RubyMonk clarification

Ruby
1

Would someone please clarified this solution:

module Perimeter
def perimeter
sides.inject(0) { |sum, side| sum + side }
end
end

class Rectangle
include Perimeter

def initialize(length, breadth)
@length = length
@breadth = breadth
end

def sides
[@length, @breadth, @length, @breadth]
end
end

class Square
include Perimeter

def initialize(side)
@side = side
end

def sides
[@side, @side, @side, @side]
end
end

I don’t understand point of module Perimeter

2

It’s code refactoring - all 2D shapes need the method ‘perimeter’.
Instead of implementing the method separately for each shape - and it is
the same for each shape - you put it inside a module an simply add the
module to each shape.

If you ever need to change the ‘perimeter’ method, you need to do so
only once.

Btw, sides.inject(0) { |sum, side| sum + side } can be shortened to
sides.inject(&:+)

3

I don’t understand def sides in class Rectangle class Square.

what does def sides have to do with def perimeter?I don’t understand
the connection

4

If you include the module, all methods are added to the class. So this
code

module Perimeter
def perimeter
sides.inject(0) { |sum, side| sum + side }
end
end
class Rectangle
include Perimeter
end

is the same as this:

class Rectangle
def perimeter
sides.inject(0) { |sum, side| sum + side }
end
end

The only connection between the methods ‘perimeter’ and ‘sides’ are that
they are methods of the same class - and thus they can access all class
variables and all methods of the object.

‘perimeter’ can use the method ‘sides’ because it has been included in
the class Square/Rectangle.

5

sides.inject(0) { |sum, side| sum + side }

is short for

sides().inject(0) { |sum, side| sum + side }

  1. method sides gets called with no arguments, returning an array
  2. method inject of the array gets called, passing the number ‘0’ and
    the block ‘{|sum,side|sum+side}’ as arguments.
  3. the block gets called, by method inject, once for each element of the
    array, adding its value to the sum
6

I’m confused with sides in def perimeter with def sides.

def sides in classes Square/Rectangle is method so when sides.injection
in def perimeter it calls method def sides with an array as argument

Am i right?

7

Just to be sure:

sides in sides.inject is def sides from class Square/Rectangle

thanks for the rest of explanation

8

Yes, it is method sides from Square/Rectangle (there is no other one).

By including the module, you add the method perimeter to the
Square/Rectangle class.