Hi Romain,
thanks for this precise answer. So the suggested methods below will work
without making a copy of the object. What is about the implicit call of
Rcpp::as<>() inside arma::mat()? It is a very convenient way to create an arma
object reusing memory and it seems to work just fine.
Best
Simon
On Jun 7, 2013, at 3:47 PM, Romain Francois <rom...@r-enthusiasts.com> wrote:
> Le 07/06/13 15:14, Simon Zehnder a écrit :
>> Hi Romain, hi Dirk,
>>
>> sorry for posting here again, but I found something in some way connected to
>> this discussion - and pretty interesting concerning the Rcpp::as<>()
>> function:
>>
>> 1. I create a class containing a list:
>>
>> setClass("myclass", representation(par = "list"))
>>
>> l <- list(lambda = array(0, dim = c(10,2)))
>>
>> mclass <- new("myclass", par = l)
>>
>> 2. I compile a C++ function, that should reuse the memory of 'lambda' inside
>> the list:
>>
>> library(inline)
>> cfunction <- cxxfunction(signature(myClass_R = "array"), body = 'Rcpp::S4
>> myclass(myClass_R); Rcpp::List parL((SEXP) myclass.slot("par")); arma::mat
>> armaPar(parL["lambda"].begin(), 10, 2, false, true); armaPar(0,0) = 1.2
>> ;return Rcpp::wrap(myclass);', plugin = "RcppArmadillo")
>>
>> I get an error:
>>
>> Error in compileCode(f, code, language = language, verbose = verbose) :
>> Compilation ERROR, function(s)/method(s) not created! file6fc2ab39965.cpp:
>> In function ‘SEXPREC* file6fc2ab39965(SEXP)’:
>> file6fc2ab39965.cpp:30:109: error: ‘Rcpp::Vector<19>::NameProxy’ has no
>> member named ‘begin’
>> Rcpp::S4 myclass(myClass_R); Rcpp::List parL((SEXP) myclass.slot("par"));
>> arma::umat armaPar(parL["lambda"].begin(), 100, 10, false, true);
>> armaPar(0,0) = 1.2 ;return Rcpp::wrap(myclass);
>
> What you get from doing parL["lambda"] is an object of class NameProxy, it
> will be converted to the appropioate class later. NameProxy have no .begin
> method.
>
> Do this:
>
> NumericVector tmp = parL["lambda"] ;
> arma::mat armaPar(tmp.begin(), 10, 2, false, true);
>
> This will not make copies of the data.
>
> If thius feels like too much to type, just stuff this into a function:
>
> inline arma::mat convert( SEXP x ){
> NumericVector tmp = parL["lambda"] ;
> return arma::mat(tmp.begin(), 10, 2, false, true);
> }
>
> and call :
>
> arma::mat armaPar = convert( parL["lambda"] ) ;
>
> There is a chance convert won't make a copy thanks to RVO.
> ^
>> make: *** [file6fc2ab39965.o] Error 1
>> In addition: Warning message:
>> running command '/Library/Frameworks/R.framework/Resources/bin/R CMD SHLIB
>> file6fc2ab39965.cpp 2> file6fc2ab39965.cpp.err.txt' had status 1
>>
>> -----------------------------------------------------------
>>
>> It seems, that the SEXP inside the List is not yet an Rcpp::NumericMatrix,
>> so I make it explicit to the arma::mat() constructor:
>>
>> cfunction <- cxxfunction(signature(myClass_R = "array"), body = 'Rcpp::S4
>> myclass(myClass_R); Rcpp::List parL((SEXP) myclass.slot("par")); arma::mat
>> armaPar(Rcpp::as<Rcpp::NumericMatrix>(parL["lambda"]).begin(), 100, 10,
>> false, true); armaPar(0,0) = 1.2 ;return Rcpp::wrap(myclass);', plugin =
>> "RcppArmadillo")
>>
>> That compiles.
>>
>> If we let it run it gives us:
>>
>> cfunction(mclass)
>> An object of class "myclass"
>> Slot "par":
>> $lambda
>> [,1] [,2]
>> [1,] 1.2 0
>> [2,] 0.0 0
>> [3,] 0.0 0
>> [4,] 0.0 0
>> [5,] 0.0 0
>> [6,] 0.0 0
>> [7,] 0.0 0
>> [8,] 0.0 0
>> [9,] 0.0 0
>> [10,] 0.0 0
>>
>> So, we can see, that implicitly calling Rcpp::as<>() inside the constructor
>> arma::mat() avoids a copy by Rcpp::as<>() - which is just beautiful!
