def diff(infile, outfile, columns)
	h = File.open(outfile, 'w')
	last_values = [0.0] * columns.length

#	counter = 0
	IO.foreach(infile) { |row|
#		break if counter == 100
#		counter += 1
		data = row.split(',')
		new_columns = []
		columns.each_with_index { |c_id, i|
			value = data[c_id].to_f
			new_columns << value - last_values[i]
			last_values[i] = value
		}
		h.write "#{row.strip},#{new_columns.join(',')}\n"
#		puts "#{row.strip.gsub ',', "\t"}\t#{new_columns.join("\t")}\n"
	}
end