All language subtitles for 14. [Dart] Maps

All right.
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Let's talk quickly about Dart maps and how they work and how we can create them and what we can use them
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for. When we're talking about Dart maps
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it kind of refers to not like a map of the world, but more like how something can map onto something
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else.
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So for example if I type this last June longitude into Google maps, it will map onto a specific location.
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And even better than that, you should think of maps as almost like a phone book
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right?
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Let's say that I have three friends.
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I hope you have more friends than I do.
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Amy, James and Tim and I keep their phone numbers in a book because I'm old school like that.
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Now a phonebook is kind of like a very basic sort of map.
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It's able to associate two pieces of data together.
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So usually you would have a key, in this case it'd would be the names of my friends,
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and then you would have some values and these would be the values that are associated with the keys.
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This is how maps are. They're kind of like a collection type just like lists which we saw earlier.
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But unlike lists, maps are unordered. So they don't go from zero to one to two because it doesn't make
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sense to order your names in your phone book unless you're super OCD and you really want them to be
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alphabetized and you would rewrite your phonebook every single time you had a friend that had a name
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beginning with A. But essentially all that maps are is we have a key and we have a value and the key
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will map onto a value and we can pull out the value if we know the key. In code,
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what this would look like is something like this.
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So they would be contained inside a set of curly braces.
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We have a key
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and then after a colon we have the value that's associated with that key. And all of these entries are
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separated by a comma.
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So this is how we would create it
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and this is how we would use it. If we wanted to create a map, we can specify the data type as a map data
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type, and then we can have a set of angle brackets and just like when we had lists where we put in the
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data type all of the values that are in the list,
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in this case we can specify the data type of the keys and the data type of the values so that when we
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create our map or when we try to add values to our map our compiler can check to make sure that the key fits
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the initial key type and the value fits the value type.
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And if we want to use our map we simply refer to the name of the map,
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use some square brackets and provide a key. And this will end up giving us the value that's associated
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with that key.
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So let's create a new map from scratch.
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I'm going to start out by providing the data type which is going to be a map data type. And I could just
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leave it at that and get Dart to figure out what are the data types of the entries in the map.
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But if I wanted to help it along and also help other programmers who I'm working with to be able to
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see at a glance what is contained inside this particular map, then I could add some angle brackets. And
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inside, I could provide the data types of my key first which in my case is going to be a string, the names
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of my friends. And then the value is going to be an integer,
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the phone numbers of those people.
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And so this is how that full data type looks like. I'm creating a collection of type map
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and it has a key that will be string, and it'll have associated values which will be an integer. And
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then I get to give it a name.
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So let's call it I did a phonebook
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right?
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And it's gonna be set to equal a new map.
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So I'm going to open out with some curly braces and inside here is where I will specify my keys and
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values.
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Now in this case my keys have to be a string.
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So let's say that we create a key called
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I don't know,
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Kyle, and then we would add a colon to say that this is the end of the key.
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And now I'm going to write the value that's gonna be associated with Kyle, so let's say that he's at
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some random phone number. And then I can separate each of the entries in my collection with a comma.
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So I'm going to paste in the ones from our previous keynote just for time sake.
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And now if we cap off the end of our map with a semicolon, we can reformat our map to make it look pretty.
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So here we have a map of strings and integers,
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it's called phone book,
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it's contained inside a set of curly braces and it's got four entries.
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So now what I can do is I can start using my map.
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So for example if I wanted to print out the value that is in my phonebook for maybe let's say Amy right?
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Then I could write phonebook, add a set of square brackets denoting that I'm trying to pull something
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out of a collection, and then I would provide the key which obviously has to match one of the keys in
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my map.
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So let's try and get Amy's phone number printed into the console. So let's hit run and we can see that
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we get 123456789, which is the value that's associated with the key Amy.
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Now if I tried to pull out a key that doesn't actually exist in the map, let's say try to get my own
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number out of this phonebook, then all we'll get is null. When it tries to pull out of value for a key
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that doesn't exist
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you will get the value
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null.
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So it's easy to check against this if you wanted to see if a particular key existed. You could just see
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if phone book at a particular key is equal to null, do this.
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Else do that.
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Now we can also use this particular syntax,
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the square brackets, to add new values into our phonebook,
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that doesn't yet exist.
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So let's say I write phonebook and I'm going to add a value for my own name now.
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So I'm going to add phonebook for the key
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Angela is now going to be equal to,
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let's add some random numbers.
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And now if I tried to again print phonebook
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for the key Angela, you can see that this now works and I no longer get null printed in here. So the cool
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thing about maps is that it's unordered.
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It doesn't really matter at which point your entries go in because you don't need to pull them out in
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order, as in 0 is Kyle,
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1 is Amy. And instead you can actually get the values out of the map using something that's quite easy
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to read and easy to understand when you're looking at it at a glance.
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This syntax is very expressive.
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Now there's a number of methods that make it easy for you to use a map.
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So for example you could write phonebook.length to see how many entries you currently have in the
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phonebook.
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So that's four plus this one which is five.
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Or you could write phonebook.keys to print out all of the keys that's in the phone book. And it will
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print out everything it's currently got. Or you can do the same for values as well and it'll print out
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all the values it has. So we're going to be seeing maps a lot more in the coming lessons.
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And it's just a convenient way of putting items into a collection not in any particular orde,r but searchable
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by their keys.