[Rd] R-devel does not update the C++ returned variables
Dénes Tóth
toth.denes at ttk.mta.hu
Mon Mar 2 21:18:15 CET 2015
On 03/02/2015 04:37 PM, Martin Maechler wrote:
>
>> On 2 March 2015 at 09:09, Duncan Murdoch wrote:
>> | I generally recommend that people use Rcpp, which hides a lot of the
>> | details. It will generate your .Call calls for you, and generate the
>> | C++ code that receives them; you just need to think about the real
>> | problem, not the interface. It has its own learning curve, but I think
>> | it is easier than using the low-level code that you need to work with .Call.
>
>> Thanks for that vote, and I second that.
>
>> And these days the learning is a lot flatter than it was a decade ago:
>
>> R> Rcpp::cppFunction("NumericVector doubleThis(NumericVector x) { return(2*x); }")
>> R> doubleThis(c(1,2,3,21,-4))
>> [1] 2 4 6 42 -8
>> R>
>
>> That defined, compiled, loaded and run/illustrated a simple function.
>
>> Dirk
>
> Indeed impressive, ... and it also works with integer vectors
> something also not 100% trivial when working with compiled code.
>
> When testing that, I've went a step further:
>
> ##---- now "test":
> require(microbenchmark)
> i <- 1:10
Note that the relative speed of the algorithms also depends on the size
of the input vector. i + i becomes the winner for longer vectors (e.g. i
<- 1:1e6), but a proper Rcpp version is still approximately twice as fast.
Rcpp::cppFunction("NumericVector doubleThisNum(NumericVector x) {
return(2*x); }")
Rcpp::cppFunction("IntegerVector doubleThisInt(IntegerVector x) {
return(2*x); }")
i <- 1:1e6
mb <- microbenchmark::microbenchmark(doubleThisNum(i), doubleThisInt(i),
i*2, 2*i, i*2L, 2L*i, i+i, times=100)
plot(mb, log="y", notch=TRUE)
> (mb <- microbenchmark(doubleThis(i), i*2, 2*i, i*2L, 2L*i, i+i, times=2^12))
> ## Lynne (i7; FC 20), R Under development ... (2015-03-02 r67924):
> ## Unit: nanoseconds
> ## expr min lq mean median uq max neval cld
> ## doubleThis(i) 762 985 1319.5974 1124 1338 17831 4096 b
> ## i * 2 124 151 258.4419 164 221 22224 4096 a
> ## 2 * i 127 154 266.4707 169 216 20213 4096 a
> ## i * 2L 143 164 250.6057 181 234 16863 4096 a
> ## 2L * i 144 177 269.5015 193 237 16119 4096 a
> ## i + i 152 183 272.6179 199 243 10434 4096 a
>
> plot(mb, log="y", notch=TRUE)
> ## hmm, looks like even the simple arithm. differ slightly ...
> ##
> ## ==> zoom in:
> plot(mb, log="y", notch=TRUE, ylim = c(150,300))
>
> dev.copy(png, file="mbenchm-doubling.png")
> dev.off() # [ <- why do I need this here for png ??? ]
> ##--> see the appended *png graphic
>
> Those who've learnt EDA or otherwise about boxplot notches, will
> know that they provide somewhat informal but robust pairwise tests on
> approximate 5% level.
> From these, one *could* - possibly wrongly - conclude that
> 'i * 2' is significantly faster than both 'i * 2L' and also
> 'i + i' ---- which I find astonishing, given that i is integer here...
>
> Probably no reason for deep thoughts here, but if someone is
> enticed, this maybe slightly interesting to read.
>
> Martin Maechler, ETH Zurich
>
>
>
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