ruby-dev

In article <e0b1e5700905132145i32bed2f0y80faef19c119824f@mail.gmail.com>,
  Yusuke ENDOH <mame@tsg.ne.jp> writes:

> 現在、Time#- が Rational を返すようになっています。
>
>   $ ./ruby -e 'p Time.now - Time.now'
>   (-7/156250)
>
> しかし、Time.now のように inexact な時刻の差分が、exact の
> ように見える Rational になるのはよくないと思います。

一般に数値計算では、値が持つ誤差よりも充分に高い精度を持って
いる数値表現を使う必要があるんじゃないでしょうか。

たとえば、比較的短い時間を扱う用途では、double は妥当な選択
肢かもしれません。

しかし、異なる Epoch へ変換するという用途では double では充
分でないこともあるでしょう。

どちらも Time 同士の引き算で実現できますが、必要な精度は異な
ります。

ちなみに、Unix Epoch からナノ秒単位で数えると、今の時刻は
double で下のほうのビットを表現できません。なので、double 経
由で他の Epoch に変換して、変換した側で計算すると、精度が落
ちていて適切な値にならない、という可能性もあります。まぁ、具
体的な Epoch と変換のやりかたにもよりますが。

inexact, exact という単純な区別ではなくて、用法によってどの
くらいの誤差が許容できるか、という点を議論する必要があるので
はないでしょうか。
-- 
[田中 哲][たなか あきら][Tanaka Akira]

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[#38362] [Bug #1427] Difference between Array#to_s and Array#inspect — "ujihisa ." <redmine@...>

[#38364] [Bug #1429] An arbitrary number of arguments of SortedSet#initialize — "ujihisa ." <redmine@...>

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[#38370] Re: [ruby-core:23338] [Bug #1425] ruby/1.8/drb/drb.rb:1480: [BUG] Segmentation fault — Masatoshi SEKI <m_seki@...>

[#38371] Re: [ruby-cvs:30538] Ruby:r23320 (trunk): * lib/set.rb (SortedSet#add): Do not let an uncomparable object — "Yugui (Yuki Sonoda)" <yugui@...>

[#38372] making install-sh more descriptive — "Yugui (Yuki Sonoda)" <yugui@...>

[#38374] [Bug #1433] test_sprintf_p fails — Yui NARUSE <redmine@...>

[#38375] [Bug #1435] README.ja: 「defined(THINK_C)」 — Kazuhiro NISHIYAMA <redmine@...>

[#38376] [Bug #1435](Closed) README.ja: 「defined(THINK_C)」 — Nobuyoshi Nakada <redmine@...>

[#38377] Marshaling URI between 1.8 and 1.9 — Tanaka Akira <akr@...>

[#38382] [Bug #1442] indentation check and coverage for toplevel do not work — Yusuke Endoh <redmine@...>

[#38390] [Bug:1.8] Tempfile and extended Enumerable — Tanaka Akira <akr@...>

[#38392] Enumerable#gather_each — Tanaka Akira <akr@...>

[#38398] [Bug #1451] dl in ruby-1.9 can't parse signatures correctly. — Takashi Tamura <redmine@...>

[#38403] [Bug #1452] DL::CPtr.new doesn't check the type of the 3rd arg. — Takashi Tamura <redmine@...>

[#38404] [Bug #1453] dl in ruby-1.9 must check whether block is given to TempFunction — Takashi Tamura <redmine@...>

[#38423] longlife gc — Narihiro Nakamura <authornari@...>

[#38426] [Bug #1456] invalid $LOAD_PATH when configure with --program-suffix — Kazuhiro NISHIYAMA <redmine@...>

[#38434] [Bug #1464] backport request to ruby_1_9_1 — Usaku NAKAMURA <redmine@...>

[#38443] Re: [ANN] Ruby 1.9.1-p129リリース — Takahiro Kambe <taca@...>

[#38446] [Bug:1.9] exact Time and inexact Time — Yusuke ENDOH <mame@...>

[#38448] GCの問題 — keiju@... (Keiju ISHITSUKA)

[#38456] [Bug #1470] class definition under the anonymouse module dumps core — Nobuhiro IMAI <redmine@...>

[#38463] SQLiteライブラリ — "NARUSE, Yui" <naruse@...>

[#38474] [Bug #1477] DL::Function#unbind raises TypeError — Takashi Tamura <redmine@...>

[#38486] [Bug #1483] some commands installed without program-suffix — Kazuhiro NISHIYAMA <redmine@...>

[#38488] [Bug #1485] warning in pathname when verbose mode — Kazuhiro NISHIYAMA <redmine@...>

[#38492] [Bug: trunk] test_encode_xmlschema depends upon system's time zone — zunda <zunda616e@...>

[#38493] [Feature:trunk] enhancement of Array#drop — "U.Nakamura" <usa@...>

[#38496] [Bug #1489] Netbeansからruby-debug-ideを利用した際 Segmentation faultが発生。 — Kiyokazu Hayashi <redmine@...>

[#38498] [Bug #1491] 「attr_reader :○○」と「def ○○ ; @○○ ; end」で動作が異なる — 慎一 江口 <redmine@...>

[#38508] overflow on LP64 — Nobuyoshi Nakada <nobu@...>

[#38518] [Bug:1.9] Enumerator.new { }.take(1).inject(&:+) causes stack overflow — Yusuke ENDOH <mame@...>

[#38524] [Bug #1503] -Kuをつけた時、/[#{s}]/n と Regexp.new("[#{s}]",nil,"n") で実行結果が異なる — sinnichi eguchi <redmine@...>

[#38531] [Bug #1075](Rejected) \r\n と \n が混在した CSV がエラーになる — James Gray <redmine@...>

[#38533] [Bug #1505] DL::CPtr#<=> doesn't return -1 — Takashi Tamura <redmine@...>

[#38535] [Bug #1506] irbで日本語を入力できない — Nobuhiro IMAI <redmine@...>

[#38537] [Bug #1508] Recursive arrays with the same structure are not eql?. — Yuki Sonoda <redmine@...>

[#38538] [Bug #1509] typo in CGI::HTTP_STATUS — Nobuhiro IMAI <redmine@...>

[#38539] [ANN] RubyKaigi2009: Commiter Invitation — SASADA Koichi <ko1@...>

[#38544] [Bug #1523] typo in sample/optparse/opttest.rb — Kazuhiro NISHIYAMA <redmine@...>

[#38545] [suggestion] sorted flag for Array — Yusuke ENDOH <mame@...>

[#38549] Re: [ruby-cvs:30701] Ruby:r23482 (ruby_1_8): * ext/thread/thread.c (unlock_mutex_inner): should adjust join — Tietew <tietew@...>

[#38550] チケットBackport #1091 [ruby-core:21762]のステータスについて — Takahiro Kambe <taca@...>

[#38554] [Bug #1534] Inconsistency Between Struct#to_s and Struct#inspect — "ujihisa ." <redmine@...>

[#38555] [Bug #1537] The Feature of Hash#to_s was Changed — "ujihisa ." <redmine@...>

[ruby-dev:38451] Re: [Bug:1.9] exact Time and inexact Time

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[#38496] [Bug #1489] Netbeansからruby-debug-ideを利用した際　Segmentation faultが発生。 — Kiyokazu Hayashi <redmine@...>

[#38498] [Bug #1491] 「attr_reader :○○」と「def ○○ ; @○○ ; end」で動作が異なる — 慎一江口 <redmine@...>