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In this video, we'll learn how to declare and initialize variables.

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Then we'll head over to CodeLite and declare and initialize some variables in live code.

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The syntax for a variable declaration in c++ is very simple.

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First, you tell the compiler what the type of the variable is.

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Then you tell the compiler what the name of the variable is.

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And finally, you terminate the declaration with a semicolon.

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Later in this section, I'll go over some of the basic c++ types,

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but here are a few as a preview. Int is an integer or a whole number.

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A double is a floating point number.

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And a string is a sequence of characters like you would see in my name Frank.

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Also notice that we can declare variables of non-c++ built-in types.

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One of the very powerful features of c++ is object-oriented programming,

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which allows us to create our own types.

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So we can create variables that represent an account or a person

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using the same syntax as we used for integers and doubles.

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This is great since it makes our program so much more readable.

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In the next, slide we'll talk about some of the rules that you must follow

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when you name variables.

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Remember, in c++ we must declare our variables before we use them.

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So let's take a look at some of the variable naming rules.

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The rules for naming c++ variables are very very simple.

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C++ variable names can contain letters, numbers and underscores.

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However, the first letter of the name cannot be a number,

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it can be an underscore or a letter.

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As discussed previously, c++ keywords are also reserved

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words so you can't use those names.

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You can't use int, you can't use double as your own names.

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Also you cannot declare a variable name that's already been declared in the same scope.

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So if you declared x to be an integer, you can't do that again.

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Remember, c++ is case-sensitive.

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So the name, age in uppercase is considered

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different from the name, age and lowercase. Let's take a look at a few examples.

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In the left column of the table, you can see some legal c++ variable names.

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Since c++ is case-sensitive,

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age beginning with an uppercase A is considered

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a different name from age beginning with a lowercase a.

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We can begin our variable names with underscores or letters.

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And we can also have underscores and numbers within the variable name.

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The right column has some examples of the legal variable names.

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The first example is lowercase int.

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This is not allowed since lowercase int is a reserved word in c++.

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We cannot have special characters like the dollar sign or mathematical characters

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like the plus sign.

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Also notice that variable names cannot have spaces in them.

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Now let's talk about cout. Cout is a little different.

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Cout is a legal name because it conforms to the variable naming rules.

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However, if you include iostream in your program,

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then cout is already defined somewhere else. It's defined by the standard library.

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So it no longer becomes a valid name

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since it's been defined somewhere you cannot re-declare it yourself.

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So the rules that the c++ compiler enforces for variable names

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are pretty simple as you can see.

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Now let's take a look at variable naming style and best practices.

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The most important thing you can do is to be consistent with whatever

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naming convention you choose to use.

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There are many style guides and naming conventions out there.

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In many cases, you'll be working for a company,

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and they have a specific style and any convention that you have to use.

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This has nothing to do with the c++ compiler, but it's still very important.

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For example, google has a published c++ style guide

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as do many other companies.

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One of the first decisions you make with regard to variable naming

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is whether you want to use camel case or underscores to separate words.

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Both styles make the text more readable by avoiding long streams

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of lowercase letters which are harder to read.

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CamelCase capitalizes the first letter of every word in your name

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and doesn't use underscores.

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The other style is to use underscores to separate words.

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Either style is fine, the important thing is to be consistent.

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In this course, I'll use the underscore style.

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Please use meaningful names. Don't use obscure abbreviations. For example,

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instead of using the variable name m-o-e to represent the mass

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of the earth, just use the real name mass of earth,

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with underscores or CamelCase either way.

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Sure it's a little bit more typing but your program becomes so much more readable and maintainable.

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Never use variable names before initializing them.

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Uninitialized variables can cause all kinds of unexpected results in c++.

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I'll show you an example of this when we head over to the IDE.

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Finally, it's best practice to declare your variables

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as close to when you need them as possible.

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In other words, if your program uses 10 variables,

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don't declare all 10 of them at the top of the file and then use them later.

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Instead declare them right before you use them.

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There are many, many more programming best practices, and we'll discuss them throughout the course.

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So now let's talk about initializing variables.

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Notice the first line of code in this example, int age.

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This is a variable declaration that's uninitialized.

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You have no idea what the value of age is.

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And that's the problem. If you try to display age

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or perhaps use it in some calculation,

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you'll run into problems because it was never initialized, so its value could be anything.

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Uninitialized variables are a very common source of programming errors.

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C++ provides three ways to initialize variables.

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All three are equivalent and, they're related to the way that c++ has evolved

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over the years.

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The first is to use an assignment operator. That's the equal sign.

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This will assign 21 to age right when it's created,

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so it will be immediately initialized.

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The second uses parentheses around the value, in this case 21.

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This is a constructor-style initialization.

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We'll talk about constructors when we talk about object-oriented programming.

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The final initialization statement

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is the c++11 initialization syntax.

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This style of initialization is consistent across different types in c++

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and has some benefits that we'll discuss later in this course.

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I encourage you to use this style of initialization.

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Now let's head over to CodeLite and see some of these examples in live code.

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Okay. So now I'm in CodeLite.

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And let's write a real simple program that calculates the area

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of a room in square feet.

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So the idea is we'll ask the user for the length of the room and the width of the room

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and then we'll multiply those two together to calculate the square footage of the room,

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the area of the room. Okay.

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Now obviously, we need two variables. But before we do that,

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let me show you one of the issues with uninitialized variables

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as we talked about in the previous slides.

