CS50軌跡2019-iOS-第1課時 (CS50 Tracks 2019 - iOS - Lesson 1)

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TOMMY MACWILLIAM: So let's get started writing our first iOS app.
So all of the iOS apps that we write are going
to be written in a program called Xcode.
So Xcode is an IDE or Integrated Development Environment
that's put out by Apple, and it contains all of the tools
that you're going to need to write apps for iOS.
But before we get into that, let's first take a look at the programming language
that you'll be using to write all of your iOS apps.
So all of iOS apps are written in Swift.
You might see online that there is another language out
there called Objective-C that you can also use, but it's a bit outdated.
And Swift is what Apple currently recommends
to write all of your iOS apps in.
So Swift is pretty similar to C, but there
are a few differences that we're going to go through right now.
The first is variables.
So when you're writing in C and you create a variable,
every variable is what's called mutable.
And a mutable variable means once you have a variable,
you can change its value.
You can create an integer called x.
You can set it equal to 5.
And if you want to change it later to be 50, you can do that,
and that's no problem.
In Swift, there's a distinction between mutable and immutable variables, where
an immutable variable once you declare it,
you can never change that value again.
So think about it as a constant.
And, actually, a lot of the variables that you're
going to create while you're writing iOS are going to be constant.
So if you're creating a constant variable
in Swift, which we're going to do a lot of,
you are going to start the variable with the word let.
If instead you want to make it mutable, you
can start the variable with the word var.
So if you're ever looking at some source code
and you want to know if a variable is mutable or not,
you just have to find where it's declared.
If it says let, it's not.
If it's says var, it is.
So Swift just like C has a bunch of built-in data types,
and they're pretty similar to what you've seen before.
You've got Integers, Doubles, Floats, Strings,
and you notice that all of these are capitalized.
And Swift also has a few other built-in data
types that we'll take a look at like Arrays, Dictionaries, and so on.
So here's our first line of Swift that we can write.
So you'll see here that we have a variable called title.
Because it starts with let, this is an immutable variable,
which means we can't change its value.
If we try, then the Swift compiler is going to yell at us.
And then after the variable's name, we have a colon.
And then we have the type of the variable.
So it's a little bit different than C.
In C, we put the type first and then have the variable name.
Here we have the name of the variable first, a colon, and then
the type of the variable.
So this is a String, and then we have the equals sign and then
the value of the variable.
So in the next line, we're declaring a mutable variable.
So we started it with the word var.
We call this variable count.
And you'll notice here that I didn't give this variable a type.
And this is one of the first new features of Swift
that we'll see compared to C is that Swift can
infer the type of the variable for you.
So because it says count equals 50 and that right hand side of the assignment
is an integer, the Swift compiler can look at that
and say, OK, count must be an integer because it's
being assigned to an integer value.
So you don't need to tell me that it's an int.
I can figure that out for you.
And then in the next line just as we did in C,
we can just change the value of that variable and keep going.
So, next, let's look at conditions.
They're pretty similar to what we've seen before.
So in this first line, we're just creating a new constant.
It's a string variable of the value of iOS.
And then here we have a condition.
You'll notice here that in Swift we don't need
to put parentheses around conditions.
This is kind of nice.
Swift basically says, well, we know the thing after the word
if is going to be a condition so there's no need to put parentheses around that.
So you're not going to see them in any of the code that we write.
You can express or and and in the same way
you did in C with a double ampersand or a double pipe.
And then in Swift to print something to the screen,
you can just use the print function.
It's similar to printf, but you can print out a whole bunch of different
values without needing to do %d, or %f, or any of that stuff.
And then else's work the same way.
Next, let's look at arrays.
So same syntax as before for declaring a variable.
We can say let.
Then we have the name of our variable, which
is values, a colon, and then the type.
And you can see here to create an array of integers,
I'm going to use brackets and just put Int inside of those brackets.
And that tells the Swift compiler that this is an array of integers.
And then to declare the array, just open bracket,
have your values, and then a close bracket.
Again, if we wanted to omit the type of this variable, that would be OK
because a Swift compiler can see the right-hand side of this variable
is a list of Ints.
So that's the type of that's variable.
