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[MUSIC]

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Now that we finished up the Invader class,
let's move on to the Tower class.

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So far the Tower class is completely
empty, let's fill it out.

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First each Tower will need to
have a location on the map, so

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let's add a field for that.

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Nothing outside the class needs to
know the location of the Towers,

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so we won't create a MapLocation property.

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This will just be a field, because each
tower needs to have a location on the map,

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it should be a parameter
in the constructor.

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And we'll need to use it
to set the location field.

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Now most tower defense games don't
allow placing towers on the path.

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We could add some validation here
to ensure the towers can't be

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placed on paths.

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In fact, you know what?

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You already saw an example of this
validation pattern when we created

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the MapLocation class.

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I'll leave that as a challenge for
you to do on your own.

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Now we need to think about what other
behaviors and attribute Towers will need.

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One behavior I can think of is
the ability to shoot at invaders.

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Let's create a method
called FireOnInvaders.

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This Method will go through all
of the Invaders on the map and

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check to see if they are in range.

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If they are, the Tower will shoot
at them and decrease their health.

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This method will need to have access
to all the Invaders on the map.

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So we'll pass them in as an array.

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Now we'll need to determine
which invaders are in range.

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To do that we'll need to look
at each invader in the array.

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For that we'll need a loop.

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Let's look at how we would
use a while loop for this.

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If you're unfamiliar with while loops
check out the teacher's notes for

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a link to a Treehouse
course that covers them.

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We'll need to loop through
each index in the array.

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So we'll need a variable to keep
track of where we are in the array.

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Let's create an integer variable called
index here and initialize it to zero.

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Now let's create a while loop that will
stop when it gets to the end of the array.

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To do that, we'll check if the index
is less than the length of the array.

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The last index in an array is
always one less than the length of

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the array because we
start counting at zero.

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If index equals the length of the array,

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then we know that it's
time to exit the loop.

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Now we can use the index
variable to index into the array.

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We'll assign it to a variable so
we don't have to index into the array

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every time we want to reference
this specific Invader.

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Now that we have
an Invader from the array,

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we can do whatever we want with it here.

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Finally we'll need to add
one to the index variable so

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that the next time through the loop we're
getting the next invader in the array.

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This way, we can do something with
each of the items in the array.

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This is called incrementing
the index variable by one.

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Incrementing is so common that there
is some syntactic sugar for it.

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Instead of saying index plus equals one,
we can just type index++.

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This is shorthand for
adding one to the index variable.

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Sometimes you may see the ++ at
the start of the variable name.

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There's a very subtle difference
between these two forms.

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I've included a description of these
differences in the teacher's notes.

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In most case, you want to put the ++
on the right side of the variable.

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Our while loop as it's written will
loop through each item in the array

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one at a time.

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This allows us to do something with each
item by adding some code right here where

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I've typed this comment.

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This works perfectly, but I want to
show you another way to write this

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that's both easier to write and
to understand.

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Let's take a short break and
then we'll learn about for loops.