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Chaining Functions
6:49Standalone functions are great but when we're writing small functions, we often need to do more work than we can achieve in a single function. Luckily, we can chain functions so long as we write them correctly.
I mentioned a couple of videos ago that
map and filter work really well together.
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Let's look at a few ways to combine
these two small handy functions for
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some useful results.
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So, I handed this earlier and
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you probably figured it out if
you've been playing around.
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But you can combine map and
filter, and our other functions.
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So long as your function and
in that case I mean map, filter sorted,
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reversed, et cetera, returns an iterable,
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these things can all be chained together
because they all work together.
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So let's look at these books.
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And there's a lot of, I mean we have
a lot of subjects in all these books.
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But I don't know how many of them
all contain the same thing, but
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I know that Roland here
is in several of them.
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So let's use Roland.
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And let's look up all the books
that have Roland in them, alright?
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So let's write a little function here,
has_roland.
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That's gonna take a (book).
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And we're gonna return
["Roland" in subject for
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subject in book.subjects.
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And now this isn't gonna work, because if
"Roland" isn't in any of the subjects,
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it's gonna return false.
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So, what we need to do is we're gonna
wrapped it in a new function any.
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And any returns true or
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false if any of the things in
the iterable that it's passed are true.
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So, if we get nothing but falses,
then it comes back to false.
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If we get some trues,
then it will come back true.
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Even one true makes it true.
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Alright, and then, if we look at our
data here, we see the title "Wizard and
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glass." "g" is lowercase, but on "The
Waste Lands," it's all title cased.
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You know we got others here that are,
seen this one is like upper and lower.
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So let's make it where it
always uppercases the book.
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Okay, so
again we want to do copy book, and
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then book.title equals book.title.title.
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Title gives up back the title
case version of the title.
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We're going to return book.
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All right, cool.
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So, we have two functions.
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Our first function is going
to be a filter function.
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We know that because we have this has,
or is, or are or whatever.
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And, it's gonna give us back books
that have Roland as a subject.
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And then title case gives us back
the title cased titles of all the books.
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So let's print the list version and
now we can do the filter or
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the map in either direction that we want.
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We could do the filter first or we could
do the map first, but if we do the filter
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first, that means we're only title-casing
books that are correctly filtered.
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So that means we have to do the map on
the outside cuz these things are weighted
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from the inside out.
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So we're gonna map titleCase to
whatever comes out of this filter.
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And the filter's gonna be has_roland.
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And that's gonna be all of our books.
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So we close our filter, close our map,
close our list, close our print.
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Fingers crossed that this works.
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Look at that.
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We've got, we should have one, two,
three, four, five, six I left out a book!
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I left out book four.
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Oh well, anyway, there's our books that
we have that are all about Roland.
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We don't have any copies of
the fourth book in the series, so
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if you're trying to read the series and
bought them at our store,
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you're gonna get really disappointed
in the middle of the series.
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But, there's how we can
chain this stuff together.
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Now I want to show you one more and
I think this is kind of where the power of
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functional programming first like
really hits people is in this chain.
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So here's what we're going to do.
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All right.
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So I think a book is a pretty good
deal if it's less than five dollars.
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Right?
If I can buy a book for
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two bucks, that's a pretty good deal.
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What I wanna know is what are all
the books that are less than
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five dollars when they're on sale.
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Tell me about all those books.
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Okay.
And I want them sorted,
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I want to know the cheapest
one I can possibly buy.
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So.
Here we go.
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We're gonna call this cheap books.
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And we're going to start with sorted.
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Right?
Cuz we're going to sort something.
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What are we going to sort?
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Well first we need to write a function.
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So let's write a function.
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We're going to say is good deal.
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All right.
A book is a good deal if it's
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less than five bucks.
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So we're going to return book.price
less than or equal to 5.
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So, $5 and one penny, not worth it.
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Just not worth it to me.
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But, $5 even or cheaper, that's worth it.
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So, we're gonna filter all these books,
is_good_deal.
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And what books we want to filter.
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Well I said I wanted to
do sales price books.
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So that means I gotta do my
sales price thing before.
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So I'm gonna map sales price and
that's gonna go to all the books.
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So we close our map we close our filter.
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And then we need a key.
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Well, I'm gonna go back
in iterable of objects.
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And each item in our
iterable is an object.
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And each of these objects has attributes.
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So we're gonna use attrgetter.
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And the attribute we wanna get is price.
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Alright, so now,
all that work later, Well that's
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print(cheap_books[0]), and
let's see what I get.
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Needful things, all right.
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And then, what is needful thing's price?
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How cheap is needful
things when it's on sale?
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$2.80, that's not bad.
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I don't mind a book for $2.80.
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But now look, let's look at how
much work that we've done here.
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So we turned all of our books
into their sales price.
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Their default 20% sales price,
we turn all of them into that.
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All right, cool.
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Then we filtered through all of these
sale price reduced price books,
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and we said, I only want the ones
that are $5 or less, right?
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Only these ones that
are super duper cheap.
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And then we sorted all of them by whatever
that new updated cheap price was.
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We did all that work in six lines,
counting a blank line.
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That's pretty amazing.
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Like peanut butter and
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chocolate, some things just get
better the more you add to them.
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Filter and
map are both useful on their own, but
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their real power comes out much
better when you mix them together.
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