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Now with our main.dart file now being totally empty, I will first of all walk through some programming
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and Dart fundamentals before we continue rebuilding our app from scratch.
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Now if you already are a more experienced developer, if you work with JavaScript or Java or C#, then
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you will already know a lot of the things I'm going to explain.
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Still, there will be some Dart specifics and therefore I would recommend that you don't skip this lecture
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but instead maybe just ramp up that playback speed if it's boring to you.
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If you're brand new to programming or you've never programmed before, then you should definitely
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follow along.
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Now for these basics, we'll not stay here in our project at the moment because we couldn't really execute that code
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here since we have no working app at the moment
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but instead you can google for DartPad and that is a web-based playground that allows you to write
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some Dart code and run it in the browser
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and that's exactly what I have here,
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this is a little demo starting project which we started with but we'll write our own code here in a
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second.
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So this is Dart code, not a Flutter app,
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just normal Dart code, you could also use Dart for web development,
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this is just some dummy Dart code where we see the output here on the right in the console and it's
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perfect for learning the basics about Dart.
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Now what do we have here?
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The overall thing we have here, this main thing, that is a so-called function. Functions are a crucial
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concept in pretty much any programming language you're going to learn,
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functions are code snippets which you can execute multiple times and at anytime you want. This function
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here is named main because the thing in front of the parentheses here is the so-called function name.
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The parentheses themselves are always required for every function you defined and they allow you to receive
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so-called arguments that would be input to your function
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and this function simply doesn't take any arguments,
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nonetheless it needs these parentheses to turn it into a function.
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Then we have curly braces,
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the green ones here right now,
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curly braces surround the so-called function body and that is the code which executes when this function
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gets called
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and I will show you how to call a function in a second. Void here,
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that is a so-called type. Dart is a typed language which simply means that everything in Dart
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has a type,
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here that is the type of data which this function returns.
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This function returns nothing and therefore the return type is void,
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it's empty so to say. Here we see a number type and this strange for thing to which I'll come back later,
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this would be the type of i, i holds a number, a so-called integer but more on that in a second,
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let's not learn too much at the same time.
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So this is a function called main,
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it has no arguments and it returns nothing and therefore has a return type of void,
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this is the function body. Main is a special function name, generally you can give your functions any
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name you want but main is a special name because main is the entry point of a Dart application. Dart
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automatically calls the main function for you when the app starts and
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therefore in a Flutter app when you add main in the main.dart file which is also a special file for
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which Flutter will look for, when you add a function called main here by adding parentheses and
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by adding void in front of it,
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this is the first function which Dart and Flutter will execute automatically when your app launches.
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So into this function,
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you need to put the code that basically kicks off that UI rendering work that brings something onto
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the screen,
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more on that in a second.
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So that's a function, that's a function
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and main is a special kind of function. Now because it's a special kind of function, you probably also
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can create other functions, right
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and you can. Let me first of all remove that code inside of the function and let's add a new function
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above the main function and now I'll name this add or addNumbers, that name is now totally up to you but
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you should follow a naming convention which is called camel case where you have only one word, so no
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blank in between,
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that would actually be a syntax error but you have one word
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and if your word actually consists of multiple words, like this one add and numbers, then all words should
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always start with a capital character even right inside of the word
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but the first word always starts with a lowercase characters. So you have addNumbers,
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if this would just be called add, then you would also start with a lowercase character but any other
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word in there has an uppercase character, that's just a naming convention which we use in Dart and
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also in many other programming languages which you should memorize and use, so that when other developers see
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your code, they have an easier time understanding it. Now as I mentioned, a function can take arguments,
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so an input.
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Now the main function didn't take any arguments but let's say addNumbers is a function which should
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hold some code to add two numbers and therefore this will now take two numbers as an input.
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So between the parentheses here, we can add num1 and num2 and just as the function name,
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these names are also up to you,
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you can name that whatever you want and as you see here, you simply add commas as a separator between
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your different arguments in that list of arguments here.
