R-devel does not update the C++ returned variables
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 (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 -------------- next part -------------- A non-text attachment was scrubbed... Name: mbenchm-doubling.png Type: image/png Size: 7244 bytes Desc: not available URL: <https://stat.ethz.ch/pipermail/r-devel/attachments/20150302/0f61efab/attachment.png>