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Map

"Map," "filter," and "reduce" are three commonly used higher-order functions in functional programming.

In Python, the built-in map function takes a function and an iterable (often a list) as inputs. It returns an iterator that applies the function to every item, yielding the results.

With map, we can operate on lists without using loops and nasty stateful variables. For example, given this code:

def square(x: int) -> int:
    return x * x


nums: list[int] = [1, 2, 3, 4, 5]
squared_nums: list[int] = []
for num in nums:
    num_squared: int = square(num)
    squared_nums.append(num_squared)

print(squared_nums)
# [1, 4, 9, 16, 25]

We could use map instead, like this:

from collections.abc import Iterator


def square(x: int) -> int:
    return x * x


nums: list[int] = [1, 2, 3, 4, 5]
squared_nums: Iterator[int] = map(square, nums)

print(list(squared_nums))
# [1, 4, 9, 16, 25]

map() returns a "map object," so the list() type constructor is needed to convert it back into a standard list.

Assignment

Markdown supports two different styles of bullet points, - and *. We prefer *, so, we need a function to convert any - bullet points to * bullet points.

Complete the change_bullet_style function. It takes a document (a string) as input, and returns a single string as output. The returned string should have any lines that start with a - character replaced with a * character.

For example, this:

- This is a bullet
- This is a bullet

Becomes:

* This is a bullet
* This is a bullet

Use .split() to split the document into a list of lines.
Use the built-in map function to apply the provided convert_line function to each line.
Use .join() to join the converted lines and return the result, preserving the original line breaks.

Examples of split and join:

# my_document is a string with newlines
lines_list: list[str] = my_document.split("\n")

rejoined_doc: str = "\n".join(lines_list)