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Groups > comp.lang.c++ > #7137
| From | Pete Becker <pete@versatilecoding.com> |
|---|---|
| Organization | Roundhouse Consulting, Ltd. |
| Newsgroups | comp.lang.c++ |
| Date | 2011-06-24 16:26 -0400 |
| Message-ID | <2011062416262439377-pete@versatilecodingcom> (permalink) |
| References | <589f8b74-d065-44a5-ab9e-81e6a6fcd88b@v12g2000vby.googlegroups.com> <iu2kvq$27a$1@hoshi.visyn.net> |
| Subject | Re: (simple?) problem with multiplication |
On 2011-06-24 14:28:09 -0400, Kai-Uwe Bux said: > Pete Becker wrote: > >> On 2011-06-24 14:07:45 -0400, Kai-Uwe Bux said: >> >>> Pete Becker wrote: >>> >>>> On 2011-06-24 13:09:56 -0400, SG said: >>>> >>>>> In case this hasn't been mentioned already (I just skimmed this >>>>> thread): >>>>> >>>>> (a*b)%B = ((a%B)*(b%B))%B >>>>> >>>> >>>> Yes, but it doesn't solve the problem. :-( Suppose a*b overflows and >>>> that B is greater than both a and b. Then a%B is a, and b%B is b; then >>>> (a%B)*(b%B) is a*b, and it overflows. >>>> >>>> Another approach is brute force: write double-precision multiplication >>>> and division routines; not too hard, but involves tedious details; >>> >>> That seems to be vicious snipping on your part. SG said: >> >> "Vicious"? Really? > > Yes. > >>> >>> (a*b)%B = ((a%B)*(b%B))%B >>> >>> The last expression can be computed with a modified double-and-add >>> algorithm for multiplication that includes the modulo reduction. >>> >>> So, he did not rely on (a%B)*(b%B) not overflowing. >> >> Um, if the expression ((a%B)*(b%B)) overflows the behavior is >> undefined. "The last expression" is that result %B. If that's not your >> interpretation, please allow for the possibility that others may have >> read it differently without being "vicious". > > A fair interpretation has to take into account the full material of the > text. How does your interpretation deal with the "can be compute with a > modified double-and-add algorithm for multiplication" part? That part makes > it perfectly clear that SG did not intend (a%B)*(b%B) to be evaluated as a C > expression. Your interpretation (forced upon readers of your posting by > leaving out the part it ignored) makes it sound as though he did. That is > why I think your quoting was vicious: it left out a part that is crucial to > the interpretation of the part that was quoted. > Okay. The signal to bullshit ratio in this newsgroup is fast approaching zero. It's not worth the time spent reading it any more. Have a good life. -- Pete Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The Standard C++ Library Extensions: a Tutorial and Reference (www.petebecker.com/tr1book)
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(simple?) problem with multiplication Ttodir <sanmrtn96@gmail.com> - 2011-06-21 11:04 -0700
Re: (simple?) problem with multiplication Ttodir <sanmrtn96@gmail.com> - 2011-06-21 11:11 -0700
Re: (simple?) problem with multiplication "Balog Pal" <pasa@lib.hu> - 2011-06-21 20:35 +0200
Re: (simple?) problem with multiplication Anonymous <anonymous@no.net> - 2011-06-21 20:45 +0200
Re: (simple?) problem with multiplication Pete Becker <pete@versatilecoding.com> - 2011-06-21 14:46 -0400
Re: (simple?) problem with multiplication Ttodir <sanmrtn96@gmail.com> - 2011-06-21 12:03 -0700
Re: (simple?) problem with multiplication Gido <gido0001@gmail.com> - 2011-06-21 14:30 -0700
Re: (simple?) problem with multiplication Anonymous <anonymous@no.net> - 2011-06-21 23:32 +0200
Re: (simple?) problem with multiplication Juha Nieminen <nospam@thanks.invalid> - 2011-06-23 06:33 +0000
Re: (simple?) problem with multiplication Pete Becker <pete@versatilecoding.com> - 2011-06-21 14:40 -0400
Re: (simple?) problem with multiplication Christopher <cpisz@austin.rr.com> - 2011-06-21 11:39 -0700
