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These are the user uploaded subtitles that are being translated: 1 00:00:02,260 --> 00:00:09,190 We had a first look at some programming basics, functions and types or some of the important types 2 00:00:09,190 --> 00:00:15,970 for the moment but there is more in programming and there are two other features which we need to understand 3 00:00:16,330 --> 00:00:19,520 before we can go back to our main.dart file. 4 00:00:19,560 --> 00:00:24,870 The first feature is the feature or the idea of variables. 5 00:00:24,870 --> 00:00:31,260 Thus far what I always did in here is, I always immediately printed the result of addNumbers. 6 00:00:31,260 --> 00:00:35,520 Now of course this is just some simple dummy code and therefore this make sense here, 7 00:00:35,520 --> 00:00:39,690 we basically just want to see what this yields us here. 8 00:00:39,720 --> 00:00:47,550 In reality however, you often write programs where you also need to store the result of some calculation 9 00:00:47,580 --> 00:00:52,730 or some user input, you want to store it somewhere so that you can use it later. 10 00:00:52,740 --> 00:00:57,900 Now with store, I don't mean in a database but in memory because you needed it later, 11 00:00:57,900 --> 00:01:00,910 a couple of code lines later in the same code. 12 00:01:01,020 --> 00:01:04,380 So we don't want to write it into a file or store it in a database, 13 00:01:04,500 --> 00:01:06,530 we just want to store it in memory 14 00:01:06,600 --> 00:01:13,830 so that when this code executes, we can take it and use it a couple of lines further down for example 15 00:01:14,310 --> 00:01:21,140 and we can store data with the help of so-called variables. Let's say that firstResult here shouldn't 16 00:01:21,140 --> 00:01:25,580 be printed immediately but instead I do want to store it in a variable. 17 00:01:25,580 --> 00:01:31,970 You create a variable by giving it any name you want, like firstResult and the naming convention here 18 00:01:31,970 --> 00:01:40,700 is just the same as for functions, you use camel case, lowercase starting character, then only one word 19 00:01:40,880 --> 00:01:45,600 and every word inside the word starts with a capital character. 20 00:01:45,650 --> 00:01:52,900 Now that's firstResult and you assign a value to that so-called variable now with an equals sign. 21 00:01:52,910 --> 00:01:57,660 Now it's called variable because the data in there is variable 22 00:01:57,670 --> 00:02:03,200 where you can change it, here I'm storing the result of addNumbers in firstResult, I could reuse that 23 00:02:04,760 --> 00:02:08,390 and now also store that result in firstResult. 24 00:02:08,420 --> 00:02:14,630 So after the first line, the result of addNumbers for 1 and 2.6 would be stored in here, after 25 00:02:14,630 --> 00:02:16,780 the second line line here, 26 00:02:16,910 --> 00:02:22,180 the result of addNumbers with 1 and 1 would be stored here and the firstResult, so this result of this 27 00:02:22,180 --> 00:02:23,540 line here would be dropped, 28 00:02:23,600 --> 00:02:28,310 we would store that result in the firstResult variable now. 29 00:02:28,370 --> 00:02:33,290 However, we have a syntax error here and that means that Dart doesn't understand this. 30 00:02:33,380 --> 00:02:39,760 This is almost the right syntax but to tell Dart that this is a variable, you need to add the var keyword 31 00:02:39,770 --> 00:02:40,140 here 32 00:02:40,160 --> 00:02:46,430 when you create that variable for the first time. So not in this line where I then use it again, the same 33 00:02:46,430 --> 00:02:47,700 variable with the same name 34 00:02:47,780 --> 00:02:55,040 but when you introduce it, then you need to add the var keyword to tell Dart that this is a variable 35 00:02:56,350 --> 00:03:04,670 and now we can for example use that result, the firstResult, the name is firstResult, 36 00:03:04,750 --> 00:03:07,240 we can use that here in a separate line. 37 00:03:07,240 --> 00:03:12,340 Sometimes you even just use a variable to make your code easier to read, so that you can split it across 38 00:03:12,340 --> 00:03:13,340 multiple lines, 39 00:03:13,360 --> 00:03:18,430 sometimes you have some other code in between that you need to execute first and then you want to use 40 00:03:18,430 --> 00:03:19,710 firstResult later. 