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Let's create a real simple variable here called age, just like we did before.

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And I'm not going to initialize it on purpose.

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And what I want to do is just output that value

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to cout. Let's build this,

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and I'll execute it. And you can see the what's printing out here.

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You can see there's age. I'm outputting age, and that's the value that's in age right now.

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Not what we expected for sure.

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The problem is that we never initialize the variable age.

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So the value that's in that variable could be anything.

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And in this case, it is anything. Okay. So

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let me show you how that works.

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So let's go back to our conceptual model. Here's our memory.

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I've got a memory location, let's say that that's maybe location 2000 that could be anything.

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And age is associated with that location.

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Now in there, there are a series of zeros and ones.

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That's how we store values in computers.

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Whatever value happens to be in there

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is the value that will be displayed on the console. Remember,

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when I output age,

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the compiler is going to grab whatever this value is in here

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and replace it right in there.

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In this case, we've got that we got that really strange, very large number.

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The idea is we really want to initialize this guy

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to something even zero

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before we use it. So you can see here, I'm using it before I initialize it.

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That's not a good idea.

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Okay. So let me clear this. And

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if we now initialize this to let's say 0 or 21 or whatever we'd like,

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I'll use 21 here.

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Now we'll build and run.

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You can see now it's printing out 21.

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Okay so that gives you an idea. Now also

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when we did not initialize it,

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the compiler did give us a warning. I'll just right click here and compile.

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Notice the warning that the compiler is giving me down here,

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it's saying warning age is used uninitialized in this function.

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That's a clue. Don't ignore those warnings.

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The compiler is telling you be careful. You're using something that you never initialized.

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Okay. So that gives you an idea of what's going on with uninitialized variables.

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All right. So let me erase this, and let's write our program.

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So what we need here is we need to ask the user for

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the length of the room and the width of the room.

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We'll grab those two values from cin,

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then we'll multiply them together, and we'll display the square footage of the room.

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Really, really simple program. But let's get started.

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So what we'll do is we'll say cout, and remember, I don't have to use standard

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scope resolution operator here because I'm using namespace.

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So I'll say enter,

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the width of the room

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and we'll assume this is in feet.

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We'll put a semicolon at the end of that line to terminate it.

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Now we need to read something into a variable, right. So we need to do

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cin into something.

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Well, we need to declare that variable. Let's declare it right up here. We could call it width.

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And it's going to be an integers, let's say. Remember, we've got to say the type,

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followed by the name of the variable, followed by a semicolon.

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This is perfectly fine.

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Width is probably not a great name. Something like room width

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is better.

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It provides much more information to whoever's going to come later and modify this code.

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So now we've got the room width

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and I'm going to enter the room width and I'm going to read it into

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room width.

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Right now, we've got our variable. Now you'll notice a couple of things.

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One is we never initialized room width.

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In this case, it's probably okay because I'm going to read something into it before I use it

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but best practice is let's initialize that.

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We'll set it to zero.

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Now we need another variable, right. We need the length of the room. So I'll just cut and paste this

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to save a little time here.

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And I'm going to say enter the length of the room.

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And we're going to read that into another variable called room length.

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And again, we'll initialize that to zero.

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In this case, zero is fine. You could initialize it to anything that makes sense in your application.

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All right, so now we'll read that one into room length. I'll just change this variable name right here.

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So look at what's going on here. We've initialized two variables,

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room width and room length. They're both integers.

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Again, notice the syntax. The type name is integer,

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followed by the name of the variable. And in this case, we're initializing it.

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I'm prompting the user to enter the width of the room and I'm reading that integer into room width.

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Then I'm prompting the user to enter the length of the room,

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and I'm reading that integer into room length. All I have left to do now is just say cout,

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and I'll say the area of the room

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is and we'll do the calculation here right in line.

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It's going to be room width times

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room length.

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And we may as well say square feet.

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That's it.

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We'll complete this with an end line,

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and I'll clean a little bit of this space up just to make it a little bit more readable.

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That's it. We're returning zero at the end.

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Now let's build and run this.

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We get a clean compile, and it says enter the width of the room. Let's say that the width is 10 feet.

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Enter the length of the room, let's say it's 5 feet. Always remember,

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we know what to expect, I expect a 50.

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Press enter. It says the area of the room is 50 square feet.

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So we get back what we expected. Always

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think about what you're going to get back before you do this. Otherwise,

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you may have errors that you didn't think about

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or press any key to continue. That's it. That's a

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pretty straightforward solution you can see how we use variables, how we can initialize variables,

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how you can read into variables.

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Now a best practice would be

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rather than declare these two variables up here,

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what we could do is we could declare them closer to where we're using them.

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So in this case, this is the first use of room width.

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So i could modify this by taking this line here and I'll just cut it

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and I'll paste it right in here.

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And I'll do the same with room length. I'll cut it from here,

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and I'll paste it right in here.

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And again, I'll just clean up the code a little bit, get rid of some of this extra white space.

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Now you can see what's happening. I'm prompting the user.

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I'm declaring that variable. I'm reading into it.

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I'm prompting the user for the length. I'm declaring the variable. I'm reading into.

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So this is basically what we talked about in the previous slides.

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You can see this is a really simple example.

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Try it out, play around with it. There'll be a challenge exercise

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at the end of this section that's going to extend this a little bit.

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So that's the end of this video. This shows you how to initialize variables,

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use the variables. In the next video, we'll talk a little bit about

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scope and global variables.