And then to iterate through this array, Swift
has this cool operator called for-in.
So we can just say for value in values and iterate through this array.
So on the first run of this loop, the value of value will be 1,
and we'll print out one.
On the second iteration of this loop, it'll be 2 and so on.
So it's just kind of nice.
You don't have to do the whole crazy syntax of Int i equals 0.
i is less than values.count. i++.
Swift has this kind of syntactic sugar to shorten that a bit for you.
So another data type that we'll see a lot in Swift is dictionaries.
So you can think of a dictionary as a mapping between keys and values.
So in this example here, I've created a dictionary
that maps string keys to string values.
So you see here in my type that I have in brackets String colon String,
and that says that the type of the keys in this dictionary
is going to be strings.
And then the type of values in this dictionary is also going to be strings.
And then when I declare these values, you
can see that I have my key, so SFO is our key.
And then our value for that key is San Francisco.
So what I'm basically doing is mapping airport codes
to the names of those airports.
So Boston's code is BOS.
So I have a key of BOS and a value of Boston.
Now to iterate through this dictionary rather,
I can use that sort of same syntax as before.
I can say for this time in parentheses code, name.
Here code is going to be the key in the dictionary,
and name is going to be the value in the dictionary.
Then I just say the name of the dictionary itself,
so I'll say airports.
And then I'm going to print out the code and the name.
And you'll see here that we have sort of a new syntax
here where you'll see this backslash and then in parentheses code
and then backslash in parentheses name.
This is something that's called string interpolation.
And what that means is rather than printing out backslash parentheses
code, Swift is actually going to take the value of the variable called code
and print that instead.
So if you're used to writing C with printf, %d,
it's sort of the same thing, just a nicer syntax.
Rather than saying %s, code, we can just put in this backslash in parentheses,
and then it'll print it out for us.
So this is just going to iterate through this entire dictionary.
So it'll first print SFO, San Francisco.
Then it will print BOS, Boston.
So now let's write some functions in Swift.
And just like before, you'll notice that there's this sort of type syntax
that occurs after the thing that you're creating.
So to create a new function, we're going to start
with the keyword func, which is kind of an interesting choice for a keyword.
But they're all going to start with func.
Then you have the name of your function just like before.
You notice here that I'm using camelCase rather than using underscores
to separate words.
Every time I start a new word, I'm going to use a capital letter.
And this is just a convention that Swift has,
and every programming language has its own convention.
But this is what Swift chose.
Then inside of the parentheses are my parameters to the function.
So this function has one parameter.
It's called name.
And just like before, we're going to put the type of this parameter
after its name.
So name is a string, and we're going to use a colon to separate those.
And, lastly, we're going to say that my function returns a value,
and that value is a string.
So we have this little right arrow operator.
It's a hyphen and then a greater than, and then we'll
have the type that this function returns.
So this function returns a string.
So the body of this function is pretty simple.
We're going to use that string interpolation again
by creating a new immutable string variable.
It's called message.
We can't change its value because we declared it with let.
Then we're giving it a value of Hello, and then adding in that name parameter.
So with that variable, we're trying to do two simple things.
First, we're just going to print it out to the screen,
and then we're going to return it.
So one other thing you might notice here is that there
are no semicolons at the ends of lines.
And this is pretty nice.
In C, you sort of had to remember every single line ends with a semicolon.
If you forgot, your compiler would yell at you.
In Swift, you don't need to do that.
Semicolons are optional, and so we're just not going to use them anywhere.
So the Swift compiler can figure out when your line ends
and then move on to the next one.
So now to call this function, the syntax is a little bit different.
So we're going to have the name of the function sayHello and parentheses
again.
But then we're going to specify the name of the parameter followed by a value.
So you can see here we have name colon Tommy.
So we're calling this function with the value of Tommy.
But in Swift by default, you're going to include the names of each parameter
before their values.
And this is kind of nice, especially we have a long function
with three or four parameters and kind of hard
to remember what the order of those things is.
The Swift is much, much easier to read that because each one is
going to be prefixed with what the parameter name is,
so it's a lot easier to read it.
And we'll see a few of those as we write more code later.
So like in C, Swift also has this notion of structs.
