R devel - Apr 2018 - mean(x) for ALTREP

Hi,

By looking at a doc about ALTREP 
https://svn.r-project.org/R/branches/ALTREP/ALTREP.html (by the way 
congratulations for that and for R-3.5.0 in general), I was a little bit 
surprised by the following example:

 > x <- 1:1e10
 > system.time(print(mean(x)))
[1] 5e+09
 ?? user? system elapsed
 ?38.520?? 0.008? 38.531

Taking 38.520 s to calculate a mean value of an arithmetic sequence 
seemed a lot to me. It probably means that calculations are made by 
running into a for loop while in the case of arithmetic sequence a mean 
value can simply be calculated as (b+e)/2 where b and e are the begin 
and end value respectively. Is it planned to take benefit of ALTREP for 
functions like mean(), sum(), min(), max() and some others to avoid 
running a for loop wherever possible? It seems so natural to me but 
after all some implementation details preventing this can escape to me.

Best,
Serguei.

Serguei,

The R 3.5.0 release includes the fundamental ALTREP framework but does not
include many 'hooks' within R's source code to make use of methods
on the
ALTREP custom vector classes. I have implemented a fair number, including
for mean() to use the custom Sum method when available, in the ALTREP
branch but unfortunately we did not have time to test and port them to the
trunk in time for this release. The current plan, as I understand it, is
that we will continue to develop and test these, and other hooks, and then
when ready they will be ported into trunk/R-devel over the course this
current development cycle for inclusion in the next release of R.

My hope is that the end-user benefits of ALTREP will really show through
much more in future releases, but for now, things like mean will will
behave as they always have from a user perspective.

Best,
~G


On Thu, Apr 26, 2018 at 2:31 AM, Serguei Sokol <sokol at insa-toulouse.fr>
wrote:
> Hi,
>
> By looking at a doc about ALTREP https://svn.r-project.org/R/br
> anches/ALTREP/ALTREP.html (by the way congratulations for that and for
> R-3.5.0 in general), I was a little bit surprised by the following example:
>
> > x <- 1:1e10
> > system.time(print(mean(x)))
> [1] 5e+09
>    user  system elapsed
>  38.520   0.008  38.531
>
> Taking 38.520 s to calculate a mean value of an arithmetic sequence seemed
> a lot to me. It probably means that calculations are made by running into a
> for loop while in the case of arithmetic sequence a mean value can simply
> be calculated as (b+e)/2 where b and e are the begin and end value
> respectively. Is it planned to take benefit of ALTREP for functions like
> mean(), sum(), min(), max() and some others to avoid running a for loop
> wherever possible? It seems so natural to me but after all some
> implementation details preventing this can escape to me.
>
> Best,
> Serguei.
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
>

-- 
Gabriel Becker, Ph.D
Scientist
Bioinformatics and Computational Biology
Genentech Research

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