Simple Riemann sum calculator in ruby

riemann_sum.rb

#!/usr/bin/env ruby

def eval_function(function, x_value)
	function.upcase!
	function.gsub!(/X/, "#{x_value}")
	return eval(function)
end

def riemann_sum(function, delta_x, lower_bound, upper_bound)
	areas = []
	x_value = delta_x+lower_bound
	while x_value < upper_bound
		areas << eval_function(function.dup, x_value)*delta_x
		x_value += delta_x
	end
	left_hand = areas[0..areas.size-2]
	left_hand_area = 0
	left_hand.each do |area|
		left_hand_area += area
	end
	return (left_hand_area+left_hand_area+areas[-1])/2
end

if ARGV.size < 4
	puts "Usage: ./riemann_sum.rb <formula> <delta_x> <lower_bound> <upper_bound>"
	exit
end
puts riemann_sum(ARGV[0].dup,ARGV[1].dup.to_f,ARGV[2].dup.to_f,ARGV[3].dup.to_f)