pyramid.out

[6] pry(main)> load 'pyramid.rb'
=> true
[7] pry(main)> pyr1 = Pyramid.new(5)
=> #<Pyramid:0x007fb08e74a9a0
 @pyramid=[[1], [2, 1], [3, 2, 1], [4, 3, 2, 1], [5, 4, 3, 2, 1]],
 @size=5>
[8] pry(main)> print pyr1
     1      
    2 1     
   3 2 1    
  4 3 2 1   
 5 4 3 2 1  
=> nil
[9] pry(main)> 

pyramid.rb

class Pyramid 

  attr_reader :size
  attr_reader :pyramid

  def initialize(size)
    @size = size
    @pyramid = _make_pyramid
  end
  
  def to_s
    @pyramid.map{|a| a.join(" ")}.map{|a| a.center(@size * 2 + 2)}.join("\n") + "\n"
  end

  private

  def _make_pyramid
    [].tap do |a|
      @size.times do |n|
        a << (1..(n+1)).to_a.reverse
      end
    end
  end

end