41 00:03:19,810 --> 00:03:24,580 Maybe you even want to combine it with some other calculation, that you add plus one to it, 42 00:03:24,610 --> 00:03:31,390 that would be possible too. firstResult simply holds the value returned by addNumbers and that number 43 00:03:31,390 --> 00:03:32,740 in our case is a double, 44 00:03:32,740 --> 00:03:36,660 so you can do anything with it which you could do with any other double. 45 00:03:36,660 --> 00:03:45,190 So now if I run this, we print three here because firstResult has the result of addNumbers with one 46 00:03:45,190 --> 00:03:48,340 and one stored which is two and then we add one to it, 47 00:03:48,370 --> 00:03:55,240 so we have 2 + 1 and we print that result and therefore see three here. Variables are a core feature in 48 00:03:55,240 --> 00:03:58,410 Dart and we'll see it a lot throughout this course. 49 00:03:58,540 --> 00:04:06,490 Now besides that syntax where I use var for creating it, you can also create it by adding the type of data 50 00:04:06,520 --> 00:04:08,880 which this variable will hold eventually. 51 00:04:08,890 --> 00:04:12,810 So in this case, we could also write double firstResult. 52 00:04:13,090 --> 00:04:17,770 However, Dart also has a feature called type inference and 53 00:04:17,770 --> 00:04:24,400 this means it's able to infer the type of data you'll store here by the value you have on the right 54 00:04:24,400 --> 00:04:31,120 side. Here since I call addNumbers which returns a double and which always returns a double, Dart is 55 00:04:31,150 --> 00:04:34,660 able to infer that firstResult will hold a double 56 00:04:34,870 --> 00:04:37,850 and therefore you don't have to tell Dart that this is the case 57 00:04:37,930 --> 00:04:44,530 and actually it's considered a better practice to then just use var. It would be a different thing if 58 00:04:44,530 --> 00:04:49,580 you created the variable like this, without assigning an initial value. 59 00:04:49,690 --> 00:04:56,680 That is possible too, the variable now exists but it is uninitialized and sometimes this is also something 60 00:04:56,680 --> 00:05:01,120 you need to do because you want to create a variable and at a later point of time, you want to assign 61 00:05:01,120 --> 00:05:08,470 different values to it. In such a case, it is a good practice to tell Dart which kind of data will eventually 62 00:05:08,470 --> 00:05:09,880 end up in this variable, 63 00:05:09,880 --> 00:05:13,060 so then you should use the type here instead of var, 64 00:05:13,060 --> 00:05:15,420 in this case, that's the double. 65 00:05:15,440 --> 00:05:20,680 If I now run this, we see the same output as before but now we created this a bit different and we are 66 00:05:20,680 --> 00:05:22,170 using double instead of var 67 00:05:22,180 --> 00:05:31,580 here. Now that's one of the important features which I wanted to mention, another important feature is that 68 00:05:31,670 --> 00:05:36,540 Dart is a so-called object oriented programming language. 69 00:05:36,680 --> 00:05:39,440 I did mention that everything a Dart is an object, 70 00:05:39,440 --> 00:05:45,170 so this number, this one here, is an object. You would say it's a number and that's true but a number 71 00:05:45,170 --> 00:05:47,220 is just object. 72 00:05:47,220 --> 00:05:49,020 Now what is an object? 73 00:05:49,410 --> 00:05:55,080 Everything in Dart is basically a data structure with a lot of different information in it. 74 00:05:55,260 --> 00:06:02,280 Now here, the information is of course that it's one but there is some other metadata which Dart uses 75 00:06:02,280 --> 00:06:03,010 internally 76 00:06:03,030 --> 00:06:08,940 you could say. Now why do we actually start expressing our data structure as objects? 77 00:06:08,940 --> 00:06:11,670 Well, that is simply related to the real world. 78 00:06:11,700 --> 00:06:15,640 If we think about the real world, we work with objects there too, right? 79 00:06:15,690 --> 00:06:21,810 You have a car and you can drive that car or your car has a certain amount of seats or a certain amount 80 00:06:21,810 --> 00:06:22,930 of horsepower. 81 00:06:22,950 --> 00:06:29,430 So we describe our real world with objects and the idea simply is to bring that way of thinking into 82 00:06:29,460 --> 00:06:33,770 development, into programming so that it's closer to the real world. 