Thread

1. BUG #5150: math bug

   Gray <gray@ms-irk.ru> — 2009-10-30T05:39:10Z

   The following bug has been logged online:
   
   Bug reference:      5150
   Logged by:          Gray
   Email address:      gray@ms-irk.ru
   PostgreSQL version: 8.2.6
   Operating system:   i686-pc-linux-gnu
   Description:        math bug
   Details: 
   
   select 1/3*3,(1.0/3.0)*3.0,floor((1.0/3.0)*3.0);
   
   returns
   0, 1, 0
   
   

2. Re: BUG #5150: math bug

   Robert Haas <robertmhaas@gmail.com> — 2009-10-31T03:10:02Z

   On Fri, Oct 30, 2009 at 1:39 AM, Gray <gray@ms-irk.ru> wrote:
   >
   > The following bug has been logged online:
   >
   > Bug reference:      5150
   > Logged by:          Gray
   > Email address:      gray@ms-irk.ru
   > PostgreSQL version: 8.2.6
   > Operating system:   i686-pc-linux-gnu
   > Description:        math bug
   > Details:
   >
   > select 1/3*3,(1.0/3.0)*3.0,floor((1.0/3.0)*3.0);
   >
   > returns
   > 0, 1, 0
   
   Well, the first answer is correct, because 1/3 is a request for
   integer division, so you get 0, and 0 * 3 is still zero.
   
   I don't believe the second answer is really what you got, because
   surely if you requested floating-point division the answer would be a
   floating point number, not just 1.  On pg 8.3.8, I get
   0.999999999999999999990, which explains why the third answer comes out
   to zero.
   
   In general, floating point arithmetic is inaccurate and sucky.  That
   has nothing to do with PostgreSQL; it's just life.
   
   http://en.wikipedia.org/wiki/Floating_point#Accuracy_problems
   
   ...Robert
   
   

3. Re: BUG #5150: math bug

   Tom Lane <tgl@sss.pgh.pa.us> — 2009-10-31T03:29:29Z

   Robert Haas <robertmhaas@gmail.com> writes:
   > In general, floating point arithmetic is inaccurate and sucky.  That
   > has nothing to do with PostgreSQL; it's just life.
   
   Actually, I think these examples are being done in "numeric" not float8.
   Your comment stands though --- 1.0/3.0 does not give the exact rational
   number 1/3, but some finite decimal approximation to it, which when
   multiplied by 3 will not produce exactly 1.0.
   
   There is special-purpose software out there that can compute exactly
   with rational numbers, but you aren't likely to find it embedded in any
   general-purpose tools like databases --- the use-case just isn't wide
   enough.  One reason why not is that it'll still fall down on irrational
   numbers.
   
   			regards, tom lane
   
   

4. Re: BUG #5150: math bug

   John R Pierce <pierce@hogranch.com> — 2009-10-31T03:51:57Z

   Tom Lane wrote:
   > There is special-purpose software out there that can compute exactly
   > with rational numbers, but you aren't likely to find it embedded in any
   > general-purpose tools like databases --- the use-case just isn't wide
   > enough.  One reason why not is that it'll still fall down on irrational
   > numbers.
   >   
   
   
   <nit>
   
   1/3 is a rational number.  however,  it is a repeating fraction when 
   expressed in decimal.
   
   </nit>
   
   
   
   

5. Re: BUG #5150: math bug

   David Fetter <david@fetter.org> — 2009-10-31T05:45:21Z

   On Fri, Oct 30, 2009 at 08:51:57PM -0700, John R Pierce wrote:
   > Tom Lane wrote:
   >> There is special-purpose software out there that can compute
   >> exactly with rational numbers, but you aren't likely to find it
   >> embedded in any general-purpose tools like databases --- the
   >> use-case just isn't wide enough.  One reason why not is that it'll
   >> still fall down on irrational numbers.
   >>   
   >
   >
   > <nit>
   >
   > 1/3 is a rational number.  however,  it is a repeating fraction when
   > expressed in decimal.
   >
   > </nit>
   
   <nit level="2">
   The set of algebraic numbers, of which rational numbers are a proper
   subset, is countable and hence has Lebesgue measure zero on the real
   line.
   </nit> ;)
   
   Cheers,
   David.
   -- 
   David Fetter <david@fetter.org> http://fetter.org/
   Phone: +1 415 235 3778  AIM: dfetter666  Yahoo!: dfetter
   Skype: davidfetter      XMPP: david.fetter@gmail.com
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6. Re: BUG #5150: math bug

   Robert Haas <robertmhaas@gmail.com> — 2009-10-31T13:10:56Z

   On Fri, Oct 30, 2009 at 11:51 PM, John R Pierce <pierce@hogranch.com> wrote:
   > Tom Lane wrote:
   >>
   >> There is special-purpose software out there that can compute exactly
   >> with rational numbers, but you aren't likely to find it embedded in any
   >> general-purpose tools like databases --- the use-case just isn't wide
   >> enough.  One reason why not is that it'll still fall down on irrational
   >> numbers.
   >>
   >
   >
   > <nit>
   >
   > 1/3 is a rational number.  however,  it is a repeating fraction when
   > expressed in decimal.
   >
   > </nit>
   
   That's true.  Nobody said otherwise.
   
   ...Robert
   
   

7. Re: BUG #5150: math bug

   Mark Kirkwood <markir@paradise.net.nz> — 2009-11-01T07:12:27Z

   David Fetter wrote:
   > On Fri, Oct 30, 2009 at 08:51:57PM -0700, John R Pierce wrote:
   >   
   >> Tom Lane wrote:
   >>     
   >>> There is special-purpose software out there that can compute
   >>> exactly with rational numbers, but you aren't likely to find it
   >>> embedded in any general-purpose tools like databases --- the
   >>> use-case just isn't wide enough.  One reason why not is that it'll
   >>> still fall down on irrational numbers.
   >>>   
   >>>       
   >> <nit>
   >>
   >> 1/3 is a rational number.  however,  it is a repeating fraction when
   >> expressed in decimal.
   >>
   >> </nit>
   >>     
   >
   > <nit level="2">
   > The set of algebraic numbers, of which rational numbers are a proper
   > subset, is countable and hence has Lebesgue measure zero on the real
   > line.
   > </nit> ;)
   >
   >   
   
   LOL - fortunately (going by the bug) he is not trying to compute a 
   measure (i.e integrate) from a set of 'em.
   
   Mark