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All right.

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So now let's talk about something
really useful in programming--

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conditional statements.

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So conditional expressions
allow your programs

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to make decisions and take different
forks in the road, something

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I alluded to little earlier,
depending on the values of variables,

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or based on what the user inputs at
the programmer, at the command line,

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or if you have a prompt
or something like that.

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C has a couple of different ways to
express conditional expressions, which

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we also sometimes will call a
conditional branch in your programs.

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And some of these are going to look
pretty familiar to you from scratch,

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so we'll even pull them
up side by side, just

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you can make that analogy in your head.

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>> So, if-- if is a pretty
simple conditional.

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If you recall from
scratch on the right here

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you could fill in that is hexagon
with a blue expression if mouse down

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or if x is less than 10,
or something like that.

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And then, if x was less than 10,
or if the mouse was in fact, down,

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all of the code inside of the
puzzle piece would execute.

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All the things that fit
inside that C shape.

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>> Similarly, do we have
if on the left there.

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If Boolean expression,
which I'm just using

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as a substitute for one of the Boolean
expressions we previously discussed,

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open curly brace, close curly brace.

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So think of open curly
brace and closed curly brace

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as sort of analogous to the sandwich
effect of the if block on the right

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from scratch.

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>> If the Boolean expression
in the if statement is true,

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then all the lines of code
between the curly braces

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will execute in order
from top to bottom.

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If the Boolean expression
is false, we'll

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skip over everything in between
the curly braces, because we only

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want to go down that fork in the road
if the Boolean expression is true.

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>> We can take this one step
further with if else.

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So this Scratch block is
pretty similar to the one

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we saw just a second ago, except
it takes two different paths based

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on what happens.

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So if the mouse was down,
or if x was less than 10,

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we'll do everything that's in between
that first fork, that first C. ,

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>> Otherwise, if the mouse is
up, or x is not less than 10,

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we will do everything in the second set.

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And that's analogous to what you see
here for C. If Boolean expression,

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do the stuff between the
first set of curly braces.

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Else, do the stuff between the
second set of curly braces.

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So if the Boolean
expression is true, we'll

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do whatever's between the first set.

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If the Boolean expression is
false, that would trigger the else,

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and we would do whatever's in
the second set of curly braces.

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Again, top to bottom, all
lines in between the braces.

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>> In C, it's possible to create
an if-else if-else chain.

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In fact you can have if-else if-else
if-else, if, and so on and so on and so

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on.

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In Scratch, this required
nesting the blocks.

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You add an if-else, and you had to
put another one inside of the else,

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and so on, and it got kind
of nested and complicated.

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But C, we don't have to do that.

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We can actually just have
it be a chain like this.

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Again, as you might expect, all of
these branches are mutually exclusive.

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You can only ever go
down one of the branch.

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If this is true.

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Otherwise, if this is true.

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Otherwise, if this is true.

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Otherwise, do this.

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So all four of the branches in this
example are mutually exclusive.

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It's an if-else if-else chain.

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>> It is possible though,
and sometimes very useful,

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to create a chain of not
mutually exclusive branches.

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In this example, only the third and
fourth branches are mutually exclusive.

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It could be that you could
satisfy the first condition,

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and you could satisfy
the second condition,

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and you could satisfy the
third condition-- in which case

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you would go down the first branch,
then you go down a second branch,

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then you would go down the third branch.

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Or perhaps you satisfy the first
condition, and the second condition,

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but you don't satisfy
the third condition.

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In this case you go down the first
branch and the second branch,

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and then the fourth branch,

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>> The reason for this is that the else
will only bind to the nearest if.

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So even though there's
an else here, that

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doesn't necessarily create a mutually
exclusive chain of everything.

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It's only the expression
there with Boolean

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expression 3-- that's the
mutually exclusive with the else.

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So it is possible, and
sometimes quite useful,

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as I said, to create a chain of
not mutually exclusive branches.

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Let's take a look at a different
kind of conditional, which

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you have not seen before in Scratch.

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There's something called
the switch statement.

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The switch statement is
kind of neat because it's

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a conditional statement that allows
you to specify distinct cases,

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instead of relying on Boolean
expressions to make decisions for you.

