, and print the front element.

DSA C++

std::queue<int> [1];
[1].[2](20);
std::cout << [1].[3]() << std::endl;

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

Aq

Bpush

Cfront

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

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

Using inconsistent variable names.

Create queue named q, push 20, then print front element using front().

Practice

1. What does the FIFO principle mean in the context of a queue?

easy

A. Last In, First Out - the last element added is the last removed

B. First In, First Out - the last element added is the first removed

C. Last In, First Out - the first element added is the last removed

D. First In, First Out - the first element added is the first removed

Solution

1. Step 1: Understand FIFO principle

   FIFO means the first element that enters the queue is the first one to leave.

2. Step 2: Match FIFO with queue behavior

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

3. Final Answer:

   First In, First Out - the first element added is the first removed -> Option D

4. Quick Check:

   FIFO = First In, First Out [OK]

Hint: Remember queue = line; first come, first served [OK]

Common Mistakes:

• Confusing FIFO with LIFO
• Thinking last added is removed first
• Mixing queue with stack behavior

2. Which of the following is the correct way to declare a queue of integers in C++ using STL?

easy

A. queue<int> q();

B. queue q<int>;

C. queue<int> q;

D. queue<int> q[];

Solution

1. Step 1: Recall STL queue syntax

   The correct syntax to declare a queue of integers is queue<int> q; without parentheses or brackets.

2. Step 2: Analyze options

   queue<int> q; matches correct syntax. queue q<int>; has wrong order. queue<int> q(); looks like a function declaration. queue<int> q[]; uses array syntax incorrectly.

3. Final Answer:

   queue<int> q; -> Option C

4. Quick Check:

   Correct STL queue declaration = queue<int> q; [OK]

Hint: Use template syntax: queue<Type> name; [OK]

Common Mistakes:

• Adding parentheses like a function
• Using brackets as if declaring array
• Swapping template and variable names

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

std::queue<int> q;
q.push(10);
q.push(20);
q.push(30);
int x = q.front();
q.pop();
int y = q.front();
std::cout << x << " " << y << std::endl;

medium

A. 10 20

B. 20 30

C. 30 20

D. 10 30

Solution

1. Step 1: Trace queue operations

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

2. Step 2: After pop and front calls

   After q.pop(), 10 is removed. New front is 20. So x=10, y=20.

3. Final Answer:

   10 20 -> Option A

4. Quick Check:

   First out is 10, next front is 20 [OK]

Hint: front() shows first element; pop() removes it [OK]

Common Mistakes:

• Assuming pop() returns value
• Mixing front() and back() values
• Confusing order of elements

4. Identify the error in the following C++ code snippet using queue:

std::queue<int> q;
q.push(5);
q.pop();
int val = q.front();
std::cout << val << std::endl;

medium

A. Accessing front() on an empty queue causes undefined behavior

B. pop() does not remove element from queue

C. push() is used incorrectly

D. Missing include for queue header

Solution

1. Step 1: Analyze queue state after operations

   After push(5), queue has one element. pop() removes it, queue becomes empty.

2. Step 2: Check front() call on empty queue

   Calling front() on empty queue is undefined and causes runtime error.

3. Final Answer:

   Accessing front() on an empty queue causes undefined behavior -> Option A

4. Quick Check:

   front() on empty queue = error [OK]

Hint: Never call front() if queue might be empty [OK]

Common Mistakes:

• Ignoring empty queue before front()
• Thinking pop() returns value
• Assuming push() syntax is wrong

5. You have a queue with elements 1, 2, 3, 4 (front to back). You perform these operations:
q.pop(); q.push(5); q.pop(); q.push(6);
What is the final order of elements in the queue from front to back?

hard

A. 2 -> 3 -> 4 -> 5 -> 6

B. 3 -> 4 -> 5 -> 6

C. 1 -> 2 -> 3 -> 4 -> 5 -> 6

D. 4 -> 5 -> 6

Solution

1. Step 1: Initial queue and first operations

   Queue: 1(front),2,3,4(back). q.pop() removes 1. Queue: 2,3,4. q.push(5) adds 5 at back: 2,3,4,5.

2. Step 2: Remaining operations and final queue

   q.pop() removes 2. Queue: 3,4,5. q.push(6) adds 6 at back: 3,4,5,6.

3. Final Answer:

   3 -> 4 -> 5 -> 6 -> Option B

4. Quick Check:

   After pops and pushes, queue front to back = 3,4,5,6 [OK]

Hint: Pop removes front; push adds to back, track stepwise [OK]

Common Mistakes:

• Forgetting pop removes front element
• Adding push elements at front instead of back
• Not updating queue after each operation