83 00:06:33,780 --> 00:06:38,640 So we work with objects in our program because of course typically with programs, like here where we're 84 00:06:38,640 --> 00:06:43,680 building apps, we also have things which we could describe as objects, 85 00:06:43,680 --> 00:06:49,440 for example in a mobile app we have a button, a button of course would be a valid object if you think 86 00:06:49,440 --> 00:06:50,280 about it like this. 87 00:06:50,610 --> 00:06:56,720 So that's why we call it object and where this idea of using objects comes from, why we group things into 88 00:06:56,730 --> 00:06:58,930 data structures that we call objects, 89 00:06:59,040 --> 00:07:05,250 that simply comes from the real world and it should help you think about and reason about your program 90 00:07:05,640 --> 00:07:12,300 and the core idea really just is that you can kind of bring the way you think about reality into how 91 00:07:12,300 --> 00:07:14,080 you think about your programs, 92 00:07:14,100 --> 00:07:18,680 that's the rough general idea and how you work with objects will of course become very clear throughout 93 00:07:18,690 --> 00:07:22,170 this course because we work with objects all the time in there. 94 00:07:22,290 --> 00:07:27,210 Now you can also create your own objects because sometimes, you're not just working with a text or a 95 00:07:27,210 --> 00:07:32,310 number but you have a more complex data structure which you want to express in your code. 96 00:07:32,340 --> 00:07:35,330 Let's say you have a person with a name and an age. 97 00:07:35,520 --> 00:07:38,780 Of course, we could create a variable here, 98 00:07:38,850 --> 00:07:41,410 String name. String, 99 00:07:41,420 --> 00:07:44,670 this is how you tell Dart that this will be of type String. 100 00:07:44,670 --> 00:07:46,440 Please note that unlike double, 101 00:07:46,440 --> 00:07:48,180 it starts with a capital character, 102 00:07:48,180 --> 00:07:49,620 that's just how it is. 103 00:07:49,680 --> 00:07:53,610 So you could have a String name and let's say an int age, 104 00:07:53,610 --> 00:07:57,270 that would be possible and you could store values in there 105 00:07:57,690 --> 00:08:02,440 and to you as a developer it would of course be obvious that these two are related. 106 00:08:02,490 --> 00:08:05,000 However for other developers, it might not be 107 00:08:05,130 --> 00:08:11,220 and even if it is clear, it's a bit annoying that you always have to work with these two separate 108 00:08:11,250 --> 00:08:12,000 variables, 109 00:08:12,000 --> 00:08:18,030 it would be nice if you could merge it into one variable and that can be done with so-called objects. To 110 00:08:18,030 --> 00:08:23,600 create an object, you first of all need a blueprint for it that tell Dart how this 111 00:08:23,610 --> 00:08:30,390 object should look like. You do this with a core Dart feature, which you also see in other programming languages 112 00:08:30,840 --> 00:08:39,060 which is a class. A class allows you to define a blueprint for objects. A class needs to have a name 113 00:08:39,090 --> 00:08:40,430 and that could be person 114 00:08:40,500 --> 00:08:46,800 and here the naming convention is not to start with a lowercase character but with an uppercase character. 115 00:08:47,670 --> 00:08:53,130 Then you add your class body with curly braces, just as you did it for the function and still this is 116 00:08:53,130 --> 00:09:00,060 no function, this is a class, it's a different thing and in between that class, you can now define how that 117 00:09:00,240 --> 00:09:04,320 class and therefore objects based on the class should look like. 118 00:09:04,470 --> 00:09:05,610 Now what does this mean? 119 00:09:05,640 --> 00:09:13,110 For example you can add variables in here, you can add a variable name and age if you wanted to, 120 00:09:13,140 --> 00:09:19,320 so what I basically had down there is now added in that class and you can also give this default values 121 00:09:19,320 --> 00:09:20,950 just as we did it before. 122 00:09:21,000 --> 00:09:26,630 Now if you have a variable in a class, this is also sometimes called a property. 123 00:09:26,730 --> 00:09:31,010 It's still a variable, it's just inside of a class and therefore we give it a special name 124 00:09:31,050 --> 00:09:38,130 so that when I say we change this property, you know that we change a variable inside of a class. A variable 125 00:09:38,130 --> 00:09:43,140 inside of a function on the other hand is called a variable and we have these different terms so that 126 00:09:43,140 --> 00:09:47,460 when I say property, it's clear I'm referring to a variable in the class and not to a variable in the 127 00:09:47,460 --> 00:09:49,080 function. 