And if you remember, a struct is basically a container for data.
It's kind of an object that says, I can have multiple different entries
or fields inside of this object, but it represents one thing.
So here we have a struct called Course.
And here it has two fields.
It has a name of the Course, and then it has the instructor for the Course.
So the syntax is very similar to what we've seen already.
Both of these fields are immutable because they're declared with let,
and their data type is String because we have a colon followed by a string.
So in addition to structs, Swift also has a more powerful concept
called classes.
So you can think about a class as basically a struct
that can also have functions.
So in C when you wrote structs, you basically
only gave them field names like integers or strings.
But it was hard to put a function inside of that struct.
With the class, it's much easier.
And when you add a function to a class, we're going to call that a method.
So let's take a look at an example.
So here we have a class called Person.
Rather than starting off our declaration with the word struct,
we're going to start it off with the word class.
Then the name of our class is person.
And it has one field.
This field is called name.
It's mutable because it starts with the var.
And the type of this field is a string.
So next is our method.
So in Swift, init is a special method that's
reserved for when you create an instance of this class.
So our constructor takes one parameter.
That parameter happens to be called name.
It's string type.
And then the body of our constructor is pretty simple.
All it does is assign this field name to whatever it is the user passed in.
And so this last line here is how we instantiate our class.
So that's how we create an instance of a person.
That instance you'll often see referred to as an object.
So, here, same syntax we've seen before.
We're saying let, which means this is immutable.
The name of our variable is a Person.
We don't need a type.
The Swift compiler will figure that out for us.
And then we're going to invoke the constructor.
So we're going to say Person, which is the name of our class,
and then we're going to pass in that value as we did before.
So here I'm passing in the name of Tommy.
And then later I can simply say person.name,
and that's going to give me back the value Tommy.
So that's how you would create a class.
But now let's add a method since we can do that with classes.
So, again, we're going to see that same syntax as before.
We're going to start off with func.
We're going to call our method sayHello.
Notice, again, I'm using camelCase to separate words.
I'm starting each new with a capital letter.
And then the body of that method is pretty simple.
We're just going to print out the value of that name.
We can sort of assume that that same constructor exists
before, so nothing new there.
But now if we look at the bottom, after I
instantiate that class to call the method,
I'm just going to say person.sayHello.
You notice that this method didn't take any arguments this time
because, instead, we're just using the value that we already
passed in in the constructor.
And so if I say person.sayHello, this is going
to print out to the screen I am Tommy.
So another feature of classes that's really powerful is inheritance.
With inheritance, you can create a subclass of a given class.
So let's suppose that you write out a class.
Here's an example here.
It's a Vehicle class.
Our Vehicle class is pretty simple.
It just has two things.
It has a function that's called wheels.
It returns an Int.
And that's just going to tell us how many wheels this vehicle has.
And then we have another method.
It's called go, and that's just going to print something out to the screen.
It goes fast, so it says Zoom!
And let's say we then want to create a class that represents a motorcycle.
And a motorcycle is really similar to this vehicle class.
We probably want to say how many wheels the motorcycle has,
and maybe this go method is actually exactly the same.
We don't want to change that at all.
So what we can do is we can create a subclass of this Vehicle class
and override just one of these methods.
So here's what that looks like.
So here we have a class Motorcycle.
And it's inheriting from this Vehicle class.
So that's what this colon is.
It's saying that Motorcycle is a subclass of Vehicle.
And as soon as I do that, every functionality
that was in our Vehicle class is now in our Motorcycle class.
So if I didn't have anything inside of this class,
I could say motorcycle.wheels.
That would give me a value of 4.
I can say motorcycle.go, and that would print something out to the screen.
But, obviously, motorcycles have a different number of wheels
than 4 or most of them do at least.
And so what I want to do is just override that one method and say,
rather than using that definition that I specified
before, I want to change that and use this new definition instead.
So you'll notice here that my method now starts with the word override.
And that tells the Swift compiler this method already existed
on that superclass or that parent class, and I know that.
And I want to change what this method does.
So then I'm going to declare the method in the same way, so func.
Make sure it's called wheels.
Make sure it also returns an Int.
