**💡 Problem Formulation:** Given a list of numbers in Python, the task is to calculate the sum of all the odd numbers contained within it. For example, if the input is `[1, 2, 3, 4, 5]`

, the expected output is `9`

, since `1 + 3 + 5 = 9`

.

## Method 1: Using for-loop and Modulus Operator

This method involves iterating over each element in the list using a for-loop and checking if the number is odd using the modulus operator (%). The odd numbers are added to a sum variable initialized to zero.

Here’s an example:

odd_numbers = [1, 2, 3, 4, 5] sum_odds = 0 for num in odd_numbers: if num % 2 != 0: sum_odds += num print(sum_odds)

Output: `9`

This approach is straightforward; it directly checks each number to see if it is odd and then accumulates the odd numbers. It is easy to understand and implement, making it a good starting point for beginners.

## Method 2: Using List Comprehension and Sum Function

A more Pythonic approach, this method employs list comprehension to filter out the odd numbers in one line and then uses Python’s built-in `sum()`

function to calculate the sum of these numbers.

Here’s an example:

odd_numbers = [1, 2, 3, 4, 5] sum_odds = sum([num for num in odd_numbers if num % 2 != 0]) print(sum_odds)

Output: `9`

This method is succinct and reduces the code required to perform the task. It takes advantage of Python’s expressive syntax and is ideal for those who prefer clean, concise code.

## Method 3: Using filter() and lambda Function

This technique leverages Python’s functional programming features—specifically, the `filter()`

function, combined with a lambda function, to isolate the odd numbers, and the `sum()`

function to calculate their sum.

Here’s an example:

odd_numbers = [1, 2, 3, 4, 5] sum_odds = sum(filter(lambda x: x % 2 != 0, odd_numbers)) print(sum_odds)

Output: `9`

This method is elegant and utilizes Python’s capabilities for creating short, anonymous functions with lambda. It’s great for those who favor a functional programming approach.

## Method 4: Using a While Loop

Alternatively, a while loop can traverse the list to calculate the sum of odd numbers. It keeps running until it has processed all elements of the list.

Here’s an example:

odd_numbers = [1, 2, 3, 4, 5] sum_odds = 0 i = 0 while i < len(odd_numbers): if odd_numbers[i] % 2 != 0: sum_odds += odd_numbers[i] i += 1 print(sum_odds)

Output: `9`

This method provides an alternative loop structure to the usual for-loop, potentially offering a clearer understanding for some of how looping over each element looks in a more manual fashion.

## Bonus One-Liner Method 5: Using a Generator Expression

For a compact and memory-efficient solution, a generator expression can be used within `sum()`

to calculate the total of the odd numbers.

Here’s an example:

odd_numbers = [1, 2, 3, 4, 5] sum_odds = sum(num for num in odd_numbers if num % 2 != 0) print(sum_odds)

Output: `9`

This one-liner is similar to using list comprehension but is more memory-efficient as it doesn’t generate an intermediate list. It is best suited for large lists where memory usage is a concern.

## Summary/Discussion

**Method 1:**For-loop with Modulus. Easy to understand. Can be verbose for simple tasks.**Method 2:**List Comprehension with Sum. Concise. May be less readable for beginners.**Method 3:**filter() with lambda. Functional programming style. May be obscure to those unfamiliar with lambdas.**Method 4:**While Loop. Good for manual control of loop indices. More verbose and may lead to infinite loops if not careful.**Method 5:**Generator Expression. Memory efficient. Similar trade-offs to list comprehension in terms of readability.