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Raise your hand if you've
ever had this happen to you.

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You have a list of things, and

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you need apply some sort of
transform to all of them.

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Maybe you need to apply sales
prices to a bunch of items, or

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maybe you need to update
an attribute in a bunch of objects.

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What did you do?

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Maybe you made a new list, then a for
loop going over the old list, and

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in the loop run the function on each
item before pending it to the new list.

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Yeah me too.

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That's where map comes into play.

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Map lets us apply a function to
every member of an iterable and

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gives us back a new custom iterable.

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In Python 2, map gave back a list but
that's no longer the case.

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So we'll have to cast it to
a list if that's what we want.

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Let's check it out in Workspaces.

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Before we can show what map does we
have to write a little function.

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So map applies a function to
every item that's in iterable.

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Everything that's inside of an iterable,
map applies to.

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So, for instance,
we could do A little thing like this.

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We have a,
let's do def double n return, n * 2.

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All right, so
we're gonna apply the function

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double to our interval which is a.

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Then we're gonna print that out and
we get 2, 4, 6.

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So, that's what map does.

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It's just super, super duper simple.

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That's not a super, wonderful, and
handy example [LAUGH] though is it?

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So let's do a sales price.

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So let's say that our books, when we get
a sales price it's always a 20% discount.

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And we wanna write a function that
will take any book that we've got and

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gives back the book, but
with the price as the sales price.

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So, before we can do that though,
we need to come up here and

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say from copy import copy.

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Copy let's us make copies and
our books are mutable.

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So, we need to make sure and

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make a copy of them inside of
our functions that do things.

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So, just to be safe,
that's what we want to do.

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All right, so let's say sales price,
and it's gonna take a book.

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And we'll say,
apply a 20% discount to the book's

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price, just so
we remember what we're doing.

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So our book is gonna
be a copy of our book.

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So whatever book comes in,
we want to make a copy of that book.

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We don't want to modify the original
book that's in the JSON file.

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Not that we would change the JSON file,
but we have it loaded to memory,

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we don't wanna change all that.

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All right, so book.price should equal,

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we're gonna round this,
book.price-book.price*.2.

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So that'll give us to 20%.

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And since here in the US at least, our
prices always have just two digits after

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the decimal point,
we're gonna round it to two digits.

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And then we're going to
return the book okay?

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Cool.

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So now, let's get our sales books.

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And we're gonna call map
on sales price on books.

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And we want to print, let's see,
this has to be a list for

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us to be able to get to
objects that are in it.

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Let's do sales_books[0].price.

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And then you know what?
Let's do print(BOOKS(0).price).

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Those should be the same book.

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We're not sorting or anything like that,
so they should be the exact same book.

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So let's.

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Cool!

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So if we take our original book,
which was $13.55,

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and we put our sales price on it,
we get back $10.84.

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So pretty cool.

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Pretty handy way of doing that.

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If you've watched the comprehensions
workshop, you might be thinking, wow

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that looks a whole lot like a list
comprehension and you'd be right.

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What map does,
is effectively a list comprehension.

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So, let's write this as
a list comprehension.

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So, sales_books2, and we're gonna call
sales_price(book) for book, and in BOOKS.

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If you haven't seen a list
comprehension before,

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what we're doing here is we're
making a list using a for loop.

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And so what we're saying is, put this into
a list for every step in this formula.

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So we're getting a book for
each book that's in BOOKS.

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And we're sending that
book to sales_price.

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And so that sends us back our new and
updated book.

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So if we run this again,
we should get the same data and we do.

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Because we're doing the same work.

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Both of these are doing
the exact same thing.

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There just doing it two
slightly different ways.

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This one is using map, whereas this
one is using a list comprehension.

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Now, which one of these that you
choose is kind of up to you.

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They both do the same work, they both
take about the same amount of time,

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they both use about the same
amount of processing power.

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So, why would you pick one over the other?

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Well, map is really,
really, really nestable.

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We're gonna get to this at the end of the
course but you'll going to be able to see,

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you can stick map around other
functionalities that we're gonna do.

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And you can't always do that
easily with a list comprehension.

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List comprehensions that are more than say
a line long, more than two lines long for

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sure, become really hard to read,
and really hard to follow.

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List comprehensions that have more than
one for in them or multiple ifs or

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something like that in it,

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they become very very difficult
to track what's going on.

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Whereas map and the other fun things that
we're gonna deal with in this course,

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like filter and reduce and stuff like
that, are much friendlier to read and

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figure out, even in a larger scale.

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So if you're doing something just once,
you just need to do this little tiny thing

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and just you're done, list comprehension's
probably what you wanna use.

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If you're going for something that scales
up to a large amount of lines of code,

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or maybe it needs to be
callable from other places or

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whatever, then the map is
probably your better bet.

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Map is a really useful utility
when you need to do some work

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on a lot of items at once.

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Usually you'll get the most benefit
from map when you combine it with

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other functions, like the next one
we're gonna talk about, filter.