And now when I specify a body, this is the body
that will be used if I recall motorcycle.wheels.
So we'll have a value of 2.
And I can say motorcycle.go.
It's going to print out the same thing to the screen.
So a few other concepts to cover before we start writing some Swift code,
and one is this notion of protocols.
When you're writing iOS apps, you're going to see not only a lot of classes
but a lot of protocols.
If you've ever used other object-oriented languages before,
these are called interfaces in most languages,
but Swift calls them protocols.
And you can think about them basically as a list of methods
that a class must implement.
So here we have a protocol called Teacher.
It starts with the word protocol and then it has a name.
And now you'll notice that in this teach function,
we actually haven't specified a body, which is really interesting.
We're basically saying that in order to be a teacher,
you need to know how to teach, but we're not
going to tell you how to teach as every teacher might
teach in sort of their own way.
But in order to be considered a teacher, you
must implement or specify a definition for some teach method.
So to use this protocol, it's kind of the same syntax as before
with inheritance where we can say here's a new class.
It's a CS50.
And a CS50 teacher is a type of teacher, which means
it has to implement this teach method.
And so here we are at implements that teach method.
We're specifying that name of that method.
And we're not going to go into the body of this method
because it's probably pretty complicated.
But, here, in order for this to compile, we have to specify teach.
If we just said class CS50Teacher colon Teacher
and tried to compile that, it wouldn't work because the Swift compiler would
tell us, hey, in order to implement the teacher protocol,
you need to specify a definition for this method.
We'll see a lot of this later.
One last concept cover is optionals.
And this is another thing you'll see a lot when you're writing Swift apps.
When you were using C, remember that you could
assign null to a lot of variables.
You could say I'm not sure what this variable is going to be yet,
so let's start it off with a value of null.
And then if you remember, if you then tried
to use that variable while it was still null, you probably had a bad time.
There is some sort of segmentation fault.
Something happened that you didn't want to.
So Swift really wants to prevent mistakes like that.
And so in Swift, you can't just assign null to any variable.
Instead, null is basically a special type of a variable called an optional.
So in this first line, I'm creating a new variable called name.
We're using var because we're going to need to change this later.
And the type of this variable is a String,
and then you'll see a question mark.
So this says the type is an optional string.
So the type of this variable could be String,
or it could be nil, which is Swift's way of saying null.
But in order to use this variable, I need to explicitly check if it's nil.
So the way to do that in Swift is with this if let construct.
So it looks a little weird the first time you see it.
But what this if let n equals name is saying if name is not nil,
then assign that value to a new variable called n.
And we know for sure that n is not nil at this point
when we're inside of this body.
So we're just going to print it out.
And you'll notice that we're going to print out n instead of printing
out name because n is our string variable, and we know it's a string.
And then inside of this else, this is going to trigger if name is nil.
So if I were to just write this code and then run it,
I'm going to be inside of this else block
because I've never assigned a value to name.
And then it's going to print out that there's no name.
So as you're writing Swift for the first time,
this can feel really annoying because you're really
used to just being able to do whatever you want.
And say, oh, I know this isn't null.
Don't worry about it.
But the Swift compiler really wants to help you make those mistakes.
And so tracking down those crazy seg faults
is not really a thing in Swift because of the compiler
is what's going to prevent you from doing that.
You woke up feeling dangerous, and you want to just say, I have this variable.
It's technically an optional.
Trust me, I know what it is.
Then you can just add an exclamation point to the end of it.
We'll actually see this a few times as we're writing iOS code just
to sort of shorten it.
This is dangerous if this name were actually nil.
At this point, your app would crash.
And Xcode would sort of show you where that line was when you crashed,
so still a little better than a seg fault.
But we'll see this a few times running iOS apps.
But try to really minimize how much you're doing this
because this is how you get app crashes is having unexpected unwrapping
with this exclamation point.
Let's look at one last way to check for optionals.
So this is called a guard clause.
So remember before when we said if let n equals name, we specified two branches.
We said if it is nil, then do this.
If it's not nil, do this other thing.
A lot of the time when we're writing Swift code, all we really care about
is handling the case if it is nil.
And if that's true, we probably want to break out of the function we're in
or display some sort of error message or something like that.
