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All right.
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So in the previous lesson, we managed get our calculators start accumulating numbers and were able to
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use any of these buttons, say,
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plus, minus, times, or divide
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to start accumulating a different number.
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So we're ready on our way to building our calculator.
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Now, in this lesson, I just want to show you there's one thing that we could do to make our code a little
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bit more secure and less prone to unexpected errors,
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and that is by making this variable which we're using to track whether if the user is finish typing number
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to a private variable. And in order to make it private, we just add the keyword private in front of the
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keyword var and this variable is now private to its current scope.
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So what that means is that if you look inside the parent block, so this variable is only accessible between
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these two curly braces.
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And if you try to access it from anywhere else,
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so, say, for example, if I was to go and create a new file, let's create a new Swift File and let's just call
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it Test because we're going to delete it very soon. And inside Test, I'm going to create a new class and
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this class is going to be called Test and we're not going to inherit from a superclass. We'll delete
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that. And inside test, I'm going to create a new object of our viewController
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and this is going to be equal to viewController.
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So in the init of this class, I'm going to tap into that brand-new viewController with the lowercase "v"
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and I'm going to try and tap into its isFinish,
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so you can see it doesn't exist, right?
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It's not letting me get into that private var at all.
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But let's see if I delete that Keywood "private," and make sure that you save this viewController file
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while your cursor is inside the file.
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And now, if I head over to my Test.swift, then I can say viewController.isFinishedTypingNumber,
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and you can see it's now accessible.
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And, again, if I just go ahead and add this private over here and hit save again,
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then over here, you'll see that this will soon turn into black.
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Then if I hit command B to force it to build, then you can see that it says, "'isFinishedTypingNumber'
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is inaccessible due to 'private' protection level."
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So what exactly is this whole private protection level anyways?
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So Swift has something called Access Levels. And a good analogy for access levels is kind of like the
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clearance level that people working in the government, for example, have.
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I don't know about you, but I watch a lot of shows that involve the CIA or the FBI or, of course, 
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Jack Bauer.
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And what you hear very often is something like, "You don't have clearance to view this file,"
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right?
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And there are certain things that our top secret and only a select number of people can have access
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to it and see it.
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So you know common things are: are there aliens?
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Did we actually land on the moon? And all of these things.
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But this is exactly what Swift Access Levels do.
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Now, while most programming languages have different access levels, they are slightly different from language
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to language.
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Now, Swift 4.2 and above have five different access levels or five different tiers of clearance if you
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will.
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And the first here is private.
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This is, as the name suggests, very, very limiting in scope and it means that when you create a property
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or method inside a block of code, then that property or method is only accessible in between the curly
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braces of that block of code.
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Now, the next least restrictive is something called fileprivate, and as it suggests, it means that your
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property or your method is only accessible inside the file that it was declared.
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So this will be your ViewController.swift or your Test.swift, and you can access a property or
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a method from a different file.
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Now, the third one is what's called internal and this means that your property or methods are accessible
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anywhere inside your current app module.
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So this is also the default level of access that all properties and methods are given.
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And it means that if you don't
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add the key keyword "private" or "fileprivate" in front of your "var' or your "let," then
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this is exactly what it will be, it will be internal access. And it kind of makes sense,
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right? By default, Xcode lets all the files in the app access all of the other properties and methods
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in the app.
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Now, the next level is public.
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And this means that access is granted to other modules.
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Now, when we were working with CocoaPods, you saw that we had a pod module and we also had our app module.
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And this is exactly what the public access level was created for.
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It's when you're creating frameworks or APIs, or SDKs, or libraries, basically, a bunch of code which
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you want other modules to be able to tap into and use.
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Now, the final one and the least restrictive access level is called open, and this is basically like public
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plus,
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right?
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This is everything that you get in the public access level, namely the properties and methods, and everything
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can be accessed by other modules.
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But also it allows your classes and functions to be subclassed and overridden.
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So it basically means that anybody can do anything they want
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when you label something as open. Now, for your standard app development, you will only use one to three.
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That's the only ones that we concern ourselves with.
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But for developers who are working on frameworks, library, SDK development, then they will be looking into
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using four and five.
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And this is because their module that they're working on their library or their framework is going to
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need to be exposed to other developers.
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And the only difference between these last two levels is that public doesn't allow the class or the
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function to be subclassed or overridden while open,
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basically, lets you do whatever it is that you want.
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So if you take a look at, for example, the Clima project that we built previously, and you look inside
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the Pods modules, so everything that has a blue icon is its own module.
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And remember, we mentioned that the internal or rather the default level of access, everything gets, is
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module white,
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right?
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So that means everything that you declare within Clima will have access to everything else that is
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inside Clima.
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Now, in this case, when we're using pods, we actually have two modules now, right?
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Pods and Clima.
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And inside the Pods, one of the ones that we've incorporated is Alamofire.
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And if you look inside the Alamofire code, you can see that there's public all over the place.
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Literally, all of the methods that are needed when somebody incorporates the Alamofire pod is going
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to be marked with public to allow this module to tap into this file
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and these methods.
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So coming back to our code, the best practice when you're thinking about access levels is to by default
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just give everything a private keyword whenever you write a var or let or func,
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so any of your own properties and methods, just write a private in front of it as well.
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Now, in a lot of cases, it can't be private because you need to use it somewhere else outside its current
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scope.
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But if you start off with private and you level up to file private or to internal, then that means that
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you are starting out by being extremely restrictive and extremely safe,
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and you'll only expose your properties and your methods a little bit more as and when necessary.
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So what are the advantages of marking something private anyways?
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Well, it means that you can't accidentally modify something that is not meant to be modified by a different
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class or from a different place.
