From edd at debian.org  Mon Mar  2 20:39:48 2015
From: edd at debian.org (Dirk Eddelbuettel)
Date: Mon, 2 Mar 2015 13:39:48 -0600
Subject: [Rd] R-devel does not update the C++ returned variables
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On 2 March 2015 at 16:37, 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:

As you may know, int can be 'casted up' to double which is what happens
here.  So in what follows you _always_ create a copy from an int vector to a
numeric vector. 

For pure int, use eg 

    Rcpp::cppFunction("IntegerVector doubleThis(IntegeerVector x) { return(2*x); }")

and rename the function names as needed to have two defined concurrently.

Dirk

| 
| ##---- 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
| 
| [DELETED ATTACHMENT mbenchm-doubling.png, PNG image]

-- 
http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org



[Rd] R-devel does not update the C++ returned variables
Dirk Eddelbuettel