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So the last of Python's data
structures that we're gonna look at

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in this part of the course, is sets.

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Now if you're familiar with
mathematical set theory,

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then Python's sets will make
immediate sense to you.

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Now, a set in Python is unordered and
doesn't contain duplicates, so

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in that respect it's
similar to a dictionary.

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However, unlike a dictionary,
item's aren't accessed via a key.

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In fact, a set is probably more similar
to a collection of dictionary keys.

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And the set members are hashed in
the same ways dictionary keys are.

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Now, the elements in a set
must be immutable objects.

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Again, just like dictionary keys.

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They also support the union and

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intersection operations
that can be performed.

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On sets.

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Now sets are generally used less often
than the other data structures we've

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looked at so far, but that said, they
can be very useful for cleaning up data.

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And we'll look at that after we've seen
how to create and manipulate some sets.

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Here in Python 3,

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actually since Python 2.6, you can
define a set by using curly braces.

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Which is the same way
you define a dictionary.

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So let's go through it
doing it two ways so

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you can see the two ways to create sets.

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I'm gonna start with farm

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animals equals

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[NOISE]
Run that first of all.

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Nothing particularly new there.

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And, of course we can go for
animal in farm_animals,

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print(animal).

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Let's make a bit of a gap here, print And
let's create a set a different way.

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So we'll do wild_animals = set.

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It's a built-in function and
we'll add some, so a lion.

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Oops Lion, tiger, panther,

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elephant and hare.

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Hare print are animals and

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we can do the same for
animal in wildlife animals,

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print animals.

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So I can run that.

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You can see the two ways to create sets.

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The first one is just again
very similar to dictionary.

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The form for creating dictionary but
we're not actually Supply key there,

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so that's how Python knows that this
is in fact a set, and a second way,

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what we do is we actually pass the list
to the set built-in function, and

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created a set that way.

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And not surprisingly, as you saw on
the screen there using the For loop,

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we can iterate through
the members of a set and

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then ultimately print them out as we
saw on the example on the screen.

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Now, actually, adding a new member
to a set is also pretty easy,

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using the add method, so
let's go back up to here.

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So what we'll do is we'll start the int.

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So we'll type farm_animals.

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Let's add one for
every set that we've created.

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Let's add a horse to our
[NOISE] Animal set and then for

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our wild animals let's add
a horse to that as well.

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[NOISE]
Okay then we can just oops

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print farm animals print wild animals.

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Sign we can run that And,
not surprisingly, you can see we now have

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the horse added both to the farm animal
set and also to the wild animal set.

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And the other thing I want to show you
there as well, if we just check the order

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because there is no inherent
ordering with sets, as I mentioned.

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So if we run it again, Notice how
the order of these are changing each time.

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And that's because of course
the sets are actually un ordered.

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And they can't really
confirm a particular order

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with sets because they are by
their very nature un ordered.

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Now incidentally you work in an era to
use [...] in the right hand margin.

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Set, function, method, call.

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That the set function call is
used to create the wild set.

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The wild animals.

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I'll have you click on that and select it.

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It's basically saying you
could use a literal for that.

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So we're just going or more information
he'll be replaced with a literal.

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So it's just basically saying
if use is set here but

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you could do something like that and

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replace it with that now there are cases
where we have to use the set function.

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And one is if we want
to create an empty set.

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We can't actually create an empty set
using just the empty braces because that

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actually creates an empty dictionary So
in that instance, we have to use set.

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And just to show you what I mean,
we'll comment this code out for now.

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So we can start off by typing empty_set
= set() and that creates an empty set.

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But if we try and
do it this way, empty_set_2 =

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Like so, and we do empty_set.add, A,

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empty_set_2.add, noting
there's no method there.

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If we run this, we get an error,

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and just to confirm,
we can see the error should be on line 22.

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You can see it's on line 22 there.

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And that's because the line on 20,
it actually created a dictionary.

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Which, of course,
doesn't have an add method.

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So that's why you need
to use the set there.

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If you were creating an empty set,
you'd actually physically need to call

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the in-built function for
set to create that for you.

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And when we used the set constructor,
when we used it earlier.

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But you can see online 9
which I've commented out.

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We passed the list parameter but
you can actually use any [.] object

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such as a [.] or a range to do the same
thing so what we do is something like and

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even equals set, range, 042.

