Queue Implementation Using Array in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the queue size.

DSA Python

queue = [None] * [1]

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

A10

B5

C0

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 or None will not create a usable queue array.

The queue is initialized with a fixed size of 10 to hold elements.

2fill in blank

medium

Complete the code to check if the queue is empty.

DSA Python

def is_empty(front):
    return front == [1]

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

A0

Bfront

C-1

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for 0 or None will not correctly detect empty queue.

The queue is empty when the front index is -1.

3fill in blank

hard

Fix the error in the enqueue function to update rear correctly.

DSA Python

def enqueue(queue, rear, item, max_size):
    if rear == max_size - 1:
        print('Queue is full')
        return rear
    rear = rear [1] 1
    queue[rear] = item
    return rear

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

A-

B*

C/

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using - will move rear backward causing errors.

To add an item, rear index must increase by 1 using + operator.

4fill in blank

hard

Fill both blanks to correctly dequeue an item and update front.

DSA Python

def dequeue(queue, front, rear):
    if front == [1]:
        print('Queue is empty')
        return front, None
    item = queue[front]
    if front == rear:
        front = [2]
        rear = -1
    else:
        front += 1
    return front, item

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

A-1

B0

Cfront

Drear

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong values for empty queue check or reset.

If front is -1, queue is empty. When last item is dequeued, front resets to 0 and rear to -1.

5fill in blank

hard

Fill all three blanks to implement a function that prints the queue elements.

DSA Python

def print_queue(queue, front, rear):
    if front == [1]:
        print('Queue is empty')
        return
    i = front
    while i != rear:
        print(queue[i], end=' -> ')
        i = i [2] 1
    print(queue[[3]])

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

A-1

B+

Crear

Dfront

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong empty check or wrong index for last element.

Check if front is -1 for empty. Increment i by 1 to traverse. Print last element at rear index.

Practice

1. What is the main characteristic of a queue implemented using an array?

easy

A. It follows First-In-First-Out (FIFO) order

B. It follows Last-In-First-Out (LIFO) order

C. It allows random access to elements

D. It automatically sorts elements

Solution

1. Step 1: Understand queue behavior

   A queue adds elements at the rear and removes from the front, following FIFO.

2. Step 2: Compare options

   LIFO is for stacks, random access and sorting are not queue properties.

3. Final Answer:

   It follows First-In-First-Out (FIFO) order -> Option A

4. Quick Check:

   Queue = FIFO [OK]

Hint: Queue always removes the oldest element first [OK]

Common Mistakes:

• Confusing queue with stack (LIFO)
• Thinking queue allows random access
• Assuming queue sorts elements automatically

2. Which of the following is the correct way to initialize an empty queue using an array in Python?

easy

A. queue = ()

B. queue = {}

C. queue = set()

D. queue = []

Solution

1. Step 1: Identify array type in Python

   Python lists are used as arrays and initialized with [] for empty list.

2. Step 2: Check other options

   {} is dict, () is tuple, set() is a set, none are arrays.

3. Final Answer:

   queue = [] -> Option D

4. Quick Check:

   Empty array = [] [OK]

Hint: Use [] to create empty list (array) in Python [OK]

Common Mistakes:

• Using {} which creates a dictionary
• Using () which creates a tuple
• Using set() which is unordered

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

queue = []
queue.append(10)
queue.append(20)
queue.append(30)
print(queue.pop(0))
print(queue)

medium

A. 30
[10, 20]

B. 10
[20, 30]

C. 10
[10, 20, 30]

D. 20
[10, 30]

Solution

1. Step 1: Trace queue operations

   Append adds 10, 20, 30 to queue: [10, 20, 30]. pop(0) removes first element 10.

2. Step 2: Check print outputs

   First print outputs 10, second print outputs remaining queue [20, 30].

3. Final Answer:

   10 [20, 30] -> Option B

4. Quick Check:

   pop(0) removes front = 10 [OK]

Hint: pop(0) removes front element in queue [OK]

Common Mistakes:

• Using pop() instead of pop(0) removes last element
• Confusing append order
• Expecting pop(0) to remove last element

4. Identify the error in this queue implementation using an array:

queue = []
queue.append(1)
queue.append(2)
queue.pop()
print(queue)

medium

A. pop() removes last element, not front

B. append() is used incorrectly

C. queue should be a dictionary

D. print statement syntax error

Solution

1. Step 1: Understand pop() behavior

   pop() without index removes last element, not front as queue requires.

2. Step 2: Check queue correctness

   Queue must remove front element, so pop(0) should be used instead of pop().

3. Final Answer:

   pop() removes last element, not front -> Option A

4. Quick Check:

   Queue removal = pop(0) front [OK]

Hint: Use pop(0) to dequeue from front [OK]

Common Mistakes:

• Using pop() removes last, not front
• Confusing append usage
• Thinking queue is a dictionary

5. Given a fixed-size array of length 3, what happens if you try to enqueue 4 elements in a queue implemented using this array without resizing or circular logic?

hard

A. The queue automatically resizes to fit 4 elements

B. The 4th element replaces the first element

C. The queue overflows and cannot add the 4th element

D. The queue removes the last element to add the 4th

Solution

1. Step 1: Understand fixed-size array limits

   An array of length 3 can hold only 3 elements; no extra space for 4th.

2. Step 2: Check enqueue behavior without resizing

   Without resizing or circular logic, adding 4th element causes overflow error or failure.

3. Final Answer:

   The queue overflows and cannot add the 4th element -> Option C

4. Quick Check:

   Fixed array size = no extra elements [OK]

Hint: Fixed array size limits enqueue capacity [OK]

Common Mistakes:

• Assuming automatic resizing happens
• Thinking 4th element replaces first
• Believing queue removes last element automatically