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In this video, we'll look at the
last looping construct in c++.

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It's hard to do while loop.

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The do while loop begins with
the do keyword followed by block

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statement than the wild keyword
followed by the loop control

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expression in parentheses.

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Notice that you need a semicolon at
the end of the do while statement.

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In a do while loop, you execute
the body of the loop while

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the conditional expression is
true, pretty straightforward.

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Note that the condition is checked at
the end of each iteration that makes

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the do while loop a post-test loop.

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It also means that you're
guaranteed that the loop body

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will execute at least once.

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This comes in very
handy in many cases.

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We don't have to force our
way into the loop like we had

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to do with the while loop.

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Let's see an example of using the
do while loop for input validation.

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In this example, we execute the body
of the loop then test a condition.

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So we asked to use it for a
number between 1 and 5, and

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assign it to the variable number.

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Then we test to see if
number is out of range.

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If it is, then we loop again.

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If it is in range, then
the loop terminates.

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You can see how much cleaner
the logic is in this case.

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Also, let me talk for a minute
about the variable number.

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Notice how we declared
it outside the loop body.

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Since number is used in the while
loops condition expression at

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the bottom, it cannot be declared
within the body of the loop.

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If you do so, you'll
get a compiler error.

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In this example, we're declaring
selection as a character type.

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It's outside the loop because
we're using it in the loop

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condition at the bottom.

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So let's look at the loop.

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I always start reading loops at
their conditional expression.

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In this case, we know We're
looping while the selection is

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a lowercase and uppercase y.

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If the selection is any other
character, the loop terminates.

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Now let's see what's
going on in the loop body.

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We see we have 2 doubles with an
height declared and initialized to 0.

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And we asked to use her
for these values and read

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them in from the keyboard.

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Then were calculating the product
of the width and the height

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and initializing another double
named area with that result.

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We display the area to the
user, and then we asked if

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they want to calculate another.

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Note that we have executed
that first generation of

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the loop unconditionally.

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That means we always execute
this loop at least once.

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This makes sense in this case since I
want to be able to calculate the area.

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Now we ask the user if they want to
calculate another, and we read the

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response into the selection variable.

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Then we test this variable
in the loop condition.

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If they entered a lowercase
or an uppercase y, we perform

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another iteration of a loop
and calculate another area.

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If they answer anything else, the
loop terminates and we display thanks.

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The choice between a while
loop in a do while loop is

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it always crystal clear.

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However, if you know that you must
perform at least one iteration of the

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loop, then you should consider the do
while loop as your first alternative.

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Let's head over to the IDE, and
we'll see how we can use a do loop

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to do a real simple menu system.

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Okay, so I'm in the IDE, and
let me share an example of the

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application we're going to build.

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I'll run it now, and you can see it's
a simple menu-driven application.

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There's the menu, press 1, 2, 3 or Q.

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If you press 1., you do this.

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If you press 2, you do that.

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If you press 3, you do something else.

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Obviously these selections
could do any kind of behavior

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depending on your application.

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So in this case, I enter
selection, I press number 1.

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And it says you chose 1, doing this.

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If I press 2.

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It says you chose 2, doing that.

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I'll press 3.

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You chose 3, doing something else.

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Obviously, if I press a lowercase q or
an uppercase q to quit, but anything

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else should give me an illegal entry.

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So let's say I pressed 4, it says
your unknown option, try again.

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Let's say I I pressed Pete.

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Again, unknown option, try again.

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The only valid choices will be 1,
2, 3 in lowercase or uppercase Q.

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So now if I type of Q, the program
terminates and says goodbye.

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Okay, so that's what
we're going to build.

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And in the section challenge at the
end of the section, you're going

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to be using this kind of logic to
build a more complex application.

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All right, so let's start.

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Where do we start?

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Well, we know we need to display
a menu at least one time,

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right because the user has
just like Q or x or whatever

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we choose is the exit criteria.

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So that's a good choice
for a do while loop.

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So basically, we're doing the do
part which is display the menu while

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they haven't quit, makes sense.

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So that's what we're going to build.

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Obviously, we need a
character variable, and

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I'll just call it selection.

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And whatever the user types in will be
stored into that character variable.

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Now let's do the do logic here.

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We've got a do loop.

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And we want to do the body of the
loop while a condition is true.

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Now what is that condition?

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Well, we want to loop as
long as they haven't typed

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a lower or an uppercase q.

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So we can do something like
while selection is not equal

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to a lowercase q and selection
is not equal to an uppercase q.

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Okay, if you were thinking or
here, that would not be right

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and you would test your code and
see that that doesn't work right.

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So let's just write a real
simple test here before we

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even bother with anything else.

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Let's just say cin selection.

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No, man, you know promptly no nut.

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We're just going to grab a bunch of
characters here and see what happens.

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So let's run that.

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Okay, let's type in an a.

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We loop up q, we stopped.

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Cool.

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That's what we want.

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Let's try that again.

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A 1, we loop.

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A 2, we loop.

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A 3, we loop.

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A 4, we loop, again.

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The only time we should stop
is when we get an lowercase

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or an uppercase q, we stop.

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So our logic is correct here.

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Now what we can do as we can
actually prompt the user.

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And with the menu, we can say
cout, and let's just use put a

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new line at the beginning, so it
gives us a new line every time.

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We do a dash of characters here.

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And will do that.

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And then we'll give
them the menu option.

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So you can press 1.

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And if you press 1,
I'll go through this.

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Obviously, this would be add a
record or delete a record or do

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something in the real application.

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So that would be that.