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Remember that from our
discussion of ranges.

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Print even.

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We can run that.

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Whoops, I'll need to get
rid of that other error,

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so I'll just comment that second add out,
and then run that again.

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You can see that's created a set based on
those numbers, based on the range there.

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And we're gonna do something
like squares_tuple.

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= (4,6,9, 16, 25) and squares = set

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(squares_turple) then print (squares), so

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I can do it that way,

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and you can see me correctly get
a set there as well So when you're

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creating sets from tuples, ranges, lists,
or literally any other type of object,

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you have to use the set constructor,
rather than using curly braces.

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Now obviously in this simple
example I've got on screen,

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we could do away with the tuple and just
enter the numbers within curly braces, but

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the example is showing you that
a tuple can be used to generate a set.

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And it's most likely you'd probably,
at some point,

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be wanting to do that when
you're programming in Path.

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So hopefully, now you're getting an idea,
seeing these numbers consistently come up.

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There really is no guarantee that any
item's gonna be printed in order,

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and it's really emphasized the unordered
nature of sets in general.

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And as you saw in the first
example with the even numbers

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They're not even necessarily
appearing in order,

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even though they were
actually generated in order.

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A very important point there.

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So, unfortunately their lack of an order
makes it slightly harder to see

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what's going on.

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But, let's continue on anyway, and have a
look at it, now using union, intersection,

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and difference, as well as subsets.

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So, what we're gonna start
with is using union.

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So, I'm just gonna comment this other
code out Union being a function of sets.

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So we'll put our even in again so even
equals set range, so we'll write 42 again.

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Print even, print len even to
establish how many entries there are,

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then we'll put squares.

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Probably should of just copied and
pasted some of these.

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Squares underscore tuple equals 4,
6, 9, 16, 25 again.

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Squares = set(squares_tuple).

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And we'll just do a print(squares) and

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you'll see why we're
doing this in a minute.

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Squares_tuple, print(squares) and

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print(len(squares)) because the number
of elements is going to make sense and

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it's going to be important shortly.

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Now, if we go ahead and introduce the
union commands, we're gonna talk print,

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even dot union squares, and

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print len Even.union(squares).

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So if you run that, and
scroll this across, so

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that you can see that the new set
that we've created on line 38 and

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39 now contains the numbers 9 and 25.

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And they obviously weren't in
the original set of even numbers.

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And incidentally that's exactly
the same set of squares dot union even.

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So we're working back the other
way just to confirm that.

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We can also do a print squares.union

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even You can see everything exactly
the same output by doing that.

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00:09:38,750 --> 00:09:40,290
So that's using a union.

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Now the intersection of two sets includes
members that are present in both sets.

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And again they only appear in
the intersection set once.

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Now you can perform intersection using the
and instead of calling the intersection

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method Well I wouldn't have used the and
on sets part and

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just calls intersection, the in-built
intersection function for you.

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So just to see how that would work, let's
just close down the run window again.

155
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So let's just put a bit
of a print here to put

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a bit of a gap between the other output,
times 40.

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So if we do print Even .intersection

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squares, print.

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Again, we can do it this way, as well.

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We can, using the ampersand, squares.

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But again, using the ampersand internally,

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Python is just using
the .intersection function anyway.

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And likewise, if you do a print squares.

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Thought intersection.

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Even print squares.

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So if you run that code.

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combination.

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We'll run that again.

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You can see we've got
exactly the same output.

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These are the numbers that
are present in both sets.

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And again looking up here and our original
even set, the number 16 appeared there and

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in the second output number 16
appeared and likewise 4 and 4 and 6.

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And all the other numbers only appeared
in 1 but not both of the original sets.

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So, bottom line here is
using the ampersand.

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It really doesn't matter what you
call the intersection method.

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You still get the same result either way.

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Now, sets can also be subtracted, and
there's two ways to specify a subtraction.

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Which way you use is really
just a matter of preference.

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Now, we'll cheat a little bit before we
check that because all though Python's

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quite happily dealing
with unordered lists,

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Us humans find them hard to read when of
course there's more than a few members.

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So to make it easier to see what's going
on, what we're gonna do is use the sorted

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function to sort the results
before printing them out.

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Now this is not necessary
when dealing with sets.

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I just wanna make that clear.