Re: (simple?) problem with multiplication Anonymous <anonymous@no.net> - 2011-06-21 23:32 +0200
Re: (simple?) problem with multiplication Kai-Uwe Bux <jkherciueh@gmx.net> - 2011-06-21 23:41 +0200
Re: (simple?) problem with multiplication Rune Allnor <allnor@tele.ntnu.no> - 2011-06-21 21:33 -0700
Re: (simple?) problem with multiplication Ttodir <sanmrtn96@gmail.com> - 2011-06-22 04:59 -0700
Re: (simple?) problem with multiplication gwowen <gwowen@gmail.com> - 2011-06-22 06:55 -0700
Re: (simple?) problem with multiplication Pete Becker <pete@versatilecoding.com> - 2011-06-22 10:10 -0400
Re: (simple?) problem with multiplication Leigh Johnston <leigh@i42.co.uk> - 2011-06-22 15:19 +0100
Re: (simple?) problem with multiplication Dilip <rdilipk@gmail.com> - 2011-06-22 07:33 -0700
Re: (simple?) problem with multiplication Leigh Johnston <leigh@i42.co.uk> - 2011-06-22 15:39 +0100
Re: (simple?) problem with multiplication Pete Becker <pete@versatilecoding.com> - 2011-06-22 11:02 -0400
Re: (simple?) problem with multiplication "Paul" <pchristor@yahoo.co.uk> - 2011-06-23 23:45 +0100
Re: (simple?) problem with multiplication gwowen <gwowen@gmail.com> - 2011-06-22 23:59 -0700
Re: (simple?) problem with multiplication gwowen <gwowen@gmail.com> - 2011-06-22 23:50 -0700
Re: (simple?) problem with multiplication "Paul" <pchristor@yahoo.co.uk> - 2011-06-23 09:26 +0100
Re: (simple?) problem with multiplication "Paul" <pchristor@yahoo.co.uk> - 2011-06-22 21:24 +0100
Re: (simple?) problem with multiplication "Paul" <pchristor@yahoo.co.uk> - 2011-06-24 00:00 +0100
Re: (simple?) problem with multiplication gwowen <gwowen@gmail.com> - 2011-06-24 01:42 -0700
Re: (simple?) problem with multiplication "Paul" <pchristor@yahoo.co.uk> - 2011-06-24 10:47 +0100
Re: (simple?) problem with multiplication SG <s.gesemann@gmail.com> - 2011-06-24 10:09 -0700
Re: (simple?) problem with multiplication Pete Becker <pete@versatilecoding.com> - 2011-06-24 13:32 -0400
Re: (simple?) problem with multiplication Kai-Uwe Bux <jkherciueh@gmx.net> - 2011-06-24 20:07 +0200
Re: (simple?) problem with multiplication Pete Becker <pete@versatilecoding.com> - 2011-06-24 14:14 -0400
Re: (simple?) problem with multiplication Kai-Uwe Bux <jkherciueh@gmx.net> - 2011-06-24 20:28 +0200
Re: (simple?) problem with multiplication Pete Becker <pete@versatilecoding.com> - 2011-06-24 16:26 -0400
Re: (simple?) problem with multiplication "Paul" <pchristor@yahoo.co.uk> - 2011-06-25 03:01 +0100
Re: (simple?) problem with multiplication "Paul" <pchristor@yahoo.co.uk> - 2011-06-25 18:20 +0100
Re: (simple?) problem with multiplication "Alf P. Steinbach /Usenet" <alf.p.steinbach+usenet@gmail.com> - 2011-06-25 04:21 +0200
Re: (simple?) problem with multiplication "Paul" <pchristor@yahoo.co.uk> - 2011-06-25 10:12 +0100
Re: (simple?) problem with multiplication SG <s.gesemann@gmail.com> - 2011-06-25 02:51 -0700
Re: (simple?) problem with multiplication "Paul" <pchristor@yahoo.co.uk> - 2011-06-25 11:19 +0100
Re: (simple?) problem with multiplication "Paul" <pchristor@yahoo.co.uk> - 2011-06-25 11:19 +0100
Re: (simple?) problem with multiplication Gareth Owen <gwowen@gmail.com> - 2011-06-26 07:59 +0100
Re: (simple?) problem with multiplication "Paul" <pchristor@yahoo.co.uk> - 2011-06-26 12:34 +0100
Re: (simple?) problem with multiplication SG <s.gesemann@gmail.com> - 2011-06-25 02:37 -0700
Re: (simple?) problem with multiplication Gareth Owen <gwowen@gmail.com> - 2011-06-25 08:58 +0100
Re: (simple?) problem with multiplication "Paul" <pchristor@yahoo.co.uk> - 2011-06-25 10:00 +0100
Re: (simple?) problem with multiplication Kai-Uwe Bux <jkherciueh@gmx.net> - 2011-06-25 10:29 +0200
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