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We just programmed the heart of our app,
the random number generator.

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Now we need to use that generator
to help us pick a random fact

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on each button click.

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But before we can do that,
we'll need a pool of facts to choose from.

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Let's start by creating
an array to hold our facts.

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Remember, an array is just a container
that holds values of a specific type.

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Let's add two lines to the top
of our OnClickListener, and

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then create a new val named facts.

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Now, for the assignment side of things.

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For the sake of time,
I'm going to paste in ten facts and

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then we'll go over the syntax.

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You can type these out if you want, but

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I've put this code in the teacher's notes
so you can just copy and paste as well.

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Let's do it, I'll select everything
after the word facts, then I'll

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copy this and paste it after our facts
variable back in the OnClickListener.

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And now we can see that one way
to initialize an array in Colin,

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is just to type arrayOf, and
then add each item as a parameter.

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Now that we have an array
of strings to select from,

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let's use our random number generator
to help us pick one at random.

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Our array has ten elements in it,
lets start

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by changing our random number generator
to have a range of 10 instead of 3.

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If we clicked on our button now, we'd
see random numbers ranging from 0 to 9.

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And what else do we have
that arranges from 0 to 9?

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That's right,
the indices of our facts array.

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This first fact about ants is at index 0,
and

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this last fact about
mammoths is at index 9.

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So if we want to select a random fact,
we just need to select the fact

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from our facts array that's at the index
of the random number we generated.

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Seems easy enough,

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let's start by getting rid of what
we're currently setting our fact to.

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And then type, facts [randomNumber] and

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make sure you have a right
bracket to close it off.

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This selects the fact at
the index of our random number.

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All right, that should do it,

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but before we test our app, there's
a potential bug we should fix first.

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What happens if we add
a new fact to the array?

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Can you tell where the code will break?

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I'll add another fact and
then let's talk about it.

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I'll say,
Treehouse is not actually in a tree.

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Now that we have 11 elements in our array,
we need to change 10 to 11 down here,

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and our call to nextInt.

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But if we forget to change it,
we'll never see our new fact.

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Wouldn't it be nice if we could somehow
use the number of elements in the array

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as our parameter,
instead of a hard coded number like this?

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Then we wouldn't have to
change it every time we add or

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remove elements in our array.

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Sure enough, Colin has a way for
us to do this.

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Let's delete the number 10,
and type facts.size.

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This gives us the size of the facts array,

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which is another way to say
the total number of elements.

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By using this property of the array,

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we will always pass in
the right number of elements.

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This saves us the work of
manually changing numbers, and

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it also protects us from errors.

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For example, if we had deleted an element,
I'll delete this one that I just added

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But we left this number as 11,
then our app might try to reference

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an element that doesn't exist,
and it would crash.

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Looks good to me, let's click on
the run button to test our app.

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Then we can tap the button, and
we should see some of the new facts.

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And there we go, we got something new.

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Olympic gold medals are actually
made mostly of silver?

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And if we keep clicking,
we should eventually see all of our facts,

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very cool!

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Awesome work, don't worry if all these
new Android words aren't sticking yet.

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We've done a lot, and
we still have a lot to do.

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But if you stick with it,
I'll explain everything you need to know.

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And if you wanna talk about
anything you're stuck on,

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head over to the community for
questions or discussions.

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It's easy to feel overwhelmed
with this stuff, but

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trust me, things will start to make
more sense the more you practice.

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On that note,
why don't you try some more practice?