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[MUSIC] In this video, we're going to talk
about changing the flow of control of your

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program.
So far, we've been talking about programs

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that execute statements one after another
after another.

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So, you can just take a look at the code
and you can scan down the page and you're

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going to see how the program executes.
Now, that's not quiet true, we have

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introduced functions and functions change
the flow of control slightly.

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When you get to a function call, you
actually go off, you execute the

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statements inside the function one by one,
then you come back and you keep executing

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one by one.
Now,

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This makes your programs a little bit more
interesting but I still can't change the

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flow of control of my program based on
values in the program.

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I might want to do something different
based on data that I have available to me

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inside the program, okay?
So, let me illustrate this with a very

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silly example, okay?
Imagine that I'm a computer and I'm

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walking around running a computer program
and that computer program tells me to give

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away $twenty, alright?
So, I walk up to you, I say, hey, here's

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$twenty, okay, and then I walk to the next
person, hey, here's $twenty.

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I walk to the next person,
Hey,

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Here's $twenty.
Now,

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If I run this program, I'm going to go
broke really quickly,

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Okay?
So, this is not a very intelligent

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program.
I'd like to behave differently based on

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who I'm talking to.
So perhaps I want to come up to you,

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You're my friend.
I know you need $twenty I say, here, go

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ahead, have $twenty, Okay? Then, I walk up
to the next person.

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They're a stranger.
I don't know them.

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I don't want to give them $twenty so I
don't give them $twenty.

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Okay?
I want to change my behavior be, based on

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who I'm talking to.
Now, this is what conditionals accomplish

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in computer programs.
You are checking the value in the program,

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in this case, it would be the person that
I'm talking to in my silly analogy, okay

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Based on that value, you execute different
lines of code.

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So, let's see how that works Alright. To
illustrate conditionals, I want to use my

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silly giveaway $twenty program.
So, you can see here I have a function

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called Greet that takes two arguments.
One is going to be a Boolean, whether or

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not you're my friend.
So, this is going to be true or false.

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The other is the amount of money I have,
okay And then, I'm going to do whatever I

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do when I greet you and I'm going to
return the amount of money I have when I'm

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done and so, you can see down here, I have
three calls to greet and the way I call

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greet is I pass true or false, so this
first call,

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I'm greeting my friend.
And I have, however much money I have.

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When I'm done, I update my money, and then
I print it,

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Okay?
Then, I would greet someone who's not my

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friend over here.
So, this is false.

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And then, I greet someone who is my
friend.

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Okay.
So, I start out,

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Let's say, I start out with $fifteen and I
walk down the street, and I meet three

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people.
What happens?

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Well, I go into debt.
At least I'm friendly.

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I say hi to everyone.
And I'm in debt for $five and then I'm in

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debt for 25,
And then I'm in debt for 45.

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Okay.
I'm not a very good money manager

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obviously, right.
So, let's improve this a little bit.

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Let's make me slightly more intelligent
when I greet people.

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I'm going to check if you are my friend.
And if so, I will do these things, okay?

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So, in Python, the way that we can alter
the flow of control based on the value of

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data is to use an if statement, alright?
And as you would expect from the English

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word if, if this is true, then I execute
the code in this block.

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Like all other Python sort of constructs
here, I end this with a colon, and then, I

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have a block of indented code.
That block of indented code executes,

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alright, if the predicate, which in this
case, is friend, is true, okay?

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So, I have a, the word if, then I have
some Boolean logic expression, in this

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case, it's just a Boolean variable, then I
have a colon, then I have a block of code

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that executes.
Now let's try to run a program again.

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What happens when I run it now?
Well.

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If you're my friend, I would say hi to you
and give you $twenty and go into debt, but

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I meet someone who's not my friend I don't
even say hi, I'm not very friendly to the

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strangers then I say hi to my friend again
and go further into debt.

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Okay.
That program is getting better.

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Now, I don't have to just put a Boolean
variable here.

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Okay.
We talked about Boolean logic last time,

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and there's a reason for that.
I can put an arbitrary logic expression

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here, so let's be even smarter about our
money.

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Let's not just give it away to friends.
Let's only give it away if we have it.

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So, if my, if I have more than $twenty,
alright,, I will say hi to you, and I will

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give you some money,
Alright? Well, let's try this.

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Hm.
I am no longer very friendly, right?

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I didn't give anybody money.
Why?

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Cuz I never have $twenty.
So, this statement always ends up being

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false,
Right?

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The friend may be true or false but
because it's in an and, if money is not

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greater than, true than twenty then, this
is going to be false, always making the

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and expression false.
Alright, this isn't really what I wanted.

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So, I can put.
Something else, right?

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I want to do, I know what I want to do if
you're my friend and I have more than

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$twenty, Butif you aren't my friend or I
don't have more than $twenty, maybe I

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still talk to you.
Don't have the exclamation point.

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I'm not as happy.
I don't have as much money,

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Alright? I'm going to do this otherwise so
else is a Python construct with a colon

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after it again, right, and after the
colon, I can have an arbitrary block of

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code that can be as many statements in
here as I want.

