test.rb

# Recang method calculation
class RectangleMethod
  include Enumerable
  def initialize(a, b, n = 1000)
    @a, @b, @n = a, b, n
  end

  def calc_rectang
    # @result = 0
    interval = (@b - @a).fdiv @n
    @n.times.inject(0) {|data, exp| data + yield(@a + interval * exp)} * interval
    # @n.times do |i|
    #   @result += yield(@a + interval * i)
    # end
    # @result * interval
  end

  def calc_lambda(arg) 
    @result = 0
    interval = (@b - @a).fdiv @n
    @n.times do |i|
    @result += arg.call (@a + interval * i)
    end
    @result * interval
  end

  def calc2_lambda 
    @result = 0
    interval = (@b - @a).fdiv @n
    l = lambda { |func| Math.sin(func) * Math.cos(func) }
    @n.times do |i|
    @result += l.call (@a + interval * i)
    end
    @result * interval
  end
end

puts RectangleMethod.new(0, 2).calc_rectang { |func| Math.sin(func) * Math.cos(func) }
puts RectangleMethod.new(0, 2).calc_lambda -> func { Math.sin(func) * Math.cos(func) }
puts RectangleMethod.new(0, 2).calc2_lambda