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Reduce

functools.reduce() takes a function and a list of values, combining them from left to right, accumulating a single result as it goes.

# import functools from the standard library
import functools


def add(sum_so_far: int, x: int) -> int:
    return sum_so_far + x


numbers: list[int] = [1, 2, 3, 4]
total: int = functools.reduce(add, numbers)
print(total)
# 10

reduce starts by calling add with the first two numbers. Moving left to right, it passes each result and the next number to add:

Call	Result
`add(1, 2)`	3
`add(3, 3)`	6
`add(6, 4)`	10

We pass add without () so reduce can call it for us. We're handing over the recipe, not the finished dish.

With just one item, reduce returns that item without calling add:

print(functools.reduce(add, [5]))
# 5

Assignment

Complete the join and the join_first_sentences functions.

Complete the join function. It's a helper function we'll use in join_first_sentences.
1. It takes two inputs:
  1. A doc_so_far string – the combined sentences so far, similar to sum_so_far in the example above.
  2. A sentence string – the next sentence to add.
2. Returns the result of concatenating the "doc" and "sentence" strings together, with a period and a space in between. For example:
```
doc: str = "This is the first sentence"
sentence: str = "This is the second sentence"
print(join(doc, sentence))
# This is the first sentence. This is the second sentence
```
Complete the join_first_sentences function.
1. It accepts two arguments:
  1. A list of sentence strings
  2. An integer n
2. Only use the first n sentences from the list. If n is zero, just return an empty string.
3. Use functools.reduce() with your join function to combine the sliced sentences into a single string.
4. Add a final period without a trailing space and return this string.

Use list slicing to get the first n sentences.

Here's an example of the expected behavior:

joined: str = join_first_sentences(
    [
        "This is the first sentence",
        "This is the second sentence",
        "This is the third sentence",
    ],
    2,
)
print(joined)
# This is the first sentence. This is the second sentence.