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A map is a collection of key pair values.

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No entries can have the same key.

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And when you want to look up an entry, you just need to know the key for a quick lookup.

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Rust gives us two types of maps hash map and a B tree map, also known as a binary tree map.

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These two maps are very similar.

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The key difference is how the two maps keep entries arranged for their lookup.

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Hash maps are stored in a single heap allocated table, whereas tree maps are stored in a tree like

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structure growing from the top down into many leaves, also called nodes.

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I don't want to get too into the data structure set up behind these, but I do want to point out that

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I eventually plan to add a data structures section to this course to where we can see how data structures

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are set up and then also improve in our rest skills.

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But again, these two maps have very similar methods, so I am only going to create and use a hash map,

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but it is similar to a B tree map.

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So the first thing we need to do is bring a hash map into our scope and it's going to be in standard

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collections, hash map, as was the binary heap.

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And now we can create a mutable hash map using hash map loop.

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So like a B binary heap, did we have the line method for length and the is empty method as well, which

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is remember going to return a boolean on if there are values or not in there.

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So let's add a couple of values using the insert method, which as you can see, it's going to accept

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two parameters the key and the value designated by K and V.

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And so we can both see that they are generic types.

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So let's add in a key of one and then also a value of one.

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So then we can add a couple more in and we'll say insert five.

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And to.

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And then we'll say insert and we'll give a key of 30 and a value of three.

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So now let's print out our hash map just to kind of see how it is structured.

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Cargo run and we can see that we have 1 to 1.

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So key one value one, key five value two and then key 30, value three.

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So now earlier I mentioned that you cannot have multiple values mapped to a key.

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So what happens if I say at key 30, I want you to have.

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The value for because we can see that the ID does not give us any error codes, so it looks valid.

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Well, what's actually going to happen here is the key is going to update

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the old value of three to the new value of four.

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It's also going to return the old value so we can actually say let hold equals and then to the insert.

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So now if we come down here and we try to print this out and we'll say we want to print out the old

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value.

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And now if we run this, we see that the key of 30 updated to our new value of four and then.

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We got.

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The old variable was assigned some three.

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So it's also going to return an option of.

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So great.

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So we're able to actually update values using the insert.

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That way we don't generate any bugs trying to act to insert a value at a key that already exists.

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So we can check to see if a key exists using the contains key method and that looks like this and we

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can see that it is looking for a reference to a key value.

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So in this case we'll say a reference to five.

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And if we run this, we would expect true to be printed out, which is what we got.

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So if we reference a key that does not exist, such as eight false is returned.

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So that's great.

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So now we can check to see if keys exist inside of our hash map.

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But how about getting.

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Values back.

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So we can say.

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Brit.

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And then we want to print out the value of the key and we'll say key five again.

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So when we run this, we expect.

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Two to be returned and we get some too.

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So we are returned and option type T.

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So that that means that if that key does not exist, then none would be returned.

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Otherwise we are going to return some t if that key does exist.

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So now we have two ways that we can remove values.

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The first way is remove and we're going to pass in the key that we want removed.

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So in this case, we'll pass in the key of one.

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If we now print out, ah, I'm just going to copy this.

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If we now print out our hash map, we expect the 1 to 1 to be gone.

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Which as you can see.

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So it's up here when we create it.

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And now down here are key of one and value one is gone.

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But this also returns.

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Value for us.

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So if we run it this way, then we see we get some one return.

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So it does return a value if we decide to do it that way.

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Now, another way we can do it is we can say remove entry.

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So we can also.

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You also get a returned value from this, but it is a returned paper value.

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So remove entry is going to actually return the key and the value.

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So let's remove the key of five and make sure that the key of five is returned as well as the value

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so we can see that we are returned.

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The key of five and the value of two, which is what we initially inserted in.

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So the difference between remove entry and remove is that remove entry is also going to return the key

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alongside the value, whereas remove is only going to return the value.

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And then if we quickly want to clear out our hash map, we can use clear and then to verify that this

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was cleared out, we can use our method is empty and since we ran clear, we would expect this to return

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true, which is what we got.

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So these are just some of the methods associated with the hash map.

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Again, a B tree map and a hash map are extremely similar.

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So I encourage you to create a B tree map and then just basically do what we did here just to make sure

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everything works the same.

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And that you are also familiar with how a B tree map works.