Comparing and sorting elements with Comparable

Our previous feature "Building a collection of
Kittens"
was a huge success for our application. So, the product owner decided to bring
this collection of kittens to a brand new level of awesomeness. Our mission is
to add a level property to a Kitty, and allow to sort kittens based on this
property.

Because everyone wants to know who's the cutest between Maru, Kitty and Tard.

Levels are defined in a very scientific way, as you can see below:

LEVELS = [ "not remotely cute", "cute", "kawaiiiii", "awwwwwwwwwwwwwwww"]

First thing first, we need to define a level property on the Kitty class.

Kitty = Struct.new(:name, :age, :cuteness, :level)

Now, let's create some kittens and let's try to check who's the cutest.

maru = Kitty.new("maru", 8, 64, "awwwwwwwwwwwwwwww")tard = Kitty.new("tard", 13, 42, "not remotely cute")kitty = Kitty.new("kitty", 37, 97, "kawaiiiii")puts maru > kittyputs tard > kitty# => undefined method `>' for #<Kitty:0x007fa99eaaa588> (NoMethodError)'`

Obviously, Ruby does not know how to compare two kittens. But, it provides us
with tools to do just that.

Comparable

Comparable is a module which
provides the following methods:

<
<=
==
>
>=
between?

Exactly what we needed to sort out and compare our kittens.

In the same way that Enumerable needs us to define each, Comparable needs
us to define <=>. Some modifications to the Kitty class must be made:

Kitty = Struct.new(:name, :age, :cuteness, :level) do include Comparable def <=>(other) LEVELS.index(level) <=> LEVELS.index(other.level) endend

We include the Comparable module, and implement a <=> method comparing the
position of the kitty level to the LEVELS definition.

maru = Kitty.new("maru", 8, 64, "awwwwwwwwwwwwwwww")tard = Kitty.new("tard", 13, 42, "not remotely cute")kitty = Kitty.new("kitty", 37, 97, "kawaiiiii")p maru > kittyp tard > kittyp kitty.between?(tard, maru)# => true# => false# => true

Obviously, Maru is the cutest, and Tard is not.

Comparable and Enumerable

By adding Comparable to Kitty, we can now sort kittens from the Kitten
class of our previous blog post.

class Kitten include Enumerable attr_reader :results private :results def initialize(results) @results = Array(results) end def each results.each {|item| yield item } endendkittens = Kitten.new([maru, tard, kitty])puts "Sorting"puts kittens.sortputs "Cute"puts kittens.maxputs "Less cute"puts kittens.min# => Sorting# => #<struct Kitty name="tard", age=13, cuteness=42, level="not remotly even cute"># => #<struct Kitty name="kitty", age=37, cuteness=97, level="kawaiiiii"># => #<struct Kitty name="maru", age=8, cuteness=64, level="awwwwwwwwwwwwwwww"># => Cute# => #<struct Kitty name="maru", age=8, cuteness=64, level="awwwwwwwwwwwwwwww"># => Less cute# => #<struct Kitty name="tard", age=13, cuteness=42, level="not remotly cute">

Thanks to modules like Enumerable and Comparable, Ruby provides us with some
tools to easily model our application domain.

Original Article:

Comparing and sorting elements with Comparable