Write QueueUp(Client) and QueueDel(Client) methods/functions in Python to add a new Client and delete a Client

Question

Q. Write QueueUp(Client) and QueueDel(Client) methods/functions in Python to add a new Client and delete a Client from a List of Clients names, considering them to act as insert and delete operations of the Queue data structure.

(OR)

Write PushOn(Book) and Pop(Book) methods/functions in Python to add a new Book and delete a Book from a List of Book titles, considering them to act as push and pop operations of the Stack data structure.

CBSECBSE Class XII Board 2019Subjective· 4mImportance★★★★★

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Part (a)Concept understanding — Queue Operations

Let’s begin with something you already know: a stack of plates in a cafeteria. You can only take the top plate off, and you can only put a new plate on top. You never pull a plate from the middle or the bottom. That everyday action is the core idea behind a stack in computer science — a list where all additions and removals happen at one end, called the top.

Now, in your NCERT textbook for Informatics Practices or Computer Science (Class 11 or 12), when you study List Stack Operations, you are learning how to treat a Python list as if it were that stack of plates. A normal list lets you add or remove items from anywhere. But a stack is a restricted list: you can only work with the most recently added item. This restriction is not a weakness — it is a deliberate design that makes certain tasks simple and fast.

The two fundamental operations are:

Push – adding an element to the top of the stack.
Pop – removing the top element from the stack.

There is also a third operation, Peek (or Top), which just looks at the top element without removing it. And you will often check if the stack is empty before trying to pop.

Note

In Python, you can implement a stack using a list. The append() method acts as push, and pop() (with no argument) acts as pop. Python’s list already has these built-in, so you don’t need to write any special code — you just use the list in a disciplined way.

Why does this matter? Because stacks are everywhere in real computing. When you press the Back button in a web browser, it pops the previous page off a stack of visited pages. When you undo an action in a word processor, it pops the last change off a stack of edits. Even the way your computer calls one function from another uses a stack behind the scenes. So learning stack operations is not just about lists — it is about understanding a pattern that appears again and again. …

Part (b)Concept understanding — Stack

The Stack: A Simple but Powerful Idea

Think about a stack of plates in a cafeteria. You place a clean plate on top. When someone needs a plate, they take the one from the top — the last one you put there. You cannot pull a plate from the middle or the bottom without causing a mess. That everyday action is the exact logic of a stack in computing.

A stack is a collection of items that follows one strict rule: the last item added is the first item removed. This is often called LIFO — Last In, First Out. The only item you can access at any moment is the one on top. To get to anything below, you must first remove everything above it.

How It Works

A stack has two fundamental operations:

Push: Add an item to the top of the stack.
Pop: Remove the item from the top of the stack.

That is all. There is no way to peek at the middle, no way to remove from the bottom. The stack only cares about what is on top.

Note

Some systems also allow a Peek operation — you look at the top item without removing it. But the core idea remains: only the top is visible.

Why Does This Matter?

You might wonder: why would anyone want such a restrictive structure? The answer is that many real-world processes naturally work this way.

Consider the Undo feature in a word processor. Every action you perform — typing a word, deleting a sentence, changing a font — is pushed onto a stack. When you press Undo, the most recent action is popped off and reversed. You cannot undo an action from an hour ago without first undoing everything that came after it. That is the stack at work.

Another example is a web browser's Back button. Each page you visit is pushed onto a stack. Clicking Back pops the current page and takes you to the previous one. You retrace your steps in reverse order — exactly LIFO.

Important

The stack is not just a data structure; it is a model for how many systems manage order and priority. Whenever you need to remember a sequence of events and later reverse through them, a stack is the natural choice. …

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Part (a)

A queue is a FIFO structure: insert at the rear (append), delete from the front (pop(0)).

def QueueUp(Queue, Client):        # insert (enqueue) at rear
    Queue.append(Client)

def QueueDel(Queue):               # delete (dequeue) from front
    if Queue == []:
        print("Queue Empty (Underflow)")
    else:
        return Queue.pop(0)

Clients = []
QueueUp(Clients, "Ravi")
QueueUp(Clients, "Sara")
print(QueueDel(Clients), Clients)

Ravi ['Sara'] …

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Part (a): a queue is FIFO — QueueUp adds at the rear with append(), QueueDel removes from the front with pop(0).

Part (b): a stack is LIFO — PushOn adds at the top with append(), Pop removes from the top with pop().

Part (a)

A queue follows First-In-First-Out (FIFO), like people in a line: the first to join is the first served. New clients are added at the rear and removed from the front. Using a Python list, append() adds at the end and pop(0) removes the first element. Always guard the delete against an empty queue (underflow).

def QueueUp(Queue, Client):
    "Insert a new client at the rear of the queue."
    Queue.append(Client)

def QueueDel(Queue):
    "Delete and return the client at the front of the queue."
    if Queue == []:
        print("Queue Empty (Underflow)")
    else:
        return Queue.pop(0)

# demonstration
Clients = []
QueueUp(Clients, "Ravi")
QueueUp(Clients, "Sara")
QueueUp(Clients, "Tom")
print("Queue:", Clients)
print("Served:", QueueDel(Clients))
print("Queue:", Clients)

Queue: ['Ravi', 'Sara', 'Tom']
Served: Ravi
Queue: ['Sara', 'Tom'] …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2023Set 91/11 markMCQ
$Q. Q. 17 and 18 are Assertion (A) and Reasoning (R) based questions. Mark the correct choice as$
(a) Both (A) and (R) are true and (R) is the correct explanation for (A).
(b) Both (A) and (R) are true and (R) is not the correct explanation for (A).
(c) (A) is true but (R) is false.
(d) (A) is false but (R) is true. Assertion (A): A stack is a LIFO structure. Reason (R): Any new element pushed into the stack always gets positioned at the index after the last existing element in the stack.
›Reveal solutionSolution

A stack is indeed LIFO, but the reason given confuses implementation detail (index positioning) with the defining principle (last-in, first-out access), so both are true yet the reasoning does not explain the assertion.

A stack is one of the most elegant data structures in computer science, defined entirely by how you interact with it rather than how it is built underneath. The assertion calls it a LIFO structure—Last In, First Out—which captures the essence perfectly. Imagine a stack of plates: you add a new plate on top, and when you need one, you take from the top. The last plate you placed is the first one you remove. That access discipline, that rule of engagement, is what makes a stack a stack.

The reason statement shifts the conversation to implementation. It says that when you push a new element, it gets positioned at the index after the last existing element. This is true if you implement a stack using an array or list, where elements occupy consecutive indices and the "top" pointer advances with each push. But here is the subtle disconnect: the LIFO nature of a stack is not because elements are stored sequentially in memory. LIFO is a logical property, a contract about the order of retrieval. You could implement a stack with a linked list where elements are scattered across memory, each pointing to the one below it, and it would still be LIFO. The sequential indexing is an artifact of one common implementation, not the reason the structure behaves as last-in, first-out.

Note

The defining characteristic of a stack is the restriction on access—only the top element is reachable for removal. Whether that top is tracked by an index, a pointer, or any other mechanism is secondary. …

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Q.Write QueueUp(Client) and QueueDel(Client) methods/functions in Python to add a new Client and delete a Client from a List of Clients names, considering them to act as insert and delete operations of the Queue data structure.

Part (a)

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