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Now you also need a function body and you add that by using curly braces,
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that simply tells Dart that in between the curly braces, you have the code that should execute when this
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function gets called,
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so when you instruct it to execute the code inside of addNumbers.
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Now you do instruct Dart to execute this function by using its name,
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now in the main function which is the code that first runs when the app starts.
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So if here, in main, you simply call addNumbers by its name
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and now here you also need to add parentheses.
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You always need to do this, even if the function takes no arguments,
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this function however does take arguments so now between these parentheses, you add the concrete values
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which will be passed to addNumbers.
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So the num1 and num2 do have actual data with which they can do their work,
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so here we could add 1 and 2.
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Now of course this function doesn't do anything at the moment though,
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I call it with 1 and 2 here
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but we have no logic in there to really add these numbers.
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So what we can do in here is we can call num1 + num2 and now we would add that.
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Now we also need to add a semicolon, that's another thing in Dart, for every expression you have in your
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code, for every code line, you have to add a semicolon in the end, exceptions from the rule are the definition of
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functions where you don't add a semicolon after this curly brace here but you do add it here for all
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the code inside of your functions
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so to say. And in general, for all these block statements as they are called,
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so these statements where you have code between curly braces, you don't add a semicolon after the curly
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braces, you only add semicolons after the expressions between curly braces.
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So here we have num1 + num2, I added the semicolon here
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but the problem we now have is we add two numbers here
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and that would generally work but we're not doing anything with the result of this operation.
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Now you could think that it should automatically be output here on the right,
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well let's see.
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Let's click run,
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what happens is that we have an empty output here on the right.
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Huh,
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so that is not really helpful.
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Now in programming, you always have to be very clear about what should happen because the programming
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language, Dart in this case, does not make any assumptions.
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Why would it print it here,
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you don't tell it to do that and therefore it doesn't do it.
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Now you can print the result by using the print function, print is not a function we defined, it's a function
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that's built into Dart
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and you can wrap
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num1 + num2 with print
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and if you now hit run, you will see three here on the right,
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so now this works and now we're outputting the result of num1 + num2. So this is our first little piece
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of Dart code and yet we can improve that,
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for example we don't use types here but Dart is a strongly and a strictly typed programming language.
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That means that you have to be clear about which type of data your arguments are or this function returns,
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that is something which helps the Dart compiler to yell at you when you write code that violates that. If
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you don't assign your own types which will do in the second, then Dart will assume that this is of the
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dynamic type, a type built into Dart which as the name suggests doesn't tell too much about the type,
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it basically accepts any value and therefore it won't really help you that much with checking that you're
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writing proper code.
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So if possible, you should avoid that dynamic type and assign explicit types when working with Dart. Now
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there are a couple of built-in types in Dart.
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Everything in general is a so-called object, which is a data structure that has some complex
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logic inside of it,
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possibly at least and we'll create such objects later too
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but there are certain special types of objects you could say. There is text, for example and you create
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text by using quotation marks.
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So here if I type hello and I print this, please note the quotes, you can use single or double quotes,
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that doesn't matter but you shouldn't mix them,
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so if you open it with a double quote, you also have to close it with a double quote. I will use single
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quotes in this course though and this creates a so-called string data type,
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so this is a value which is of type string, text simply is a string.
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If we now hit run, you will see hello being printed because
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I'm printing this here and therefore we see the output here. So text strings are one data type.
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Besides the text, for our numbers we have so-called integers and we have floats or doubles. Integers are
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numbers without any decimal places, like 29 or 43 or -10. Floats or doubles are
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numbers with decimal places like 29.99, -10.56, something like that. So
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here, these two numbers would actually be integers.
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Now in this function here, we should be clear about the fact that this only works with integers and
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we do that by adding int in front of each argument.