And that's where the guard clause can just kind of shorten the amount of code
you have to write.
So this is pretty similar syntax as before.
Instead of saying if let, we're going to say guard let.
Same as before.
We're saying n equals name, which means that if name is not nil,
we're going to assign it to this new variable called n with the same type.
And then we just have one branch here.
So we're just shortcutting one.
We're saying else do this other thing.
In this case, we're just going to print out there's no name
and break out of the function we're in.
And then after this guard clause, we have
this new variable n that we know is of type string, and we can keep using it.
So you'll see a lot of guard clauses as I'm writing Swift.
I kind of prefer it because let's say that you have got a bunch of optional
and you have a bunch of if lets and then you
know you're sort of indented six levels deep and it can be kind of crazy,
with guards, you can basically say at each point, let's just make sure
that everything is sort of what we expect it to be.
And if it's not, break out of the function.
But if it is, just sort of keep going linearly down the function
rather than having a bunch of these different branches.
So this is sort of a nice syntax to handle optionals and my preferred one.
OK, so let's open up Xcode and write our first few lines of Swift together.
So I'm just going to open up Xcode.
And when I do, I'll see this little splash screen.
And what I want to do is click on Create a New Xcode Project.
So if I click that, then I'll have this nice screen.
In here, you can see Xcode actually has a bunch of different templates
that's built in for me to sort of get me off the ground running with an app.
So we'll take a look at some of these later.
But for now, we're going to come over here to macOS
and click on Command Line Tool.
And what this is going to do is basically
give us the equivalent of those C programs
we had with a little main method that we can just click on Run.
Since we're not going to worry about some of the iOS stuff yet,
let's just focus on Swift for now.
So we're going to do that.
Let's click Next.
And so here is now some options for your project.
So that's going to ask for a product's name.
So we'll just call this a Swift example.
And then it's going to ask you for an organization name and an organization
identifier.
So if you're publishing an app to the App Store,
this is basically where you'd specify the company you're writing the app for
and this organization identifiers, this unique thing
that you'll get when you register an App Store account.
Because we're not worrying about the App Store right now,
don't even worry about that.
So I just put CS50 for my organization name.
And then a common convention with organization identifiers
is it's your domain name backwards.
A lot of programming languages do this, and Swift is among them.
So CS50's domain is cs50.harvard.edu, so I've done edu.harvard.cs50.
And that's my identifier.
For language, we're going to make sure that Swift is selected.
You notice you can actually write iOS apps in C if you really want to.
I wouldn't.
We're going to use Swift.
Then we're going to come over and click Next.
And then it's just going to open up a file finder so you can pick
where you want to save your project.
You notice here there's this Create Git Repository.
If you want to use Git and you're familiar with that,
you can leave this checked.
If you don't, you can leave it unchecked.
Doesn't really matter either way.
So I'm just going to leave it checked.
And I'm just going to put this on my desktop.
And so I will click Create.
And so here we are.
So this is what Xcode looks like.
So in this main area in the middle, this is where
we're going to be writing our code.
On the left-hand side here is where all of our different files in the project
are going to be.
And in the right-hand side, you'll see we'll
use this to change some properties of different objects
we'll create in the project.
So this project is really simple.
It just has one Swift file called main.swift.
If I click on that once, it's going to open it up,
and you'll see here that Swift generated a bunch of code for me.
It has this sort of nice header comment at the top
with the name of the file, the name of the project, my name,
it's copyrighted if you're concerned about that,
and then some really simple Swift.
So the Swift program just starts off this one line import Foundation.
This basically just pulls in a bunch of libraries if we want to use them,
and we will later, and then just a simple print function.
So we know what that's doing.
So let's just run this project that Xcode generated for us.
So up in the top left here is this little Play button.
So if you just click the Play button, what this is going to do
is compile this Swift code into an executable.
And then it's going to run it for us.
So when we're writing iOS apps, this is going to run in a little iOS simulator.
Right now, we're just writing sort of plain Swift, so it's not.
And down here, Xcode just opened up this little thing for me.
I can drag to make it a bit bigger.
And this is the output of this terminal app.
