BUG #5150: math bug

[Gray-14]

The following bug has been logged online:

Bug reference:      5150
Logged by:          Gray
Email address:      gray@...
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

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[Robert Haas]

On Fri, Oct 30, 2009 at 1:39 AM, Gray <gray@...> wrote:

>
> The following bug has been logged online:
>
> Bug reference:      5150
> Logged by:          Gray
> Email address:      gray@...
> 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

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[Tom Lane-2]

Robert Haas <robertmhaas@...> 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

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[John R Pierce]

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>



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[David Fetter]

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.
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[Robert Haas]

On Fri, Oct 30, 2009 at 11:51 PM, John R Pierce <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>

That's true.  Nobody said otherwise.

...Robert

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[Mark Kirkwood]

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

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