Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #127063

Re: Catching overflow

From Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups comp.lang.c
Subject Re: Catching overflow
Date 2018-02-26 01:25 +0000
Organization A noiseless patient Spider
Message-ID <878tbgtudk.fsf@bsb.me.uk> (permalink)
References <p6vf4p$ot1$1@dont-email.me> <87efl8two2.fsf@bsb.me.uk> <p6vl8r$2c3$1@dont-email.me>

Show all headers | View raw

jacobnavia <jacob@jacob.remcomp.fr> writes:

> Le 26/02/2018 à 01:35, Ben Bacarisse a écrit :
>> jacobnavia <jacob@jacob.remcomp.fr> writes:
>>
>>> I want to do the computation of catalan numbers in 480 bits (my qfloat
>>> extension to C)
>>
>> Are approximate values OK?  I'm not sure what role floating point is
>> going to play here.
>>
>>> But I would like to do as much calculations in 128 bits only (at the
>>> start of the series) as possible.
>>>
>>> Possible means until before the computation overflows.
>>>
>>> How can I know if a multiplication will overflow in 128 bits?
>>>
>>> Let's see:
>>>
>>> I want to know when
>>>
>>> 	A x B > pow(2,128)
>>>
>>> (A and B < pow(2,64), unsigned long long data)
>>
>> Is that a typo?  If A and B are both < pow(2,64), A*B will not be >
>> pow(2,128).
>>
>> <snip>
>>
> You arez right. I wanted to go a bit further, so I casted into 128
> bits, so A and B could be bigger than 2^64

Doing some of the calculation in 128 bits may be a needless
optimisation.  With a series that grows this fast, a table is almost
always going to be the best way to go.  A table of just 70 numbers can
hold all the 128-bit Catalan numbers.  Even if you don't pre-compute
any, just caching these as they get computed would probably mean you can
use qfloats from the start.

-- 
Ben.

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Catching overflow jacobnavia <jacob@jacob.remcomp.fr> - 2018-02-25 23:59 +0100
  Re: Catching overflow David Brown <david.brown@hesbynett.no> - 2018-02-26 00:32 +0100
    Re: Catching overflow jacobnavia <jacob@jacob.remcomp.fr> - 2018-02-26 01:45 +0100
  Re: Catching overflow Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-02-26 00:35 +0000
    Re: Catching overflow jacobnavia <jacob@jacob.remcomp.fr> - 2018-02-26 01:44 +0100
      Re: Catching overflow Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-02-26 01:25 +0000
        Re: Catching overflow jacobnavia <jacob@jacob.remcomp.fr> - 2018-02-26 10:53 +0100
          Re: Catching overflow Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-02-26 15:21 +0000
            Re: Catching overflow jacobnavia <jacob@jacob.remcomp.fr> - 2018-02-26 18:36 +0100
              Re: Catching overflow Ike Naar <ike@iceland.freeshell.org> - 2018-02-26 22:55 +0000
                Re: Catching overflow Tim Rentsch <txr@alumni.caltech.edu> - 2018-02-26 21:59 -0800
                Re: Catching overflow David Brown <david.brown@hesbynett.no> - 2018-02-27 09:30 +0100
                Re: Catching overflow Tim Rentsch <txr@alumni.caltech.edu> - 2018-03-03 02:17 -0800
                Re: Catching overflow Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-03 03:19 -0800
                Re: Catching overflow asetofsymbols@gmail.com - 2018-03-03 03:20 -0800
                Re: Catching overflow Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-03 03:50 -0800
                Re: Catching overflow David Brown <david.brown@hesbynett.no> - 2018-03-04 13:48 +0100
                Re: Catching overflow asetofsymbols@gmail.com - 2018-03-04 07:11 -0800
                Re: Catching overflow Ike Naar <ike@iceland.freeshell.org> - 2018-03-11 09:21 +0000
                Re: Catching overflow Ike Naar <ike@iceland.freeshell.org> - 2018-03-11 09:26 +0000
                Re: Catching overflow Tim Rentsch <txr@alumni.caltech.edu> - 2018-03-26 22:36 -0700
                Re: Catching overflow David Brown <david.brown@hesbynett.no> - 2018-03-27 15:15 +0200
                Re: Catching overflow Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-27 06:26 -0700
              Re: Catching overflow Ike Naar <ike@iceland.freeshell.org> - 2018-02-27 19:41 +0000
                Re: Catching overflow Ike Naar <ike@iceland.freeshell.org> - 2018-02-27 20:14 +0000
                Re: Catching overflow jacobnavia <jacob@jacob.remcomp.fr> - 2018-02-27 22:03 +0100
            Re: Catching overflow jacobnavia <jacob@jacob.remcomp.fr> - 2018-02-26 18:40 +0100
              Re: Catching overflow Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-02-26 18:31 +0000
      Re: Catching overflow Steve Carroll <frelwizzen@gmail.com> - 2018-03-01 08:45 -0800
  Re: Catching overflow bartc <bc@freeuk.com> - 2018-02-26 01:12 +0000
    Re: Catching overflow Richard Damon <Richard@Damon-Family.org> - 2018-02-25 23:10 -0500
  Catching overflow asetofsymbols@gmail.com - 2018-02-25 22:09 -0800
  Catching overflow asetofsymbols@gmail.com - 2018-02-25 23:30 -0800
    Catching overflow asetofsymbols@gmail.com - 2018-02-25 23:31 -0800
    Catching overflow asetofsymbols@gmail.com - 2018-02-28 00:03 -0800
      Catching overflow asetofsymbols@gmail.com - 2018-02-28 22:49 -0800
  Re: Catching overflow "Mr. Man-wai Chang" <toylet.toylet@gmail.com> - 2018-02-26 23:37 +0800
    Re: Catching overflow jacobnavia <jacob@jacob.remcomp.fr> - 2018-02-26 18:33 +0100
      Re: Catching overflow "Mr. Man-wai Chang" <toylet.toylet@gmail.com> - 2018-02-28 22:31 +0800
        Re: Catching overflow "Mr. Man-wai Chang" <toylet.toylet@gmail.com> - 2018-02-28 22:33 +0800
      Re: Catching overflow already5chosen@yahoo.com - 2018-03-01 02:31 -0800
        Re: Catching overflow asetofsymbols@gmail.com - 2018-03-01 05:45 -0800
          Re: Catching overflow already5chosen@yahoo.com - 2018-03-01 08:24 -0800
            Re: Catching overflow asetofsymbols@gmail.com - 2018-03-01 09:03 -0800
              Re: Catching overflow Steve Carroll <frelwizzen@gmail.com> - 2018-03-01 09:32 -0800
            Re: Catching overflow jacobnavia <jacob@jacob.remcomp.fr> - 2018-03-01 19:47 +0100
              Re: Catching overflow already5chosen@yahoo.com - 2018-03-01 10:58 -0800
                Re: Catching overflow jacobnavia <jacob@jacob.remcomp.fr> - 2018-03-01 20:08 +0100
                Re: Catching overflow already5chosen@yahoo.com - 2018-03-01 11:14 -0800
                Re: Catching overflow jacobnavia <jacob@jacob.remcomp.fr> - 2018-03-01 20:50 +0100
                Re: Catching overflow already5chosen@yahoo.com - 2018-03-01 12:18 -0800
            Re: Catching overflow supercat@casperkitty.com - 2018-03-03 15:08 -0800
  Re: Catching overflow jladasky@itu.edu - 2018-02-26 13:07 -0800
    Re: Catching overflow jacobnavia <jacob@jacob.remcomp.fr> - 2018-02-26 22:44 +0100

csiph-web