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We're only doing it here to
make it easier for us to see

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what the various methods are actually
doing with the actual sets themselves.

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So subtracting set b from set
a removes any item that exists in

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set b from set a, if that makes sense.

190
00:12:04,130 --> 00:12:07,470
Just to say that again, so
subtracting set b from set

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00:12:07,470 --> 00:12:12,900
a removes any item that exists in
set b from The first set said a.

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00:12:12,900 --> 00:12:15,610
So with our example, so
let's just comment this code out.

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00:12:15,610 --> 00:12:18,520
And type some code in.

194
00:12:18,520 --> 00:12:21,770
And what I'll do is I'll just
copy this first bit of code.

195
00:12:24,270 --> 00:12:26,210
We do need a little bit of that, and

196
00:12:26,210 --> 00:12:28,640
we'll just make a little space down the
end here, so we can see what we're doing.

197
00:12:28,640 --> 00:12:32,740
So we'll un-comment that.

198
00:12:35,600 --> 00:12:37,300
So we're gonna have our even set again,
but

199
00:12:37,300 --> 00:12:43,140
this time we're gonna do print sorted
even to see what it looks like sorted.

200
00:12:43,140 --> 00:12:47,770
Then we'll leave our square tuple
in again, and we'll put Squares =

201
00:12:47,770 --> 00:12:53,790
setsquares_tuple and
then we'll also print.

202
00:12:58,300 --> 00:13:00,820
Source it squares.

203
00:13:00,820 --> 00:13:03,600
So, you've got a handle
on what it looks like.

204
00:13:03,600 --> 00:13:05,930
All right.
So, let's now do a,

205
00:13:05,930 --> 00:13:07,980
we'll put a heading here just
to show what we're about to do.

206
00:13:07,980 --> 00:13:09,230
So, even.

207
00:13:09,230 --> 00:13:10,060
Money squares.

208
00:13:12,810 --> 00:13:15,200
So do that first, print, sorted.

209
00:13:16,320 --> 00:13:21,530
Even dot difference, since this is what's
subtracted, is difference squares.

210
00:13:21,530 --> 00:13:27,650
So, and print sorted even takes squares,

211
00:13:27,650 --> 00:13:33,550
so the .difference and the minus,
the subtract, does the same thing.

212
00:13:33,550 --> 00:13:39,580
And we'll try squares,
cuz we'll do a heading first, so squares

213
00:13:42,950 --> 00:13:47,800
Minus even, so first, why don't we print

214
00:13:47,800 --> 00:13:54,520
squares.difference even,

215
00:13:54,520 --> 00:13:59,780
and print Squares tack even.

216
00:13:59,780 --> 00:14:03,550
The same deal here using
the dot difference or

217
00:14:03,550 --> 00:14:07,600
using the subtract, the difference
methods exactly the same as using minus.

218
00:14:07,600 --> 00:14:12,120
So which way you use depends on which you
find more readable, in general probably

219
00:14:12,120 --> 00:14:15,040
the recommendation would be that
the difference method is better because it

220
00:14:15,040 --> 00:14:18,290
makes it clear that you're working
with sets which you may not know.

221
00:14:18,290 --> 00:14:19,670
If you're just looking at, say, line 62,

222
00:14:19,670 --> 00:14:21,740
you wouldn't know those
variables are sets.

223
00:14:21,740 --> 00:14:23,210
Or if you saw a .difference,

224
00:14:23,210 --> 00:14:26,380
you'd know that that method
is only applicable to sets.

225
00:14:26,380 --> 00:14:27,250
So let's run this.

226
00:14:28,540 --> 00:14:33,040
So what you can see in the first example
is the subtracting Squares from even,

227
00:14:33,040 --> 00:14:37,390
removes the value 4, 16 and
26 from even, leaving only 9 and 25 and

228
00:14:37,390 --> 00:14:41,800
go the other way subtracting even
from squares, remove 4,16 and

229
00:14:41,800 --> 00:14:44,630
36 from squares, again leaving only 9 and

230
00:14:44,630 --> 00:14:48,500
25 So I'm going to stop the video here
cuz it's getting a little bit long.

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In the next video we'll continue our
discussion, and we'll move forward and

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we'll talk about the update difference
method as well, which is very similar

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to the difference method and the minus,
that we've talked about just now.

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So, we'll see you in the next video.