All language subtitles for 7. [Dart] Variables

In the last lesson, we saw that we could create a variable to hold a value and insert that value inside
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here by using that dollar sign.
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This is called string interpolation. But if you're new to programming and you don't know what variables
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are, what strings are, and how these things work together to produce this effect, then I want to quickly
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spend this lesson talking about variables in Dart.
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So if you're an experienced programmer and you know all about variables, then feel free to skip the next
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two lessons where I talk about variables and data types and head straight to the next lesson
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when we continue building out our dice app. But for everybody else who's with me,
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let's head over to our slides, and let's talk more about variables in Dart. Now,
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variables at the end of the day, it's just a container.
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It's just a fancy way of saying a data container.
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So we've known each other for a little while now and it might be time for me to introduce myself. My
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name is Angela.
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Now that's a piece of information that you might hear as a human
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very frequently. My name is this or my name is that. Or I might tell you that I live in London and my
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life ambition is to eat my own body weight in Sushi.
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But if you a computer, all of this information would be best stored as a variable. So you would have a
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variable that's called name, and you would store the value Angela in that variable. So the full line of
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code would look something like this. Var is a keyword that says I'm creating a variable, then I give that
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variable a name.
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So in this case it's called myName. And I set what that variable's equal to. So in this case, it's Angela.
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And all the rest of it such as the semicolons and the equals sign,
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they're all just syntax. And syntax is just computer grammar. Just as we would have commas or full stops
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or call ons in the English language,
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Computers also have their own grammar. And this grammar differs from programming language to programming
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language. But in Dart, this is what it looks like when you create a new variable.
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Now what's actually happening here?
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Well, essentially when I write the keyword var, the computer builds a box. And then it looks to see what
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name it should give that box.
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So this one's called myName. And then it looks at the right hand side of the equal sign to see what
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it should put into the box.
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In this case it's the word Angela. And the semicolon just says, 'well this is the end of my line of code.
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So you can close the box now.'
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So now I have this shelf and as you create more and more variables, you build up shelves full of these
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boxes. And whenever you need one, you can go ahead and pull it off the shelf and open it to see what's
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inside.
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So for example, if I wanted to print the value of the variable i just created, myName,
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this is how I would write it.
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I would use the print function and inside the parentheses,I would tell it where I want to print.
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And in this case, it's not actually the words 'myName' that will be printed.
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It's the value of that variable.
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So by running this line of code, the computer is going to pick up that box that's labeled myName from
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the shelf.
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It's going to open it up, see what's inside.
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And it's going to put that into the console.
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And in reality, what you'll see is the output from Flutter, saying the words
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Angela. In order for you to try this out yourself and not just take my word for it, head over to dart
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pad.dartlang.org and I'll include a link to this in the course resources as well.
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And I want you to delete everything inside the DART file here
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other than the void main.
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So from our functions lesson, you know that this is a function.
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And this is the function that acts as the entrance point to all of our DART programs.
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And at the moment, I'm telling it to do nothing when it starts.
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And so when I hit run, nothing happens. But if inside here, I told it to maybe create a new variable that's
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called a variable my name.
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And that said it equaled the string, that's Angela.
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So strings have these single quotes around them in Dart.
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If you come from other programming languages, you might see double quotes.
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But this is the convention that we work with when we're writing Dart code. And then I'm going to tell
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the computer that I'm done with my line of code by closing it off with a semicolon.
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So now that I've created my variable, I can use it and I can use it by referring to its name which is
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this part right here.
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Let's go ahead and write a print statement. And inside the print statement, let's get it to print myName
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and close off that line of code as well.
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And now let's click Run, and you will see the word Angela which is the value of my name printed in our
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console.
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So now if I'm late today, I decided that I didn't really want to have the same name anymore or if I wanted
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to change the value that's stored in my variable, I can simply reassign it. So I can say now, my name is
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equal to Jane.
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And what will happen is that will fetch that box that's labeled myName, it'll will take out Angela from inside
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it and we'll put the new value inside.
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So now myName is equal to Jane.
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So now if we head back to our dartpad, and I change the variable my name to equal the new value right?
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Let's say I'm now called Jack Bauer. And I go ahead and move my print from that line to this line.
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What do you think will be printed now?
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Will it be Angela or will it be Jack Bauer?
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All right.
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So hurray for those guys who got it right. Now in Dart, when you're inside a function,
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so inside a set of curly braces, the code runs from top to bottom.
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So at this stage if I had a print statement, then it would print out Angela because my name is still
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equal to Angela.
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But at this point I modify my name to make it equal to Jack Bauer.
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So when I print it here, it's now equal to Jack Bauer.
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And when I click run, you'll see first my name is Angela
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will get printed,
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and then Jack Bauer will get printed after it's been changed.
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Now, what if I decided to change this variable to something a little bit different?
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What if I try to make it equal a number say 123?
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Well now you can see already, the Dart compiler's warning us that the value of type 'int' cannot be assigned
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to a variable of type 'String'.
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And if I click run, then everything crashes and I've got some errors inside my console telling me pretty
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much the same thing.
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So what is an int and why can't it be assigned to a string?
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Well if you're interested in finding out about that, head over to the next lesson where we discuss about
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Dart data types.
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So hopefully I'll see you there.