threeposition.rb

#!/usr/bin/env ruby
# -*- coding: utf-8 -*-

# threeposition.rb by takehikom

# ruby threeposition.rb 541    => "541-145=396, 396+693=1089"
# ruby threeposition.rb

# Inspired by
# 藤井斉亮「数学的問題解決過程における文字式の役割と機能」,
# 『続・新しい算数数学教育の実践をめざして―杉山吉茂先生喜寿記念論文集』
# (isbn:9784491028538), pp.198-208.

def reverse3(n, flag_comp = false)
  n = n.to_i
  if flag_comp && (n < 100 || n > 999)
    raise "outlying number"
  end
  na = (n + 1000).to_s.split(//)[-3, 3].map {|i| i.to_i}
  # puts "#{n}:#{na.join(',')}"
  if flag_comp && na[0] <= na[2]
    raise "wrong number"
  end
  na[2] * 100 + na[1] * 10 + na[0]
end

def threeposition(n)
  begin
    n_rev = reverse3(n.to_i, true)
    m = n.to_i - n_rev
    m_rev = reverse3(m)
    p = m + m_rev
    puts "#{n}-#{n_rev}=#{m}, #{m}+#{m_rev}=#{p}"
  rescue RuntimeError => msg
    # puts "#{n}: #{msg}"
  end
end

if __FILE__ == $0
  if ARGV.empty?
    100.upto(999) {|i| threeposition(i)}
  else
    ARGV.each do |n|
      threeposition(n)
    end
  end
end