You can use the fibonacci method on a positive integer and it will return the corresponding Fibonacci number

fibonacci_sequence1.rb

class Integer
  def fibonacci
    if self.kind_of? Integer
      if self < 0
        "You can only use the fibonacci method with positive integers"
      else
        current = 0
        successor = 1
        return self if (0..1).include? self
        (1..self).each do |counter|
          current, successor = successor, current + successor
        end
        curr
      end 
    else
      "You can only use the fibonacci method with integers"
    end
  end
end


puts "'#{8.fibonacci}' should be '21'"
puts "'#{0.fibonacci}' should be '0'"
puts "'#{1.fibonacci}' should be '1'"
puts "'#{2.fibonacci}' should be '1'"
puts "'#{3.fibonacci}' should be '2'"
puts "'#{4.fibonacci}' should be '3'"
puts "'#{5.fibonacci}' should be '5'"
puts "'#{-1.fibonacci}' should be 'You can only use the fibonacci method with positive integers'"
puts "'#{10.fibonacci}' should be '55'"