128 00:09:49,080 --> 00:09:55,320 So here we have these two pieces of data now in our class and it is worth mentioning that just like 129 00:09:55,350 --> 00:10:00,990 with normal variables in a function, if you are assigning a value which I'm doing here with Max and 30, 130 00:10:01,290 --> 00:10:05,750 then you should actually use var here as well, instead of string and int. 131 00:10:05,790 --> 00:10:11,010 Now I use string and int here because I'll change this later to not initialize it but if you would leave it 132 00:10:11,010 --> 00:10:16,500 like this, if you plan on initializing a property with an initial value, then you should also use 133 00:10:16,530 --> 00:10:21,660 var there instead of repeating the type. Repeating the type won't be an error but it is considered a 134 00:10:21,660 --> 00:10:26,910 better practice to use var and let type inference of Dart do its job. 135 00:10:27,210 --> 00:10:35,340 And now down there in the main function, if we need such a person, we can create a variable, p1 or whatever 136 00:10:35,340 --> 00:10:42,300 you want to name it and you create a new instance of this class because the class is just a blueprint, 137 00:10:42,300 --> 00:10:49,900 the instance of the class is the concrete object with which you're working by calling person like this. 138 00:10:50,010 --> 00:10:55,140 Now if you have some other programming experience, you might be used to using the new keyword in front 139 00:10:55,140 --> 00:11:00,780 of that, new is required in some other programming languages to create a new object based on your own 140 00:11:00,780 --> 00:11:01,970 class. 141 00:11:02,010 --> 00:11:08,700 Now that can be used in Dart 2 but since Dart 2 which we use in this course, it's not required anymore 142 00:11:08,850 --> 00:11:11,640 and therefore we just call person like this. 143 00:11:11,730 --> 00:11:19,440 So now we have person executed like a function actually with parentheses here and this will instantiate 144 00:11:19,440 --> 00:11:27,210 this class and store it in p1 and therefore now if I print p1 here and I run this code, you actually 145 00:11:27,210 --> 00:11:34,610 see instance of person being printed here. Now on this class, you can now conveniently access the 146 00:11:34,610 --> 00:11:41,660 data that's stored in here with the dot notation, so I can type p1. and now I could choose name or age 147 00:11:42,350 --> 00:11:48,560 or a couple of other things which every class has in Dart, automatically added by Dart but here I 148 00:11:48,560 --> 00:11:54,460 could print p1.age and if I now run this, we see 30 here and that's what I meant. 149 00:11:54,480 --> 00:12:00,360 Now we grouped that data together and we can create new persons by simply calling person again. 150 00:12:00,360 --> 00:12:07,640 We can also change that data on a person by accessing p2.name and changing that to Manu for example 151 00:12:07,650 --> 00:12:14,580 and now if I would print p2.name down there and I print p1.name up here, so for the two different 152 00:12:14,580 --> 00:12:20,460 persons, I print the names, then you see that here on the right, we print Manu and Max because for 153 00:12:20,460 --> 00:12:25,860 the first person, I didn't change it and therefore we have that default, for the second person I did change 154 00:12:25,860 --> 00:12:31,170 it and therefore here where I changed it, we change it to Manu, down there when I print that, I print 155 00:12:31,350 --> 00:12:38,910 Manu. This is how that works and classes are a crucial feature in many programming languages and they 156 00:12:38,920 --> 00:12:45,740 also are in Dart. Now there is more about classes and about Dart which we'll learn throughout this course, 157 00:12:45,750 --> 00:12:50,100 I don't want to bore you now with a one hour long introduction to Dart, 158 00:12:50,130 --> 00:12:56,580 I find it more interesting to learn Dart whilst we are learning Flutter but these are some basics which 159 00:12:56,580 --> 00:13:02,400 I needed to get out of the way, so that we now can continue working on our main.dart file and understand 160 00:13:02,400 --> 00:13:03,780 what's going on in there. 18298

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