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
