[datatable-help] sorting on a floating point column

[Arunkumar Srinivasan]

This is following up on a thread from a couple years ago: 
http://lists.r-forge.r-project.org/pipermail/datatable-help/2013-May/001689.html 
Things have changed A LOT! I suggest you keep up-to-date by reading the README about bug fixes and features from the github project page: https://github.com/Rdatatable/data.table

I ran into this problem myself, it took a bit of time to debug because it is so surprising. 
What’s surprising? Reproducible example please. data.table package version, R version as well please. 
Without that my best guess is for you to look at `?setNumericRounding`.

-- 
Arun

On 27 January 2016 at 21:40:23, frederik at ofb.net (frederik at ofb.net) wrote:

This is following up on a thread from a couple years ago:  

http://lists.r-forge.r-project.org/pipermail/datatable-help/2013-May/001689.html  

I ran into this problem myself, it took a bit of time to debug because  
it is so surprising.  

In my case, I was using order() to sort a list of floats.  

I expected the result to be monotonic but it wasn't!  

Then I found out that the problem was due to 'order' being part of the  
data.table library. By using base::order, I was able to get correct  
behavior.  

I don't understand why improperly ordering floating point data helps  
the data.table library accomplish anything, whether it is looking up  
keys or what.  

Also, it must be much slower to compare floats with a tolerance, than  
to just compare them. I seem to recall that floats were designed so  
that normal comparison is quite fast.  

Please fix this bug, or at least document it more visibly.  

Thank you,  

Frederick Eaton  
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