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And I'm just going to copy
this and paste it to save a

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little time here, so number 1.

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Number 2 would be do that.

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Number 3, eruptions will
be do something else.

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And then finally, we can say Q or or
x or whatever letter you like to quit.

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Perfect.

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And we'll give them one more
little prompt here that says.

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and we'll put a new line in
front just a separate,

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we'll say enter your selection.

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A colon, no endline here, so
that looks nicer for them.

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And then we can put the selection,
right at the selection, right.

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So let's run this and that
we should see a menu show up.

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And there you go.

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Do this, do that.

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Do something else or quit.

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So if I press 1, we stay in the
loop, and we display the menu again.

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Same thing with 2, same thing with 3.

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4 gives us the same issue
cuz we haven't checked for

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anything, this illegal, right.

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5P whatever.

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Now whenever I temper Q.

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I stopped.

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So what I need to do now
is decide what do user has

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typed in and what to do?

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Okay, how do I do that?

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Well, it's a lot of choices here.

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We've got literals here.

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We got 1 the 2 the 3.

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These are character literal

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So we could certainly use a switch
statement really, really easily.

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What I want to do is I'm going
to use an if else if ladder here.

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Again, I encourage you to do this with
a switch statement with all kinds of

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logic that we can play around with it.

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So let's do this.

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Let's say if the selection that the
user entered is the character 1.

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Remember, this is a character,
so we need the single quotes.

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So if the user entered
the 1, we simply want to

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say something like cout.

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Let's see "You choose 1 which was do
this or let's just say doing this.

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Put endline at the end.

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Else and we don't need
block statements here.

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We could certainly use them and
makes it easier to modify our code.

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But I wanted to try to keep as
much on the screen as I can.

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So else if the selection is a
2.,

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then what we want to do here.

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And again, I'll just copy
and paste this to save a

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little bit of typing time.

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Then you spell that correctly.

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That's better you chose
2, which was doing that.

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Else and remember your
indentation they could.

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Make sure the indentations correct as
it adds to the readability big time.

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So if the selection is equal to
3, then in this case, you chose

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3, which was doing something else.

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So now what where we at.

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Let's worry about the unknown option
as the bottom else of this condition.

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So if nothing else matches, then
we're going to say, sorry try again.

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But in this case, if you
really think about this 1, 2,

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3 and Q are all valid, right.

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So let's deal with Q.

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I'll say else if selection is
equal to a q or another sit here.

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We need an or where I lis10 to
while we need to demand or selection

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is equal to or lowercase q.

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Then 'in this case really only
want to say is goodbye because

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the loops is going to fail.

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I'm going to be out
of here in a minute.

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Okay, so that takes care of that case.

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So what's left.

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This else to here.

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So what just happened?

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Well, it's not a 1.

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It's not a 2.

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It's not a 3.

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It's not a q.

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It's got to be something that I
don't know and I really don't care.

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It's something that's illegal.

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So this case we'll just say cout.

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Let's say something
like unknown option.

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You spell that right
and maybe try again.

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We could do something like that.

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Let me clear the space up and
let's walk through this and make

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sure we're not missing anything.

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We've got our selection variable
declared that's important.

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We're in the do loop.

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We're going to execute this
code immediately, right.

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This is unconditional the
first time in we're in.

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So we're displaying the
menu, and we're reading the

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selection from the user.

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If the user selects 1,
you chose 1 doing this.

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And let's say we're out
of this if else if ladder.

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We test down here.

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And we loop while the selection
is not lowercase or uppercase q.

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We come back up.

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We display the menu
again if they typed a 3.

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We execute this code right
here, and we keep looping.

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If they type Q, what do we do?

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We simply say goodbye.

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And then this condition
Fires at the end, it'll

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stay longer out of the loop.

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If none of these are true,
we just say unknown option.

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I don't know you've entered.

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Okay, so let's try this.

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And again, I'm in the section 9
workspace in the do while project.

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So let's build and run.

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And we got to clean compile and run.

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So let's try this, press 1, do this.

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You chose 1 doing this.

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Exactly what we want 2.

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You chose 2 doing that.

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3, you chose 3 doing something else.

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Let's say we pressed 4,
unknown option, try again.

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But notice how we don't
get out of the loop.

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We don't want to get
out of the loop, right?

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We're going to ask them to
enter Q if they want to get out.

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So at this point, we can
press an uppercase q.

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And there it is.

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Goodbye and the program's finished.

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Okay.

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Really, really common use case is
this idea of menu-driven programs.

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00:12:59,470 --> 00:13:02,540
You want to display that
menu the first time, ask

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00:13:02,580 --> 00:13:04,109
the user for this election.

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00:13:04,530 --> 00:13:09,100
And basically, using if else
if ladder here or switch

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statement would work here.

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I'll try re-reading this program
using a switch statement.

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You'll see it's pretty
easy to do as well as.

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Same logic really.

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00:13:16,350 --> 00:13:17,890
Okay, so that's pretty much it.

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In the challenge exercise at the end
of the section, we're actually going

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to do stuff when the user presses 1.

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00:13:23,380 --> 00:13:24,780
We're going to use a vector.

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You'll be adding things to
the vector and displaying the

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vector elements and so forth,
depending on what they choose.

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00:13:30,600 --> 00:13:32,200
Okay, so that's the end of this video.

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I encourage you, like I said, walk
through this and we getting change it

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00:13:36,310 --> 00:13:40,260
and change this and see why it doesn't
work, understand that that's really,

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really important. That's a very common mistake.

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All right, I'll see
you in the next video.