[Rd] Long execution time for quantile() and difftime objects (PR#14091)

[Prof Brian Ripley]

Did you read the help page?

        x: numeric vector whose sample quantiles are wanted.  ‘NA’ and
           ‘NaN’ values are not allowed unless ‘na.rm’ is ‘TRUE’.

so only 'numeric' vectors are really supported, although it does say

      The default method does not allow factors, but works with objects
      sufficiently like numeric vectors that ‘sort’, addition and
      multiplication work correctly.  In principle only sorts and
      weighted means are needed, so datatimes could have quantiles - but
      this is not implemented.

There is no claim that it works (let alone works well) for class 
"difftime".  If you follow the link to 'sort' it says

      The default ‘sort’ method makes use of ‘order’ for objects with
      classes, which in turn makes use of the generic function ‘xtfrm’.

and from ?xtfrm

      The default method will make use of ‘==’ and ‘>’ methods for the
      class of ‘x[i]’ (for integers ‘i’), and the ‘is.na’ method for the
      class of ‘x’, but might be rather slow when doing so.

So, if you want this to be fast, you need to write an xtfrm method. 
There is one in R-devel

> xtfrm.difftime
function (x)
as.numeric(x)

and you can use that in your workspace (and your example is fast in 
R-devel, because of that function I think, there being other 
development work in progress in the version I tried).


On Fri, 27 Nov 2009, hong.ooi at anz.com wrote:

> Full_Name: Hong Ooi
> Version: 2.10.0
> OS: Windows XP
> Submission from: (NULL) (203.110.235.1)
>
>
> While trying to get summary statistics on a duration variable (the difference
> between a start and end date), I ran into the following issue. Using summary or
> quantile (which summary calls) on a difftime object takes an extremely long time
> if the object is even moderately large.
>
> A reproducible example:
>
>> x <- as.Date(1:10000, origin="1900-01-01")
>> x[1:10]
> [1] "1900-01-02" "1900-01-03" "1900-01-04" "1900-01-05" "1900-01-06"
> [6] "1900-01-07" "1900-01-08" "1900-01-09" "1900-01-10" "1900-01-11"
>> d <- x - as.Date("1900-01-01")
>> d[1:10]
> Time differences in days
> [1]  1  2  3  4  5  6  7  8  9 10
>> system.time(summary(d[1:10]))
>   user  system elapsed
>   0.01    0.00    0.01
>> system.time(summary(d[1:100]))
>   user  system elapsed
>   0.21    0.00    0.20
>> system.time(summary(d[1:1000]))
>   user  system elapsed
>   3.02    0.00    3.02
>> system.time(summary(d[1:10000]))
>   user  system elapsed
>  43.56    0.04   43.66
>
>
> If I unclass d, there is no problem:
>
>> system.time(summary(unclass(d[1:10000])))
>   user  system elapsed
>      0       0       0
>
> Testing with Rprof() indicates that the problem lies in [.difftime, although the
> code for that function seems innocuous enough.
>
>
>> sessionInfo()
> R version 2.10.0 (2009-10-26)
> i386-pc-mingw32
>
> locale:
> [1] LC_COLLATE=English_Australia.1252  LC_CTYPE=English_Australia.1252
> [3] LC_MONETARY=English_Australia.1252 LC_NUMERIC=C
> [5] LC_TIME=English_Australia.1252
>
> attached base packages:
> [1] stats     graphics  grDevices utils     datasets  methods   base
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595