"Stack is a linear data structure which follows a particular order in which the operations are performed." Wha

Question

Q. "Stack is a linear data structure which follows a particular order in which the operations are performed." What is the order in which the operations are performed in a Stack? Name the List method/function available in Python which is used to remove the last element from a list implemented stack. Also write an example using Python statements for removing the last element of the list.

CBSECBSE Class XII Board 2022Subjective· 2mImportance★★★★★

✓ Free question

Concept understanding — List Stack 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.

Important

The key rule to remember: Last In, First Out (LIFO). The last item you pushed onto the stack is the first one you pop off. That is the entire logic of a stack. If you ever find yourself trying to remove something from the middle, you are no longer using a stack — you are just using a plain list.

In your exam, you may be asked to trace what happens when you push and pop a sequence of items. For example, if you push A, then B, then pop, the popped item is B. If you then push C and pop again, you get C. The stack always remembers the order of arrivals, and you can only access the most recent one.

So, to sum up: List Stack Operations are simply the disciplined use of a Python list to follow the LIFO rule — push to add at the end, pop to remove from the end, and peek to see the end without removing. That is the whole idea, and it is powerful because it mirrors how many real-world processes work.

A stack follows the Last-In, First-Out (LIFO) order of operations. This means that the element which was added most recently to the stack is the first one to be removed.

In Python, when a stack is implemented using a list, the pop() method is used to remove the last element from the list, effectively removing the top element of the stack.

Here is an example demonstrating its use:

my_stack = [10, 20, 30]
print(f"Original stack: {my_stack}")
removed_element = my_stack.pop()
print(f"Element removed: {removed_element}")
print(f"Stack after removal: {my_stack}")

✓Final answer

A stack operates on the Last-In, First-Out (LIFO) principle, and in Python, the list.pop() method is used to remove the last element, simulating the removal of the top element from a stack.

A stack operates on the Last-In, First-Out (LIFO) principle, meaning the last element added is the first one to be removed.

A stack is a fundamental linear data structure, often compared to a pile of plates or a stack of books. The defining characteristic of a stack is the specific order in which elements are added and removed. This order is crucial for understanding how a stack functions in various computational tasks.

The operations in a stack are performed according to the Last-In, First-Out (LIFO) principle. Imagine you are adding plates to a stack: you place a new plate on top of the previous one. When you want to take a plate, you naturally take the one from the very top. You cannot easily access or remove a plate from the middle or bottom without first removing the plates above it.

Important

The Last-In, First-Out (LIFO) principle dictates that the element most recently added to the stack is the first one to be removed.

The two primary operations associated with a stack are:

Push: This operation adds a new element to the top of the stack.
Pop: This operation removes the element from the top of the stack.

When an element is 'pushed' onto the stack, it becomes the new top element. Conversely, when an element is 'popped', it is always the current top element that is removed. This strict adherence to LIFO ensures that the last item pushed is indeed the first item popped.

In Python, a list can be effectively used to implement a stack because lists provide methods that naturally support the LIFO principle. The list method available in Python which is used to remove the last element from a list implemented as a stack is pop(). When called without an index, the pop() method removes and returns the last element of the list, which perfectly aligns with the stack's 'pop' operation.

Here is an example using Python statements for removing the last element of a list acting as a stack:

# Initialize a list to represent a stack
my_stack = [10, 20, 30, 40, 50]

print("Stack before pop operation:", my_stack)

# Remove the last element (which is the top of the stack)
removed_element = my_stack.pop()

print("Removed element:", removed_element)
print("Stack after pop operation:", my_stack)

In this example, 50 was the last element added (conceptually, the top of the stack). The my_stack.pop() call removes 50, and the list my_stack is then left with [10, 20, 30, 40].

✓Final answer

Operations in a stack follow the Last-In, First-Out (LIFO) order. The Python list method pop() is used to remove the last element from a list implemented as a stack, as demonstrated by my_stack.pop() .

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

CBSE 2023Set 91/41 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) : In Python, a stack can be implemented using a list. Reasoning (R) : A stack is an ordered linear list of elements that works on the principle of First In First Out (FIFO).
›Reveal solutionSolution

The assertion is true, but the reasoning is false because a stack follows LIFO, not FIFO.

Let’s take this step by step. The Assertion says that in Python, a stack can be implemented using a list. That is perfectly correct. Python’s built-in list type provides methods like append() to add an element at the end and pop() to remove the last element — exactly the operations needed for a stack. Many textbooks and real-world Python code use lists as stacks, so this part is true.

Now look at the Reasoning. It claims that a stack works on the principle of First In First Out (FIFO). That is wrong. FIFO is the principle of a queue, not a stack. A stack follows Last In First Out (LIFO) — the element added most recently is the first one to be removed. Think of a stack of plates: you take the top plate first, not the bottom one. So the Reasoning is false.

Note

The NCERT Computer Science textbook for Class XII explicitly states that a stack is a LIFO data structure. The confusion between LIFO and FIFO is a common exam trap — always check which principle matches which data structure.

Since the Assertion is true and the Reasoning is false, the correct choice is (c).

✓Final answer
$(c) (A) is true but (R) is false.$

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