From: "akr (Akira Tanaka)" <akr@...>
Date: 2012-07-16T21:07:18+09:00
Subject: [ruby-dev:45960] [ruby-trunk - Bug #6741][Open] (1 << 103).to_f.to_s is "10141204801825835000000000000000.0"


Issue #6741 has been reported by akr (Akira Tanaka).

----------------------------------------
Bug #6741: (1 << 103).to_f.to_s is "10141204801825835000000000000000.0"
https://bugs.ruby-lang.org/issues/6741

Author: akr (Akira Tanaka)
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 2.0.0
ruby -v: ruby 2.0.0dev (2012-07-15 trunk 36395) [x86_64-linux]


気がついたのですが、Float#to_s が実際には 0 でない桁を 0 とした文字列を生成することがあります。

  % ./ruby -ve '1.upto(200) {|i| p [i, (1 << i).to_f.to_s] }'
  ruby 2.0.0dev (2012-07-15 trunk 36395) [x86_64-linux]
  [1, "2.0"]
  [2, "4.0"]
  [3, "8.0"]
  [4, "16.0"]
  [5, "32.0"]
  [6, "64.0"]
  [7, "128.0"]
  [8, "256.0"]
  ...
  [54, "18014398509481984.0"]
  [55, "36028797018963970.0"]
  [56, "72057594037927940.0"]
  [57, "144115188075855870.0"]
  [58, "288230376151711740.0"]
  [59, "576460752303423500.0"]
  [60, "1152921504606847000.0"]
  [61, "2305843009213694000.0"]
  [62, "4611686018427388000.0"]
  [63, "9223372036854776000.0"]
  [64, "18446744073709552000.0"]
  [65, "36893488147419103000.0"]
  [66, "73786976294838210000.0"]
  [67, "147573952589676410000.0"]
  [68, "295147905179352830000.0"]
  [69, "590295810358705700000.0"]
  [70, "1180591620717411300000.0"]
  [71, "2361183241434822600000.0"]
  [72, "4722366482869645000000.0"]
  [73, "9444732965739290000000.0"]
  [74, "18889465931478580000000.0"]
  [75, "37778931862957160000000.0"]
  [76, "75557863725914320000000.0"]
  [77, "151115727451828650000000.0"]
  [78, "302231454903657300000000.0"]
  [79, "604462909807314600000000.0"]
  [80, "1208925819614629200000000.0"]
  [81, "2417851639229258300000000.0"]
  [82, "4835703278458517000000000.0"]
  [83, "9671406556917033000000000.0"]
  [84, "19342813113834067000000000.0"]
  [85, "38685626227668134000000000.0"]
  [86, "77371252455336270000000000.0"]
  [87, "154742504910672530000000000.0"]
  [88, "309485009821345100000000000.0"]
  [89, "618970019642690200000000000.0"]
  [90, "1237940039285380300000000000.0"]
  [91, "2475880078570760500000000000.0"]
  [92, "4951760157141521000000000000.0"]
  [93, "9903520314283042000000000000.0"]
  [94, "19807040628566084000000000000.0"]
  [95, "39614081257132170000000000000.0"]
  [96, "79228162514264340000000000000.0"]
  [97, "158456325028528680000000000000.0"]
  [98, "316912650057057350000000000000.0"]
  [99, "633825300114114700000000000000.0"]
  [100, "1267650600228229400000000000000.0"]
  [101, "2535301200456459000000000000000.0"]
  [102, "5070602400912918000000000000000.0"]
  [103, "10141204801825835000000000000000.0"]
  [104, "2.028240960365167e+31"]
  ...
  [200, "1.6069380442589903e+60"]

この例では、(1 << 55).to_f から (1 << 103).to_f で、整数部分の
下のほうの桁が 0 になっています。

(1 << n) の整数部最下位桁は 0 にはならないので、これはまちがっています。

おそらく、そういう間違った桁を表示するようになってしまう状況では
指数形式を使って、そのあたりの桁は表示しないようにするべきなのだと思います。

また、実際、1.8.7 ではそのように動作します。

  % ruby-1.8.7p370 -ve '1.upto(200) {|i| p [i, (1 << i).to_f.to_s] }'       
  ruby 1.8.7 (2012-06-29 patchlevel 370) [x86_64-linux]
  [1, "2.0"]
  [2, "4.0"]
  ...
  [46, "70368744177664.0"]
  [47, "1.40737488355328e+14"]
  ...



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