[Rcpp-devel] Cast for output of Rcpp Sugar runif()

[Cedric Ginestet]

Thanks Douglas for being picky!
I have indeed noted that I needed to use b+1 in my codes to produce 
discrete uniform variates. However, I am not too sure that I understand 
how to use RNGScope properly. I originally referred to the Rcpp Sugar 
codes provided in the Rcpp-introduction .pdf document:

########################
RNGScope scope;
return rnorm(10, 0, 100);
########################

Shall I use anything else?
Cheers,


On 13/04/11 13:59, Douglas Bates wrote:
> On Wed, Apr 13, 2011 at 6:50 AM, Cedric Ginestet
> <c.ginestet05 at googlemail.com>  wrote:
>> Brilliant, Dirk. Thanks for that.
>> Next time, I'll read the Rcpp Sugar .pdf more carefully.
>> Cheers,
> Be aware that in both versions you will never return a vector
> containing b, which might be confusing.  That is, the argument b
> should be 1 greater than the maximum value in the range from which you
> are drawing.
>
> I would design it so that the upper end point in the runif range was b
> + 1, which means the arguments a and b are the minimum and maximum
> values in the range from which you wish to draw.
>
> Also you forgot to create an instance of the RNGScope class which
> causes the random number generator state to be saved and restored
> properly.
>
> Sorry for being so picky.
>
>> On 13/04/11 12:43, Dirk Eddelbuettel wrote:
>>> On 13 April 2011 at 12:19, Cedric Ginestet wrote:
>>> | Hi guys,
>>> |
>>> | I want to produce discrete uniform variates within C++ from the runif()
>>> | function available through Rcpp Sugar. However, I don't seem to be able
>>> to find
>>> | the appropriate cast in order to return an IntegerVector as opposed to a
>>> | numeric one. I have naively used int(), but this is clearly not
>>> sufficient.
>>> |
>>> | Any help would be greatly appreciated,
>>> | Ced
>>> |
>>> | ##########################################
>>> | #### Discrete uniform variate.
>>> | src<- '
>>> |   using namespace Rcpp ;
>>> |   RNGScope scope;
>>> |   int n = as<int>(xn);
>>> |   int a = as<int>(xa);
>>> |   int b = as<int>(xb);
>>> |   IntegerVector vec(n);
>>> |   for(int i=0; i<n; ++i) vec[i] = (int)floor(runif(i,a,b));
>>> | return vec;'
>>> | cxxfun<- cxxfunction(sig=signature(xn="numeric",xa="numeric",xb=
>>> | "numeric"),body=src,plugin="Rcpp",verbose=TRUE)
>>> | cxxfun(10,0,10);
>>> |
>>> | ##########################
>>> | Error in compileCode(f, code, language = language, verbose = verbose) :
>>> |   Compilation ERROR, function(s)/method(s) not created!
>>> file52d217df.cpp: In
>>> | function ?SEXPREC* file52d217df(SEXPREC*, SEXPREC*, SEXPREC*)?:
>>> | file52d217df.cpp:37: error: invalid cast from type
>>> ?Rcpp::sugar::SugarMath_1
>>> |<true, double, double, Rcpp::Vector<14>, double (*)(double)>? to type
>>> ?int?
>>> | make: *** [file52d217df.o] Error 1
>>> | ##################################################
>>>
>>> Well as a start maybe try this as your loop:
>>>
>>>     for (int i=0; i<n; ++i) {
>>>         double tmp = runif(1,a,b));     // why where you using i here?
>>>         vec[i] = static_cast<int>(tmp);  // C++ static cast
>>>     }
>>>
>>> But you do of course understand that
>>>
>>>      a) Rcpp sugar can draw an entire vector at once and
>>>
>>>      b) Rcpp sugar also has a vectorised floor() function
>>>
>>> right?   So with that try
>>>
>>> R>    src<- '
>>> +    RNGScope scope;
>>> +    int n = as<int>(xn);
>>> +    int a = as<int>(xa);
>>> +    int b = as<int>(xb);
>>> +    IntegerVector vec(n);
>>> +    vec = floor(runif(n,a,b));
>>> +    return vec;'
>>> R>    cxxfun<-
>>> cxxfunction(sig=signature(xn="numeric",xa="numeric",xb="numeric"),
>>> +                       body=src,plugin="Rcpp")
>>> R>    cxxfun(10,0,10);
>>>   [1] 8 5 1 7 0 0 6 9 8 1
>>> R>
>>>
>>>
>>> Hth, Dirk
>>>
>>> | --
>>> | Cedric Ginestet
>>> | Centre for Neuroimaging Sciences (L3.04)
>>> | NIHR Biomedical Research Centre
>>> | Department of Neuroimaging
>>> | Institute of Psychiatry, Box P089
>>> | King's College London
>>> | De Crespigny Park
>>> | London
>>> | SE5 8AF
>>> |
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