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- [Instructor] In this video, I'm going to

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focus on the mutable set
operators and methods.

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And those are operators
and methods you can use on

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a variable that refers to a set object to

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modify the set that
the variable refers to.

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In this particular slide, we're

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looking at some of the mutable operations.

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First, we have the augmented
assignments for the

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union operation, intersection operation,

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difference operation, and
symmetric difference operation.

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In the bullets below each of those,

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we have the corresponding
method names that

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can perform the same operations.

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Now, the key difference once
again between, in this case,

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the assignments and the
methods, is that the assignments

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have to have sets on both the
left and the right sides of

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the operator and in the
case of the left side of the

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operator, it has to be
something that I can modify.

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So, it's typically going to be a

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variable that refers to
an existing set object.

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And on the method case
or in the method cases,

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each of these methods not only
can receive as an argument,

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a set, but can receive any
sequence of arguments and

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turn that into a set and
then apply the operation.

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So, that's some of the operations

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we're going to look at in this video.

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We'll also look at various methods for

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adding and removing elements in sets.

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So, let's go switch over
to a terminal window.

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And, to get us started,
let me go ahead and

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create a numbers variable
that refers to a set object,

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containing the unique
values one, three, and five.

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And let's demonstrate one of
those augmented assignments.

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Now, I'm not going to demonstrate
all of them because they

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work the same basic way as the
mathematical set operations

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we just looked at a few
moments ago, two videos ago.

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And therefore, there's no
need to show them all again.

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But, let's just show that you can,

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in fact, modify an existing set.

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So here we're going to do a
union assignment operation.

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We're going to add all of the
unique values from this set

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into the existing set, as long
as they are not duplicates.

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And to confirm that, we can
evaluate numbers and see that

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the duplicated three, of course,
does not appear twice but

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all of the other unique
values do indeed appear.

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Now, just as we can do that with the

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augmented assignment, we
can also do that with the

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update method called in the
case of a union operation.

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So let's do this, let's use a range call,

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which we know produces
a sequence of integers,

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zero through nine, in this case
because the argument is 10.

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And now what's going to happen is the

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update method will create a
set out of the values from

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zero through nine, all of
which happen to be unique.

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And it will then take all of
the unique items from that set,

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combine them with the numbers set and

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remove any duplicates to
produce the resulting set.

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So, if we go ahead and say
numbers here, you can see we now

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have a set containing the
values from zero through nine.

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So, again, all of the different
augmented assignments and

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corresponding methods will
work similarly to this,

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but of course performing
their individual operations.

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Now, we also have the
ability to add items that

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are not already in a set,
using the add method.

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So, for example, if I
say numbers dot add and

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I give it as an argument,
17, which is clearly not in

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this set yet, if I evaluate
numbers after doing that,

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we can see indeed 17 was added.

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Of course, if I say
numbers dot add and I give

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it a value like three
that's already in there,

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whoops I spelled numbers
wrong, so let's do that again,

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numbers dot add and give it a three,

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and then I evaluate numbers
again, you can see that the

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set has not changed from back
up here in snippet seven.

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Now, sometimes you may wanna
remove an item from a set and

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it turns out there's several
different ways to do that.

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So, let's say we would like
to remove the value three.

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So, if I go ahead and say
numbers dot remove with three as

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an argument, now three is
in the set at the moment so

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this will work, if in fact
the value was not in the set,

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I'd actually get a key
error as a result of that.

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So you can try that on your own.

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But, let's go and evaluate
numbers and now we can see that

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the value three is no longer in the set.

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So you have the remove
method to remove an item and

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if the item is not there, you'll get

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an exception called a key error.

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If you simply want to
get rid of an item and

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you don't care whether it
was there or not, you can

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actually use a method named
discard instead of remove.

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Discard does exactly the
same thing but does not

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cause an exception if the
value is not in the set.

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Now, if you don't care
which value you remove,

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you just want to remove a value,

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perhaps you're working your
way through the set and

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you just wanna grab
one value at a time and

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do something with it, you can use

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the pop method for that purpose.

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And because sets are unordered,
you should not rely on

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what value will come out
next each time you call pop.

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In this case the value zero was returned.

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And if I now go and evaluate numbers,

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I can see that zero is
no longer in the set.

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And finally, if you ever needed
to completely clear out a

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set, you, of course, could
use the clear method as we

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have on other of the built
in collections in Python.

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And if I evaluate numbers after that,

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I can see that I get the
empty set representation.

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And again, because curly braces with

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nothing in them represent
an empty dictionary for

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an empty set, they display that as the

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word set followed by an
empty set of parentheses.