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But else is what you do if the predicate
is not true.

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So, if the predicate is true, I run this
code.

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If the predicate is not true, I run that
code.

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Okay So now, what's going to happen?
Well, at least I say hello to everybody.

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But I'm still not giving anybody any
money, right?

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I even say hello to strangers, you know,
Should I really be doing that?

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I don't, I don't really think I should say
hello to strangers.

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So, let's structure this even a little
bit, slightly more different, alright?

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There's another construct I can use, and
that's called elif,

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Alright?
Let's say, elif friend,

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Then I talk to you, I say hello.
Otherwise, you're not my friend.

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Well,
I've been friendly throughout this.

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Let's be mean.
I'm going to print uh-huh,, and I'm going

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to mug you.
No, I would never do this, okay?

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But in the program, let's just pretend I'm
going to take your money.

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Alright?
So now, what happened here?

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What did this elif do?
Okay.

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So, first, we execute the if statement.
We check if the predicate is true.

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If the predicate is true, then we execute
the block of code underneath the if.

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And that's it.
Then, we skip down here to return money,

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Okay? When the predicate is false in the
if statement, we look at the next one,

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elif, and this is a shorthand for else if.
So, else, if this predicate, which is just

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friend is true, then I execute this block
of code.

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And I can have as many elifs as I want.
I can only have one if and one else but I

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can have as many elifs in between if I
want.

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And I don't have to have any elifs and I
don't have to have an else, right?

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So, I print hello here.
If this one, this predicate was false,

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then I go down here, else, else always
works.

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So no matter what, okay,
So this means,

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If you're, if I don't have more than
$twenty and you're my friend or if you're

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not just my friend, then I'm going to take
your money, alright? That can be very nice

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alright?
Now, let's run this program, what happens

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now?
Okay, first person I walk up to is my

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friend, I say, hello, I'm a little bit
sad, I don't have a lot of money so I keep

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my $fifteen dollars then.
Second person, [laugh] I'm going to take

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your money Now, I have $25.
Last person, hey, I'm pretty happy Now, I

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have $25, I say, hi and I give you my
money alright?

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So, you can see that I can alter the flow
of control of the program based on values

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that are in the program.
So based on the value of friend, and

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money, in this case.
And it doesn't matter that there are

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arguments in the function.
These could be anywhere.

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They could be local variables, they could
be global variables.

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I can change what code gets executed.
This is a very powerful concept.

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I want to show you another example of
conditionals to help you get the hang of

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them.
Now, in this program, I basically am going

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to test if a particular year is a leap
year.

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And so, I wrote a function called
is<u>leap<u>year..</u></u>

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That's a programmer's convention when
you're just testing a predicate with a

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function, you start the function name with
the word is.

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Basically, I'm testing year and returning
true or false based on whether or not it

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is a leap year, okay?
And this function is just one conditional

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that has several cases, alright?
I'm testing three different things and

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returning true or false based on the
results of those tests.

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Now, leap year test is actually pretty
simple if you ignore the corner cases.

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I need to just check is the year a
multiple of four, if so, it's a leap year

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otherwise, it's not.
So, you can see up here.

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I test year modular four could equal zero.
That means if I divide the year by four

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and there's no remainder, well then, it's
a leap year so I return true and if it's

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not, I exceed the else clause and return
false,

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Okay?
However, there are these two extra cases

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that if the year is a multiple of hundred,
then it's not a leap year unless it's also

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a multiple of 400,
Right?

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Now, you'll notice that I have to execute
these checks in this order,

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I will not get the right answer.
I have to first check 400, then 100, then

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four, and then do the else on. I invite
you to experiment and move these around

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and find out whether or not you get the
right answer,

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Right?
You know that you won't, cuz I told you

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but you should at least convince yourself
that you understand why.

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I also have a conditional down here that
shows, allows us to print out a nice

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message based on the result of this leap
year function.

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Okay, so let's run this.
Is the year 2100 going to be a leap year?

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No, it is not.
Is the year 2000 going to be a leap year?

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Yes, it is.
Is the year 2012 a leap year?

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Yes, it is.
Is the year 2009 was that a leap year?

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No it's not.
Okay. And you can play around with this

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and see that it does catch all the cases
by executing these conditional in order

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the if, elif, else, you're going to get
the right answer in all cases.

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There are very few interesting programs
that execute directly in a straight line.

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So, the concept of conditional control
flow that I've introduced in this video

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are critically important for pretty much
all computer programs.

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You don't want to give away $twenty to
everybody you meet, do you?

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[laugh] I certainly don't.
Okay.

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So, I need to be able to have the ability
to change what I do based on the values of

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data in the program, okay?
And the ability to use arbitrary Boolean

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logic and predicates gives you a powerful
and flexible mechanism to do so.

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So, you're going to be using conditionals
to change the code that you execute in

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your programs throughout the rest of this
course and throughout your programing