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[MUSIC PLAYING]

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SPEAKER: Well, hello, one and all, and
welcome to our short on conditionals.

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We'll see here how we can use
these things called conditionals

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to ask questions in our program
and have our program take

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different paths depending on
the answers to those questions.

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Now, I have here a program
called recommendations.py.

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And the goal of this program is
to be able to recommend some card

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games to a user based on their
preferences for difficulty

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and the number of players
they want to play with.

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So it seems like our program should take
multiple paths, recommending to a user

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the right card game based on
any given input the user might

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give to our program.

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Well, the first thing
we should probably do

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is ask the user a question
themselves to get data

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on how they're feeling about what
kind of card game they want to play.

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So I'll first ask them a
question using the Python input

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function to get some user input.

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And as we've seen before, I
can give as input to input

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a string that will ask, let's
say, a question to the user.

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Maybe the first question is, do you
prefer difficult or casual card games?

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So difficult or casual?

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And why don't we store the result
of whatever the user typed in,

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in a variable, maybe this one
called difficulty, just like this.

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Now, another question would
be, well, how many players

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do you want to play with?

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Do you want maybe multiplayer,
as in more than one player?

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Or do you want a single-player game?

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Single-player, just like this.

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And why don't we, just for consistency,
capitalize these just like that?

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And now if I store this
in the players variable,

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I'm able now to see that I
have two variables storing

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both the difficulty the
user wants the players

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that they might want to play with.

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Now, the question then becomes,
how do we recommend a game based

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on the input the users gave us?

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Well, we have down here a
function called recommend.

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I can simply pass as
input to recommend a game.

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And we'll see in our terminal,
"You might like," space,

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followed by the game name itself.

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But that doesn't help us really up here.

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We can certainly call recommend.

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But how can we depend--

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how can we make sure the game actually
depends on the input the user gave us?

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Well, for that, we
can use a conditional.

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And in Python, to make a conditional,
we have this keyword called if.

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And if is followed by something
called a Boolean expression--

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more on those later--

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but essentially, a question we can
ask that has a yes or no response.

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Now, maybe the first
question I could ask

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is, is the difficulty here equal
to, in this case, difficult?

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That is, did the user type
in "Difficult," capital D?

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I'll go ahead and make sure this
goes on a new block in my code,

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just for style's sake.

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And for now, why don't I leave
this branch of code as just

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dot, dot, dot, meaning I'll get
back to it a little bit later.

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So this is our first conditional.

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We're asking the question, did the
player type in, capital D, "Difficult"

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for difficulty?

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And then whatever code
is indented will run

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if this condition, this
Boolean expression, is true.

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So what else could we do?

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Well, presumably, the user might
not always type in "Difficult."

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They could type in
"Casual," for instance.

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Well, in that case, we could actually
specify what should happen in the case

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that this condition is not true.

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Notice how on line 6, indented here, we
have a branch in our code that will run

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if this condition is true, difficulty
is equal to, capital D, "Difficult."

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But otherwise, else,
if that's not the case,

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we'll run whatever code is now indented
on line 8 and beyond, if we'd like.

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So a good first question
to ask, but now I

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think we should ask another question
too, about number of players as well.

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So if, let's say, the player
typed in difficulty "Difficult,"

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but then we might also
want to ask them, well,

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how many players do
you want to play with?

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That could inform our recommendation.

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Within this branch, I could
also ask another question.

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I could say, if players is
equal to maybe multiplayer,

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dot dot, dot, then we could probably
recommend some given game, in this case.

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But of course, similar to difficulty,
a player or a user could actually type

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in something different
than "Multiplayer."

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They could type in "Single-player."

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So we could make use
of else here as well--

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else, dot, dot, dot.

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So now we're building up the
branches of this program.

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Within this branch of difficult
games, we'll ask the player,

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do they prefer multiplayer
or single-player.

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And if multiplayer,
we'll take this path.

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If single-player, we'll take this path.

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Let's do the same thing down below here,
because even if the player specifies

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casual, well, we still want to make an
informed decision of whether they wanted

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multiplayer or single-player games too.

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So I'll go ahead and ask the same
question-- if players == multiplayer,

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just like this.

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We'll have one branch of our code.

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Else we'll have another
branch just like this.

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And notice now there are four branches,
four places we could end up with,

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which makes sense.

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If we have given the player two
options, difficulty and players,

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each of which has two
possible inputs, well, there

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are four possible games
we could recommend.

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So if a player wants a
difficult multiplayer card game,

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maybe recommend to them
poker, for instance.

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I'll recommend poker.

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If they want a difficult
single-player game,

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we could perhaps recommend
to them maybe a game

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called klondike, which is the classic
solitaire game, if you're familiar.

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But down below, let's
see down here, if they

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want a casual game that is multiplayer,
we could recommend maybe a game

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like hearts.

