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Comparable
5:14For our first stop on the Ruby Core Module tour, we're going to check out the `Comparable` module. The `Comparable` module allows you to make your classes sortable and gives you convenience methods.
Ruby as a programming language
comes with some different modules
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as part of its distribution.
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There are two parts to this,
Ruby Core and the Ruby Standard Library.
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The difference between them is as follows,
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Ruby Core contains Ruby language features,
while the Ruby standard
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library contains different libraries
you can use in your Ruby programs.
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Something in Ruby Core may only implement
a few different core pieces of behavior,
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such as file access.
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While something in the standard library
would implement a suite of behavior,
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such as logging functionality.
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We're going to take a look
now at the comparable module.
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The comparable module is a mix-in
that provides behavior to help in
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ordering your classes.
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Let's take a look at how it
works now using work spaces.
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Okay, so let's go ahead and
take a look at the Comparable module.
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Now this is part of Ruby Core, and
if you head over to the teacher notes,
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you'll find a link to the Ruby Comparable
module documentation.
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And if we look at the documentation,
it tells us exactly what Comparable does.
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It's used by classes whose
objects may be ordered, and
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it must define this operator right here.
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We haven't seen this before,
it's called the spaceship operator.
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And it says it compares the receiver
against another object,
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returning -1, 0, +1,
depending on whether or
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not the receiver is less than, equal to,
or greater than the other object.
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So what in the world does that mean?
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Well, if it is less then the other
object it's being compared to,
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we return minus one.
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If it's equal to it returns zero, or
if it's greater than, it returns plus one.
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Now using that, we get some interesting
behavior that we can view here.
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So, this documentation isn’t that great,
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let's go ahead and take a look at
how it works, using a player class.
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So, right here we have this comparable.rb
file open in a workspace, and
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we have this player class right here.
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And this is pretty simple,
the player object has a name and
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a score associated with it.
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When we initialize the player class,
we initialize the name, and the score.
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Now, all we have right here is this
simple output statement saying,
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player1 is greater than player2.
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And all this %s right here,
means it's a format string,
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and anything that's evaluated right
here will be returned as a string.
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So we're just kind of trying to evaluate
the statement, player1 > player2.
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Now let's go ahead and
run this, and see what happens.
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We have undefined method greater than for
this player class, and that makes
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sense because we haven't done anything
with the Comparable module just yet.
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So, here's how we do it.
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We can say include Comparable.
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Now, that will include the Comparable
module into our player class.
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Mixing in all of the different methods
that the Comparable module gives us.
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And let's run this again and
see what happens.
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Once again we got a failure,
but this is a different error.
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It says comparison of player with
player failed, and why did it fail?
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It's because we didn't implement
this spaceship operator.
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So, let's go ahead and implement that now,
and we define that like any other method.
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And we can do a little short circuit here,
and
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we can just say score is the spaceship
operator to the other_player.score.
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So if one of us has a score of 100 and
the other has a score of 80,
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that should return one,
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because one is greater than
the other side of this operand.
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So I'll clear my screen here and run this
again, and you'll see we now have this
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method, saying player1 is greater
than player2, that evaluates to true.
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And the same thing will work,
if we do it with the less than sign here.
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We now get that method for free, and
if we look back in the documentation for
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Comparable, we get all of these
different methods right here.
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Less than, less than or equal to,
equal to, greater than, greater than or
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equal to, and between.
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So, we could also see if something
was between something else.
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And the nice thing about this is,
we didn't have to write
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any code to see whether or
not this was greater than something else.
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By just defining this
one spaceship operator,
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we were able to get all of
this behavior for free.
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