Write a python method/function Swapper(Numbers) to swap the first half of the content of a list Numbers with s

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Q. Write a python method/function Swapper(Numbers) to swap the first half of the content of a list Numbers with second half of the content of list Numbers and display the swapped values. Note : Assuming that the list has even number of values in it. For example : If the list Numbers contains [35,67,89,23,12,45] After swapping the list content should be displayed as [23,12,45,35,67,89]

(OR)

Write a python method/function Count3and7(N) to find and display the count of all those numbers which are between 1 and N, which are either divisible by 3 or by 7. For example : If the value of N is 15 The sum should be displayed as 7 (as 3,6,7,9,12,14,15 in between 1 to 15 are either divisible by 3 or 7)

Sikkim CbseCBSE Class XII Board 2019Subjective· 2mImportance★★★★★

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Part (a)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. …

Part (b)Concept understanding — Function Call Handling

Function Call Handling

Think about what happens when you ask a friend for a favour. You stop what you're doing, explain what you need, and then wait while your friend does the work. Once they finish, they come back to you with the result, and you resume whatever you were doing. That simple back-and-forth is the essence of how function calls work in programming.

The Everyday Intuition

Imagine you're cooking a meal. You have a recipe book open. The main recipe says "prepare the sauce" — but it doesn't list every single step for making the sauce right there. Instead, it expects you to know a separate sauce recipe. So you pause the main cooking, go make the sauce, and when it's ready, you return to the main dish and continue. The sauce recipe is like a function: a self-contained set of instructions that does one specific job.

In programming, a function call is exactly this: the main program says "go do this specific task," hands over any necessary information, waits for the task to complete, and then gets back whatever result the function produced.

The Precise Meaning

A function call is the instruction that tells the computer to execute a predefined block of code — the function — and then return control back to the point where the call was made. The function itself is a named, reusable set of instructions that performs a particular operation.

When a function is called, three things happen in sequence:

The current task is paused — the computer remembers exactly where it was in the main program.
Control is transferred to the function, along with any data the function needs to do its job (these are called arguments or parameters).
The function runs its instructions, produces a result (if any), and returns control — along with that result — back to the exact spot where the call was made.

Note

The key idea is temporary suspension. The main program does not end when a function is called; it simply steps aside, lets the function do its work, and then picks up exactly where it left off. This is different from jumping to a completely new program or starting over.

Why It Matters

Function call handling is not a trivial detail — it is the backbone of how programs are organised. Without it, every program would be one long, flat list of instructions, impossible to manage beyond a few dozen lines.

Here is why this concept is so important:

Reusability — You write a function once and call it as many times as needed. The same "calculate tax" function can be called for every customer without rewriting the logic.
Modularity — A program is broken into small, independent pieces. Each function does one thing well. This makes the program easier to understand, test, and fix.
Abstraction — When you call a function, you do not need to know how it works internally. You only need to know what it does and what information to give it. This is like driving a car without understanding the engine.
Readability — A program that uses function calls reads almost like an outline: "process order, calculate discount, send confirmation." Each step is a function name that describes what happens.

Important

The most common mistake beginners make is thinking that a function call "inserts" the function's code into the main program. It does not. The function's code stays in its own place. The call merely transfers control temporarily. This distinction matters because it explains how the same function can be called from many different places without duplicating code.

How the Computer Keeps Track …

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Part (a)

def Swapper(Numbers):
    h = len(Numbers) // 2
    for i in range(h):
        Numbers[i], Numbers[i + h] = Numbers[i + h], Numbers[i]
    print(Numbers)
``` …

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Part (a) swaps the first half of a list with its second half and prints the result ([23,12,45,35,67,89]); Part (b) counts and displays how many numbers from 1 to N are divisible by 3 or 7 (7 for N=15).

Part (a)

With an even-length list, element i in the first half pairs with element i + h in the second half, where h = len(Numbers)//2. Swap each such pair in place, then display.

def Swapper(Numbers):
    h = len(Numbers) // 2
    for i in range(h):
        Numbers[i], Numbers[i + h] = Numbers[i + h], Numbers[i]
    print(Numbers)

Swapper([35, 67, 89, 23, 12, 45])

Trace (h = 3): swap index 0↔3, 1↔4, 2↔5 → [23, 12, 45, 35, 67, 89].

[23, 12, 45, 35, 67, 89] …

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Part (a)

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