#!/usr/bin/ruby
require 'mrtoolkit'
class MainMap < MapBase
  def initialize(out_fd=STDOUT)
    @out_fd = out_fd
  end
  def declare
    #usual emits and field calls here
  end
  def process(input, output)
    #Standard line-proccesing.
    #Remember to return your output object!
  end
end

action = MainMap.new
action.declare
action.prepare
items_per_line = #define this value
STDIN.each_line do |line|
  if line.scan(/\t/).size < items_per_line
    STDERR.puts "insufficient data on line #{STDIN.lineno}."
  else
    input = action.new_input(line)
    output = action.process(input, action.new_output)
    action.write_out(output)
  end
end