>>
>>
>> Best
>>
>> Simon
>>
>>
>>
>>
>>
>> On Jun 7, 2013, at 1:19 PM, Simon Zehnder <szehn...@uni-bonn.de> wrote:
>>
>>> Thank you Romain!
>>>
>>> All clear now!
>>>
>>>
>>> Best
>>>
>>> Simon
>>>
>>> On Jun 7, 2013, at 1:13 PM, Romain Francois <rom...@r-enthusiasts.com>
>>> wrote:
>>>
>>>> Le 07/06/13 13:09, Simon Zehnder a écrit :
>>>>> HI Dirk, hi Romain,
>>>>>
>>>>> allright, this is now clear to me, if I want to reuse memory, the
>>>>> allocated memory from R (so implicitly in C) must of course have the same
>>>>> type - otherwise the memory has a different size.
>>>>
>>>> so far, this is obvious.
>>>>
>>>>> On the other side I then assume, that the Rcpp:as<class M>() function
>>>>> makes a cast and therefore creates a copy with new allocated memory of
>>>>> the type included in M? Therefore a reuse of memory with this function is
>>>>> not possible.
>>>>
>>>> Depends what is M, but most of the time yes, as<> copies data.
>>>>
>>>>> Best
>>>>>
>>>>> Simon
>>>>>
>>>>>
>>>>> On Jun 6, 2013, at 8:31 PM, Dirk Eddelbuettel <e...@debian.org> wrote:
>>>>>
>>>>>>
>>>>>> On 6 June 2013 at 13:17, Dirk Eddelbuettel wrote:
>>>>>> |
>>>>>> | On 6 June 2013 at 19:05, Simon Zehnder wrote:
>>>>>> | | sorry I had overseen this message from you. Okay, so the explicit
>>>>>> cast to SEXP together with the assignment operator makes the deal. But
>>>>>> it still includes the reuse of memory right, i.e. the '=' does not call
>>>>>> the copy constructor?
>>>>>> |
>>>>>> | But how could an _unsigned int_ from Armadillo possibly have the same
>>>>>> value
>>>>>> | as a _signed int_ in R?
>>>>>> |
>>>>>> | Either you are efficient (no copy), or you are correct (with a copy).
>>>>>> I do
>>>>>> | not see how you could have both.
>>>>>>
>>>>>> Sorry -- please ignore this message.
>>>>>>
>>>>>> I had mistakenly assumed that you were still thinking about arma::umat to
>>>>>> integer. For direct int-to-int this is indeed efficient just like the
>>>>>> double-to-double is for arma::mat to NumericMatrix (and dito for
>>>>>> vectors). I
>>>>>> though that was a given -- maybe need to make this (even) more explicit
>>>>>> in
>>>>>> the documentation.
>>>>>>
>>>>>> So in sum: you get correct and efficient behaviour if and only if you
>>>>>> stick
>>>>>> with the types natively supported in Armadillo and R.
>>>>>>
>>>>>> Dirk
>>>>>>
>>>>>> --
>>>>>> Dirk Eddelbuettel | e...@debian.org | http://dirk.eddelbuettel.com
>>>>>
>>>>>
>>>>
>>>>
>>>> --
>>>> Romain Francois
>>>> Professional R Enthusiast
>>>> +33(0) 6 28 91 30 30
>>>>
>>>> R Graph Gallery: http://gallery.r-enthusiasts.com
>>>>
>>>> blog: http://blog.r-enthusiasts.com
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>>>> `- http://bit.ly/10X94UM : Mobile version of the graph gallery
>>>>
>>>
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>>
>>
>
>
> --
> Romain Francois
> Professional R Enthusiast
> +33(0) 6 28 91 30 30
>
> R Graph Gallery: http://gallery.r-enthusiasts.com
>
> blog: http://blog.r-enthusiasts.com
> |- http://bit.ly/Zs97qg : highlight 0.4.1
> `- http://bit.ly/10X94UM : Mobile version of the graph gallery
>
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