So as you'd expect, it printed out Hello, World!
And then the program ended.
This exit code 0-- if you remember back from C,
you would often return 0 from your Int main function if nothing went wrong.
Swift is basically saying the same thing.
Nothing went wrong.
This just ran, and that was the output.
So we can clear out this print line since we won't need that.
And let's write some code.
So the first program we're going to write is a little sorting hat program.
So what this is going to do is it's going to sort students
into these different CS50 tracks.
So this is the mobile track.
We also have a web track and a games track.
And so we're just going to write a little program that will randomly
assign students to tracks.
So the first thing we want to do is create
a new class that represents a track.
So I'm going to create this class.
I'm going to call it Track.
And each Track is going to have a few different fields.
It's going to have a name of the Track, so I'll say let name.
That's a string.
And you notice that Xcode has this really handy autocomplete.
Basically, as I continue to type, you'll see this.
And so if you just hit Enter, you can take the first suggestion.
It can appear as you're typing really fast if you just hit Enter a lot.
So each track has a name, and then it also has an instructor.
And let's just sort of make that a String too.
Again, we'll use autocomplete.
And then here you'll notice that this Swift compiler is already
being helpful and saying that we have a class,
but we haven't created any constructors for that class.
And because I have a few fields here that don't have a value,
I'll need to create those constructors.
So we're just going to create one constructor.
So we'll say init.
Again, Xcode sort of autocompleting for us.
And we have two parameters.
So we have one called name.
It's a String.
We have one called instructor.
It's a String.
All we want this constructor to do is assign those.
So we'll say self.name equals name.
self.instructor equals instructor.
And there we go.
We have a class.
We have a class representing a Track.
So now let's create our list of Tracks.
So we have these three different Tracks, Mobile, Web, and Gaming.
So let's create a new list called tracks.
If we want, we can give this a type of Track.
We don't need to since the Swift compiler is
going to figure that out for us.
And so we'll open up our new array.
And so now we're going to instantiate three Tracks.
So remember to do that, we're just going to say the name of our Track.
Once I hit open parentheses, Xcode is going to be really helpful
and say here's the parameters to that one constructor you created.
So I just hit Enter.
And it's going to fill all this in for me.
So let's start.
Let's say the name of our Track is Mobile.
And then if I want to jump over to this next place holder
here after instructor, if I just hit Tab, my cursor is going to jump over.
So I can say the first instructor is Tommy.
And then the next Track, let's say is Web.
That instructor is Brian.
And then the last Track is Games, and that instructor is Colton.
And so there we go.
So now I have created an array of Tracks that has three values, and each of them
represents a different instance of this class.
So if I want, I can just sort of make sure that this is compiled.
If I hit Command-B, that's going to build the project
or compile it but not run it.
So I'm just going to hit Command-B. You'll see Build Succeeded.
That means that there were no compilation errors.
Now, if I for example forgot a comma here and then I hit Command-B,
you'll notice that I have this Build Failed.
And this Xcode is going to highlight in red where my syntax error is.
So it's really neat is if I click this little red circle here,
Xcode is going to offer to fix my code for me.
So if I just click Fix, it's going to insert that comma for me,
and then I can move on with my day.
And just to sanity check, if we take out the type of this Track's variable,
if I remove that, then click Build, we're
still going to succeed because Swift is going
to say at right-hand side of this expression is a list of three Tracks.
So, obviously, the type of this has to be a list of Tracks.
OK, so we've got that.
So now let's define a list of students.
So let's say this time we have some list of students.
We know we don't have to give this a type.
So let's just call these three students three totally random names
that are really common like Harry, Ron, and Hermione.
That's our list of students, and we're also
going to want to have some variable to keep track of how we're going
to assign these students to Tracks.
And so a really nice way to do this would be with a dictionary
where the keys in our dictionary can be students
and the values of those dictionaries can be the track that they're assigned to.
So we know that we're going to need to change and update this dictionary.
So what that means, instead of using let as we've
been doing for most of the time here, we're now going to use var.
So let's say my var assignments.
And this time we do need to specify a type because initially this dictionary
is going to be empty.
So because it's empty, Swift has no idea what kind of type it is.
