On Wed, Apr 10, 2013 at 11:40 AM, Julio Trevisan <juliotrevi...@gmail.com>wrote:
> Hi Anthony
>
> Thanks again.* *If it is a problem related to floating-point precision, I
> might use an Int64Col instead, since I don't need the timestamp miliseconds.
>
Another good plan since integers are exact ;)
>
>
> Julio
>
>
>
>
> On Wed, Apr 10, 2013 at 1:17 PM, Anthony Scopatz <scop...@gmail.com>wrote:
>
>> On Wed, Apr 10, 2013 at 7:44 AM, Julio Trevisan
>> <juliotrevi...@gmail.com>wrote:
>>
>>> Hi,
>>>
>>> I am using a Time64Col called "timestamp" in a condition, and I noticed
>>> that the condition does not work (i.e., no rows are selected) if I write
>>> something as:
>>>
>>> for row in node.where("timestamp == %f" % t):
>>> ...
>>>
>>> However, I had this idea of dividing the values by, say 1000, and it
>>> does work:
>>>
>>> for row in node.where("timestamp/1000 == %f" % t/1000):
>>> ...
>>>
>>> However, this doesn't seem to be an elegant solution. Please could
>>> someone point out a better solution to this?
>>>
>>
>> Hello Julio,
>>
>> While this may not be the most elegant solution it is probably one of the
>> most appropriate. The problem here likely stems from the fact that
>> floating point numbers (which are how Time64Cols are stored) are not exact
>> representations of the desired value. For example:
>>
>> In [1]: 1.1 + 2.2
>> Out[1]: 3.3000000000000003
>>
>> So when you divide my some constant order of magnitude, you are chopping
>> off the error associated with floating point precision. You are creating
>> a bin of this constant's size around the target value that is "close
>> enough" to count as equivalent. There are other mechanisms for alleviating
>> this issue: dividing and multiplying back (x/10)*10 == y, right shifting
>> (platform dependent), taking the difference and have it be less than some
>> tolerance x - y <= t. You get the idea. You have to mitigate this effect
>> some how.
>>
>> For more information please refer to:
>> http://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html
>>
>>
>>> Could this be related to the fact that my column name is "timestamp"? I
>>> ask this because I use a program called HDFView to brose the HDF5 file.
>>> This program refuses to show the first column when it is called
>>> "timestamp", but shows it when it is called "id". I don't know if the facts
>>> are related or not.
>>>
>>
>> This is probably unrelated.
>>
>> Be Well
>> Anthony
>>
>>
>>>
>>> I don't know if this is useful information, but the conversion of a
>>> typical "t" to string gives something like this:
>>>
>>> >> print "%f" % t
>>> 1365597435.000000
>>>
>>>
>>>
>>>
>>> ------------------------------------------------------------------------------
>>> Precog is a next-generation analytics platform capable of advanced
>>> analytics on semi-structured data. The platform includes APIs for
>>> building
>>> apps and a phenomenal toolset for data science. Developers can use
>>> our toolset for easy data analysis & visualization. Get a free account!
>>> http://www2.precog.com/precogplatform/slashdotnewsletter
>>> _______________________________________________
>>> Pytables-users mailing list
>>> Pytables-users@lists.sourceforge.net
>>> https://lists.sourceforge.net/lists/listinfo/pytables-users
>>>
>>>
>>
>>
>> ------------------------------------------------------------------------------
>> Precog is a next-generation analytics platform capable of advanced
>> analytics on semi-structured data. The platform includes APIs for building
>> apps and a phenomenal toolset for data science. Developers can use
>> our toolset for easy data analysis & visualization. Get a free account!
>> http://www2.precog.com/precogplatform/slashdotnewsletter
>> _______________________________________________
>> Pytables-users mailing list
>> Pytables-users@lists.sourceforge.net
>> https://lists.sourceforge.net/lists/listinfo/pytables-users
>>
>>
>
>
> ------------------------------------------------------------------------------
> Precog is a next-generation analytics platform capable of advanced
> analytics on semi-structured data. The platform includes APIs for building
> apps and a phenomenal toolset for data science. Developers can use
> our toolset for easy data analysis & visualization. Get a free account!
> http://www2.precog.com/precogplatform/slashdotnewsletter
> _______________________________________________
> Pytables-users mailing list
> Pytables-users@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/pytables-users
>
>
------------------------------------------------------------------------------
Precog is a next-generation analytics platform capable of advanced
analytics on semi-structured data. The platform includes APIs for building
apps and a phenomenal toolset for data science. Developers can use
our toolset for easy data analysis & visualization. Get a free account!
http://www2.precog.com/precogplatform/slashdotnewsletter
_______________________________________________
Pytables-users mailing list
Pytables-users@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/pytables-users