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So for example, let's say
that I have this program,

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and I'm asking the user
to provide input to me.

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So I say, int x = Get Int(),
and if you're not familiar yet,

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get int is a function that is
also included in the CS50 library,

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so if you #include CS50.H you'll have
access to Get Int() and all of its

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cousins-- GetFloat,
GetString, and so on.

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Basically one Get function for every
data type that we've already discussed.

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>> So Int x equals GetInt.

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Basically what's happening
is I'm at the terminal.

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I'm asking the user to type in a number.

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>> And here I'm switching
what I'm doing, depending

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on what the user typed at the prompt.

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So if they typed one, I print out one.

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And then I break.

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If they type two, I print out two.

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And then I break.

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It's important to
break between each case

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because otherwise you will fall through.

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So if I didn't have any
breaks there, and the user

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typed one, what would happen is it
would print one, two, three, sorry.

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That's kind of strange behavior, right?

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You might think so.

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But there are actually some cases where
this could be a pretty useful thing.

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So here's another example of a switch
statement where I omit the breaks.

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But I do it on purpose.

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>> So what happens here?

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Think for a second.

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You may even want to pause the video.

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>> What happens here if
the user types four?

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So I've asked the user for input.

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And they provide the value 4.

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What gets printed when I do that?

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On the previous slide, there were
breaks between all the cases.

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And so it would just
print four and then stop.

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But in this case, it won't.

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What will happen is you
will fall through each case.

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>> So in this case I've organized my cases
in such a way that if the user types 4,

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I will print four, three,
two, one, blast off.

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And if they typed 5, I would start
at five and do the same thing.

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If they typed 1, I would
just do one, blast off.

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>> So in this case, I'm using
a switch kind of cleverly so

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that I do intend to fall
through all the cases.

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But generally you're probably gonna
want to break between all of them,

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unless you have a situation
like this one where you're

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kind of leveraging the fact that you'll
fall through the cases without a break.

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So that's the second of the major
types of conditional statements.

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The last of which is ?: So I
have two snippets of C code here.

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One on the left and one on the right.

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The one on the left should
probably be pretty familiar to you.

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>> I have Int x.

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And I probably should
have asked the user

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for-- this should probably be Int x
equals GetInt, or something like that.

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And then I'm making a decision.

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If some Boolean expression is
true, assign x the value 5.

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Otherwise, assign x the value 6.

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>> That on the left should probably be
pretty familiar from our discussion

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of If Else just a moment ago.

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Would you be surprised to know
that the line on the right

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does the exact same thing?

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>> So this is called ?: or sometimes
called the ternary operator.

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And it's pretty cool.

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It's usually used as a cute trick.

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>> But what it allows you to do is to
simulate an If Else with really small,

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really trivially short
conditional branches.

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You generally wouldn't use ?: if you
had six lines of code between each set

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of curly braces.

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But if you're just
making a quick decision,

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if you're going to do one thing
or the other and it's very simple,

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this might be an example of how to
do it with ?: the ternary operator.

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So Int x equals expression ?

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The thing after the question
mark is what x's value

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will be if expression is true.

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>> The thing after the
colon is what x's value

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would be if the expression was false.

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So I'm asking myself,
is the expression true?

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If it is, assign x the value 5.

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If it's not, assign x the value 6.

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Again, like I said.

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This is usually just a cute trick.

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And sometimes if you become
really comfortable with it,

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you'll do this because it looks
kind of cool in your programs.

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Generally I'm presenting it to
you now so you're familiar with it

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if you see it.

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But certainly know you don't have
to write it in any of your code.

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But it is something to be familiar with,
because you'll definitely encounter

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snippets of code here and
there where this ?: syntax,

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AKA the ternary operator, is used.

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>> So quick summary on
what conditionals are,

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and what the options are available
to you in C. You have If and if-else,

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and if else if, et cetera.

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You can use Boolean expressions
for those to make decisions.

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>> With switch statements you use
discrete cases to make decisions.

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You would specifically say, if it's
one, or if it's two, or if it's three,

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I'll do this thing, or
this thing, or this thing.

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And ?: can to be used to replace
very simple if-else branches,

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or if-else chains to make
your code look a little fancy.

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>> I'm Doug Lloyd.

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And this is CS50.

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