[datatable-help] sorting on floating point column

[Matthew Dowle]

Or, perhaps the tolerance should be a function of the range of the
column. [The range would be quick to calculate with a single C for
loop.] 

On 30.04.2013 15:09, Matthew Dowle wrote: 

> Hi, 
> 
>
data.table sorts double within machine tolerance : 
> 
>>
sqrt(.Machine$double.eps)
> [1] 1.490116e-08
>> 
> 
> i.e. numbers
closer than this are considered equal.
> 
> Otherwise we wouldn't be
able to do things like DT[.(3.14)].
> 
> I had a quick look, see
arguments of data.table:::ordernumtol which takes "tol" but there is no
option provided (yet) to change this. Do we need one?
> 
> In the
examples section of one of the help pages it has an example which
generates a series of numers very close together using pi. Note that
your numbers are both close together, and, very close to 0.
> 
>
Matthew
> 
> On 30.04.2013 14:52, Arunkumar Srinivasan wrote: 
> 
>> Hi
there, 
>> I just saw something strange when I was sorting a column of
p-values. I checked the data.table bug tracker for words "sort" and
"floating point" and there were no hits for this case. There's a bug for
"integer 64" sort on a column though. 
>> So, here's a reproducible
example. I'd be glad to file a bug, if it is and be corrected if it's
something I am doing wrong. 
>> 
>> set.seed(45) 
>> dt <-
data.table(x=sample(50), y= sample(c(seq(0, 1, length.out=1000),
7000000:7000100), 50)/1e7) 
>> head(dt) 
>> x y 
>> 1: 32 5.395395e-08

>> 2: 16 6.956957e-08 
>> 3: 12 2.142142e-08 
>> 4: 18 5.855856e-08 
>>
5: 17 6.216216e-08 
>> 6: 14 5.025025e-08 
>> setkey(dt, "y") # sort by
column y 
>> head(dt, 10) 
>> x y 
>> 1: 47 1.401401e-09 
>> 2: 12
2.142142e-08 
>> 3: 24 1.391391e-08 
>> 4: 43 9.809810e-09 <~~~
obviously false 
>> 5: 1 2.932933e-08 
>> 6: 48 2.562563e-08 
>> 7: 49
1.891892e-08 
>> 8: 40 2.182182e-08 
>> 9: 9 7.307307e-09 <~~~ obviously
false 
>> 10: 45 2.482482e-08 
>> 
>> Best, 
>> Arun

 
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