**π‘ Problem Formulation:** Consider having a nested list structure in Python, where you need to perform multiplication on elements across the sub-lists based on a custom function or rule. For instance, given a list of lists `[[1,2],[3,4]]`

, you might want to multiply each element by 2, producing an output of `[[2,4],[6,8]]`

. This article explores different methods to achieve this task in Python.

## Method 1: Using List Comprehensions

List comprehension is an elegant and concise way to create new lists by applying an expression to each element in an existing list. In the context of multiplying elements in a list of lists, it can be utilized to iterate over each sublist and multiply each element by a constant or a result of a function.

Here’s an example:

lst = [[1, 2], [3, 4]] result = [[item * 2 for item in sublst] for sublst in lst]

Output:

[[2, 4], [6, 8]]

This code snippet uses nested list comprehension to iterate through the original list `lst`

. For each sublist in `lst`

, it multiplies every item by 2 and constructs a new sublist, which forms the resulting list of lists.

## Method 2: Using the map() Function

The `map()`

function applies a given function to each item of an iterable (like a list) and returns a map object. When dealing with a list of lists, a nested `map()`

can be used where the inner map applies the multiplication and the outer map iterates over the sublists.

Here’s an example:

lst = [[1, 2], [3, 4]] result = list(map(lambda sublst: list(map(lambda x: x * 2, sublst)), lst))

Output:

[[2, 4], [6, 8]]

In this code, an anonymous function (lambda) is defined to double the value. This function is applied to each element within each sublist using the `map()`

function twice, once for the sublist and once for the elements within it. The result is then converted back into a list.

## Method 3: Using a Traditional For Loop

A traditional for loop in Python can iteratively go through each element in a list of lists, allowing us to apply a custom multiplication operation to each element. This method implies manually controlling the iteration and constructing a new nested list.

Here’s an example:

lst = [[1, 2], [3, 4]] result = [] for sublst in lst: result.append([item * 2 for item in sublst])

Output:

[[2, 4], [6, 8]]

This code uses a for loop to go through each sublist in `lst`

and a list comprehension to multiply each element within the sublist. The modified sublists are then appended to the `result`

list.

## Method 4: Using NumPy

NumPy is a Python library for large, multi-dimensional arrays and matrices, along with a collection of high-level mathematical functions to operate on these arrays. If the list of lists represents a numerical array, NumPy can be a highly optimized solution for element-wise operations.

Here’s an example:

import numpy as np lst = np.array([[1, 2], [3, 4]]) result = lst * 2

Output:

[[2 4] [6 8]]

In this code snippet, the list of lists is converted into a NumPy array, and then simple array multiplication by 2 is performed, which automatically applies to each element in the array.

## Bonus One-Liner Method 5: Using Generator Expressions

Generator expressions are similar to list comprehensions, but instead of creating a list, they generate items on the fly. This is particularly useful for large datasets where you would like to save memory.

Here’s an example:

lst = [[1, 2], [3, 4]] result = (list(item * 2 for item in sublst) for sublst in lst)

Output:

<generator object <genexpr> at 0x...>

To get the list: `list(result)`

which will output: `[[2, 4], [6, 8]]`

.

This example demonstrates the use of a generator expression. Each element is multiplied by 2 as it’s yielded from the generator. To retrieve the result as a list of lists, a conversion to a list is required.

## Summary/Discussion

**Method 1: List Comprehensions.**Strength: Concise and Pythonic. Weakness: Can be difficult to read for complex operations.**Method 2: Using the map() Function.**Strength: Functional programming style and often fast. Weakness: Less readable than list comprehensions for many Python programmers.**Method 3: Traditional For Loop.**Strength: Easy to understand and debug. Weakness: More verbose and potentially slower than other methods.**Method 4: Using NumPy.**Strength: Extremely fast for numerical operations on large datasets. Weakness: Requires the NumPy library, not suitable for non-numerical lists.**Bonus Method 5: Generator Expressions.**Strength: Memory efficient. Weakness: A generator provides items one at a time, requires conversion to list to realize all items at once.