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And if they want a casual,
maybe, single-player game,

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we could recommend one--

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maybe one like clock, which
is really just luck based.

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So here now, we have
four games to recommend

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based on the input the user gave us.

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And this is our conditional structure.

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We've used conditionals
to actually modify

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how our program runs, having it
take different pathways depending

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on the input the user gave us.

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So let's try it.

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If I were to, in this case, run
python of recommendations.py and enter

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"Difficult"--

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let's say "Difficult"--
and "Multiplayer,"

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which game do you think we should get?

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Well, probably in this case poker.

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Difficulty was "Difficult."

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So we'll be sat inside
of this branch here.

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Now, players is now going
to be "Multiplayer."

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So we'll end up at this branch here.

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I'll hit Enter.

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And we'll see "You might like Poker."

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So pretty good.

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But what could go wrong in this case?

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So maybe the user is a little more
malicious than we think they might be.

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Maybe they don't want
even a casual game.

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They want something that's like
a medium difficulty as well.

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So I'll type in "Medium."

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And maybe they want not just a
multiplayer or a single-player game.

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They actually want a two-player game.

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And notice how these inputs
are not explicitly talked

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about inside of our conditional.

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So I'm curious where we'll end up.

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If I hit Enter here, we'll
see "You might like Clock."

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Now, clock is definitively
a single-player game.

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So we should not have
ended up at clock here.

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But there is a reason that we did.

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So we asked the question here, if
difficulty is equal to, capital D,

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"Difficult," then we'll
do this branch here.

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If that's not the case, though, we'll
be in this branch down below here.

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Now, there are many other options
besides just, capital D, "Difficult"

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a user could type in.

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In this case, I typed in "Medium."

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And because that was not
equal to "Difficult," well,

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we're going to end up in
this branch down here.

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And similarly, I typed in "Two-player,"
which is definitely not "Multiplayer."

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So we then ended up at this
particular branch here.

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So else is good when there are no
other options you want to test for.

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But in this case, I'd argue we want
to test if the user actually typed

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in, in this case, "Casual" or
"Single-player" explicitly.

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So let's see how we could do
that with these conditionals.

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Well, it turns out there's one
more keyword beyond if and else,

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one called elif, which is kind
of an elision, if you will,

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a collision of both "else" and "if."

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And it's a way of asking some question
if the prior condition was not true.

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So here, I did ask if difficulty
is equal to "Difficult," capital D.

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But otherwise, if it's not, elif, we
could ask some other question as well.

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Maybe I'll ask if difficulty was
equal to, in this case, "Casual,"

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just like this, with a capital C. And
that opens up for me some new branch

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in my conditional structure.

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Now, it seems to me that I actually
want to move this conditional structure

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inside the branch that
explicitly asks for casual games.

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So I'll go ahead and copy and paste
this and move it inside this branch.

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And now what should happen if we
end up at this else branch here?

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Well, we know the player
didn't type in "Difficult."

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We know they didn't type in "Casual."

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So maybe we could print
something else entirely.

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We could say maybe "Enter
a valid difficulty,"

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letting them know that we don't really
know how to recommend games that are not

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either difficult or
casual, in this case.

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Now, what else can we do inside here?

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I think there's still the same problem
with these two conditional structures

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here.

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Let's make use of elif here as well.

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I'll say, elif players ==
"Single-player," just like this.

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That opens up a new branch that
I can then move "Klondike" into.

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And then down below, why don't I print
maybe "Enter a valid number of players,"

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just like this.

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I'll do the same thing down below.

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In fact, I can copy and paste this,
for now, down to this branch here,

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our "Casual" branch.

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And now, we want to make sure we
actually change these recommendations

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to "Hearts" and "Clock."

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And I think that should be set for us.

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So what have we done?

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We've made it able for
us to actually check

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if the user has entered a value
different from what we expected.

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We're now more flexible and
less trusting of the user.

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So let's try this.

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I'll go ahead and I will run
python of recommendations.py.

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I'll hit Enter.

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And I'll type in, let's say, a
"Casual" game, maybe "Multiplayer."

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And that should put us
at hearts, let's say.

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I see hearts.

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That's good.

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Now let's be a little more malicious.

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If I type in something like "Medium"
and maybe "Two-player," as well,

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where do you think we'll end up?

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Well, it seems like we should end
up in "Enter a valid difficulty."

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So we'll see.

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Hit Enter.

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And we will see "Enter
a valid difficulty."

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So I argue that this
program should work for us.

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If we were to enter in all combinations
of "Difficult," "Single-player,"

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"Multiplayer," or "Casual," we'd get
the right game that we would want to be

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recommending, in this case.

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Now, there are still some
improvements to be made.

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Notice one here is this
idea of copying and pasting.

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We can probably do better than that.

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But for that, we need to learn
more about this idea of a Boolean

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expressions.

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More on that another time.

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