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So inside Test that I created earlier on, let's say that I tried to change is typing number to
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true,
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so now if we didn't have this variable marked as private, then this other class Test could legitimately
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modify this property isFinishedTypingNumber to true when it's initialized.
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And this is definitely not what you want, right?
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We don't want this tracker to be modified by anybody else because nobody has any business in messing around
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with the state of this Boolean.
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It should only be accessed by this current class and only when the typing is indeed done,
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so, for example, when the calc button is pressed or when the calculator is initialized from scratch.
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So this should be something that is completely private to this class.
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And so we should, in fact, market with a private to prevent accidental modification by other classes.
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Now, this can happen accidentally.
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So, say, you forget that, you know, what was the purpose of isFinishedTypingNumber, or in another case,
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if you're working and you're collaborating with other developers, they might have used this name for
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something else in a different class, and they accidentally modify the one in the ViewController class.
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All of this can happen accidentally.
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So to prevent that and to make your code safer, try to use access levels by starting out with private
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for all of you "vars" and "lets," and methods, and leveling it up only when it's necessary.
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All right, so let's go ahead and delete that test file because we won't need it from here onwards.
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And I've got a challenge for you just so that you can play around with the code, so that you can better
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understand these ideas that we've talked about in this lesson on Swift access levels.
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So this is a Mac command line tool that I've created and it's got simply two files.
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One is called AFile and inside AFile, there is a class, and there is also another class in the same
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file.
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You can see where what I'm getting at.
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Now, there's also another file which has a class called AnotherClassInAnotherFile.
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And inside AFile, I've created a private var, fileprivate var, and a normal var which remember is going
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to default to internal.
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Now, just for comparison, I've also created aLocalVariable because, remember, when we're adding access
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levels,
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we're only able to add it to our class properties which are, in fact, Global variables.
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They're not the same as local variables.
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So I want you to go through these steps 1 through to 10 and try to complete all of the instructions.
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Now, in order to do this, you're going to need to initialize some objects, you're going to need to call
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some methods. But, essentially, through completing these 10 steps, I want you to be able to see for yourself
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just what these access levels do.
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And the ones that we're going to be exploring are the ones that you're actually going to use as a app developer.
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So from private, fileprivate, internal, and local.
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And once you've completed this exercise, you should have dissected the differences between all of these
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and understand exactly how they work and what their scopes are.
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So let's go through Step 1 together, just so I can show you how to do this.
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So Step1 the instruction is "Try to print aLocalVariable Here."
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So you can see I've already created aLocalVariable as a var inside methodA
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inside a class.
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So in order to print it here, all I have to do is say "print." Now, in my print statement, that's going to get sent
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to the bottom of my debug console.
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I want to know where it came from.
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So in addition to simply printing my aLocalVariable, I also want to say aLocalVariable printed from
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methodA in AClass.
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So, now here's my print statement and I have to make it work.
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Now, I have to make it actually run.
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So if you remember from previous lessons when we dug into intermediate Swift, we can use the Mac command
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line tool by calling commands
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inside this main.swift.
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So in order for this methodA to be carried out, I have to initialize a new object from the class AClass,
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and then call the methodA on that object.
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So I can do that by saying let aClass = AClass and we'll initialize a new object from that, and then
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I'm going to tap into that lower case aClass object that I just created and I'm going to call the method
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methodA on it.
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So, now what should happen is once I run this command line tool, it should go in and try to create a new
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object from this class, and then it will run all the code inside methodA which should, hopefully, print
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on a local variable to the debug console.
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So let's give it a go.
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All right. Great.
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So it says local variable printed from methodA in AClass program ended with exit code.
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So that's the end of our execution
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and that is exactly what we wanted to happen.
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So step one succeeds.
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So next to it will mark that it is possible to print aLocalVariable within methodA within AClass.
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And now I'm going to go ahead and either delete or just commented out so that you take it out of the
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functioning code because if we get loads of these print statements, as you can see we've got Steps 1
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through 10,
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it'll get very confusing in the debug console.
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Now, this is completely up to you whichever way you find easier,
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but I find through testing that shouldn't get a bit confused when there's like 10 print statements.
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So just go ahead and comment this out
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if that helps you.
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Now, in the next lesson, there is a quiz where we will test to see whether you've actually managed to
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make all of these work because some of these steps are indeed possible, and some of the other ones, because
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of the restrictions in access levels, are not possible.
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So the idea here is that before you start executing the code and running it to prove to yourself whether
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if it's possible or not, I want you to go through this step by step and try to predict whether if they should
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be possible or not.
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Because as always,, I think the best way to learn programming, especially, is to predict what should happen,
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and then run the code and see what actually happens.
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And that difference between your prediction and the actual reality narrowing that gap and trying to
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make it so that there is no difference between your prediction and reality is how you really level up
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as a programmer.
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So Step 1 of completing this challenge is go through these Steps 1 through 10 and see what they're
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trying to ask you to do,
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and then using your knowledge of Swift access levels, try to predict whether if they are possible or not
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in the locations that they have been specified in.
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And then the next step is to go through Steps 1 through 10 and try to carry out the instructions to print
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whatever it is that you need to print at the location where you're asked to print it. And then you might
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need to initialize objects or call methods in order to make it show up in the debug console.
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So as we're in the advance Swift section, I'm not going to give away any more solutions, and it's time
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for you to mess around with the code and really learn through doing.
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So pause the video now and download this file from the Course Resources List and go ahead and try to complete it.
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Once you are done,
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head over to the next lesson and complete the quiz, so that you can check to see if you've got all the
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answers right.
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All right.
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So have fun.
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Go ahead and do that now.