So we're going to say that this dictionary is
a mapping from String to Track.
So the keys are Strings.
The values are Tracks.
But, initially, it's just going to be empty.
So to create an empty dictionary in Swift, that's our syntax,
open bracket, colon, closed bracket.
If this was just an empty array, I could do this, but it's a dictionary.
So we need that colon.
OK, so let's think about the logic here.
So what we want to do is iterate through each element in our students array,
decide on a random Track, and then assign that Track to the student's name
in this assignments list.
So to start, let's just loop over our students.
So we can say for student in students.
Again, Xcode helped you out there with the autocomplete.
I didn't have to specify any types at all.
We just sort of understand that student is now
a String because every element in this students list is a string.
So just a sanity check and make sure on the right track,
let's just print out each student's name.
So we can say print student.
And then we're going to run it.
So if you can just hit Command-R to run.
Compiled OK.
And then, yeah, this is what we expected.
So here we have the students in the order we declared them all printed out.
So that's not super interesting.
So let's figure out how to assign them to a random Track.
So we want to say, let's create a new variable called Track.
And we want to set that equal to a random Track.
We're just going to have a really simple sort here.
And so what we want to do is generate a random number between 0
and the number of tracks that we have.
And this is where Xcode autocomplete can be really helpful.
You can just sort of start typing and then browse the autocomplete
to see what they suggest.
So I happen to know that to generate a random number I'm going to say Int dot,
and then I have this random method.
But if I was also curious what other methods are on this Int,
I can just sort of start typing and then just sort of browse.
And it's going to tell me all about what these methods do
and what their parameters are, but I wanted this random one.
So you can see here that Xcode will tell me
that this returns a random value, then a specified range.
Great, that's what I want.
So I'll hit Enter.
And now I'm going to specify a range in Swift.
So this is kind of cool, but you can say 0 dot dot less than
and then the size of the range.
So this is going to be tracks.
And I'm going to hit Enter.
And tracks.count is going to give me the total number of elements in that tracks
list.
So this syntax is basically a way of saying
I want sort of a range of numbers.
I want the start to be 0, and I want the end to be the size of the tracks array.
So what this is going to do is generate a random number between 0 and 3
exclusive or 0 and 2 inclusive.
So that's my random number.
So now I want to assign that Track to a student.
So to do that, I'm going to say assignments.
As my key, I'm going to specify student.
So this says inside of this dictionary I want
to create a new value for the key student,
and then I want to give that a value.
So then I can just say tracks, and I'm going to index
into that array just like I did in C with my new integer track,
and that's it.
So I notice that those warnings went away.
The Swift compiler is happy with me, and now I've
iterated through those students and assigned them a Track.
So, last, we want to make sure that this works,
so let's just print out the assignments.
So remember before to iterate through a dictionary,
we're going to create a new for-in loop.
But this time you need to specify the key and the value,
so let's say for student track in assignments.
So here now we have two variables.
This first variable has a type of string.
The second variable has a type of track because that's what our map was.
And let's just print out the assignment.
So we're going to say print out.
And then we're going to start using this variable interpolation.
So I'm going to say backslash and parens.
Now I can start typing student.
And what's really cool is Xcode is going to notice that you're
in this backslash parens thing.
So if you start typing, it's going to autocomplete with variable names you
might want to print out.
So if I just say student, you'll see it sort of start autocompleting for me.
I'll just hit Enter.
So there's the name of the student, and then we
can say the student got my track.
And then I don't want to just print out the object.
I want to print out one of those fields.
So I can say track.name and then say with track.instructor.
So there we go.
So here I'm just printing out one line, but I've
used this variable interpolation a few different times, the first one
to print out the value of student.
And then take this object and just convert it into this nice string
by saying track.name, track.instructor.
If we hit Run, we get Build Succeeded.
And there we go.
Nobody is taking Mobile, so we're just going to have to click Run again.
Still no one's taking Mobile.
There we go.
I think I got my favorite student here.
So each time you're running, you're going
to get a different random list of assignments.
And so that's it for our introduction to Swift.
This covers basically all of the syntax details
you're going to need to start writing some simple apps in Swift.
And so in the next video, let's start writing our first iOS app.