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This tells Dart that num1 and num2 both have to be integers.
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So if I now run this, it will still work
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but if I would pass 2.5 or 2.6 here, then we actually automatically get an error here in the bottom
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right corner
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that an argument of type double can't be assigned to a parameter, parameter is basically a different word
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for argument, of type int
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and that is why Dart does use these types, because it allows us to write cleaner and better code. If we
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know that this function should only work with integers, then we should define it as such and if we then
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later somewhere else in our code accidentally call that with a double, we get an error and we get that
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error before we ship our code into the Flutter app,
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so into the app stores for example and that is of course good, we want to catch and fix errors
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whilst we are developing.
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So here, we would get an error.
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Now we have two possible fixes,
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either we fix our code such that we only call that with an integer value or if we see no no no,
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this should actually also work with doubles,
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well in such a case, we could of course change that and accept doubles as inputs
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and now if we run this, this code would work again and it would output a double here with a decimal place
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because now we allow doubles here.
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Now of course you could think, why don't we always use a double then, it's more flexible than an int?
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Well sometimes you have situations where you really only want to have numbers without decimal places,
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let's say you have some counter, which only increment by one at a time,
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well then you should be clear about that being an int so that you can't accidentally run the code with
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an invalid value with a decimal value.
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Now side note, we could have also used num here which is the overarching number type and that would allow
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both ints and doubles but as you see, if we use double here,
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this also allows us to call this with a 1 because this implicitly is converted to a double.
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So we can really use double here if we want to allow both double and int. So this works
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and with that, we learned about text, so-called strings and about numbers where we have integers and doubles
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and that all can be used as types which we for example pass to a function. Now this function however
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might also return something or it might not
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and you should therefore also always add a type information about what type of data
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this function returns.
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Now this function prints the result and therefore, it does not return anything to the place where we call
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it,
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hence the correct type here would be void.
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Void is a special type in Dart
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which basically means nothing,
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this function returns nothing.
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Now why is that important?
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Because if we now for example wanted to print the result of addNumbers here, like this, then we get an
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error that the expression here has a type of void and therefore can't be printed, we can't print the
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result of us calling addNumbers because addNumbers doesn't return anything.
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Sure, it prints something but that is done from inside the function,
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here we would want to print what this function gives back and this function doesn't give us
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anything back.
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However, we could actually make sure that it does. Instead of printing in here,
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let me comment this out what you can do by adding two forward slashes,
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now this code won't be considered and this is a common pattern by the way, that you comment out code
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which you don't want to run but maybe you want to use it later,
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so you don't want to delete it but you simply comment it out so that it's still there but not
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executed
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and now we could return something here by using a special keyword in Dart, which is return. Now this is a
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keyword which tells Dart that this will now return something to the place where you called the function.
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So here, I return num1 + num2
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and now what this does is
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now here, this is valid code because now here we print the result of addNumbers and the result is what
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addNumbers returns
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and here we return two numbers.
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If I now run this, we get an error though that we can't return a value because this expression has a
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return type of void
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and again, this is a good thing.
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We defined that addNumbers doesn't return anything, that it returns nothing
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and yet we do return something here.
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Now the solution is to change the return type here
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and here, we can change this to double because since we add two doubles, we will always return a double.
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So now I'm returning a double here and now if I call this function and I run this,
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now we don't get an error and we see 3.6 here instead.
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This is how you should write Dart code, you should use these types,
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you have the core types of string and numbers where we have integers and doubles with a couple of other
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types too, which we'll learn about throughout this course
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and this is a core concept of Dart, just as functions are a core concept of Dart because now we can call
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addNumbers here as often as we want.
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So I can also call addNumbers here with two 1s for example and
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this is a really useful pattern and feature which you have in basically any programming language because
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it allows you to write code once,
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this logic for adding code and you can call it with different inputs
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as often as you want and therefore, functions are a really important concept in programming and also
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in Dart.
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