[Rcpp-devel] Default empty values for vector and matrices

Le 17 f?vr. 2014 ? 12:17, Alessandro Mammana <mammana at molgen.mpg.de> a ?crit :
Dear all,
I am trying to write an Rcpp function that accepts optionally a
vector, but I cannot figure out what default value I should give to
it.

First I tried the following:

// [[Rcpp::export]]
int nilVec(NumericVector v = R_NilValue){
   int sum = 0;
   for (int i = 0; i < v.length(); ++i){
       sum += v[i];
   }
   return sum;
}

but when trying:
nilVec()
I get the error "Not compatible with REALSXP", which is maybe not very
consistent with the fact that in R you can do as.numeric(NULL) and it
gives you a vector of length 0.
R?s behavior is incoherent. NULL is not a numeric vector.
Then I tried:

// [[Rcpp::export]]
int nilVec(NumericVector v = NumericVector()){
   int sum = 0;
   for (int i = 0; i < v.length(); ++i){
       sum += v[i];
   }
   return sum;
}

but it doesn't compile.
That?s the attribute parser not making sense of it. It does compile, but the default is not propagated to R.
I found the following discussion:
http://comments.gmane.org/gmane.comp.lang.r.rcpp/5922 where I
discovered that for whatever reason the following compiles and works:

// [[Rcpp::export]]
int nilVec(NumericVector v = NumericVector::create(0)){
   int sum = 0;
   for (int i = 0; i < v.length(); ++i){
       sum += v[i];
   }
   return sum;
}
Well. It might appear as something that works. It is in fact a bug. NumericVector::create(0) is supposed to create a vector of length 1 with the value of the first element set to 0. not numeric vector of length zero as it is currently the case. bug. 
https://github.com/RcppCore/Rcpp/issues/117
But the funny thing is that this does not work:

// [[Rcpp::export]]
int nilMat(NumericMatrix v = NumericMatrix::create(0)){
   int sum = 0;
   for (int i = 0; i < v.length(); ++i){
       sum += v[i];
   }
   return sum;
}
Matrix::create is nonsense as it does not create a matrix. check :
m <- evalCpp( "NumericMatrix::create(0)" )
str(m)
num 0

so creates a vector, not a matrix. Besides, what do you actually expect. What dimensions would this matrix have ?
So how should I do for matrices? I would also be happy with the
solution of defining somewhere an empty vector  or an empty matrix and
assigning it as default argument, but the following does not work:
My suggestion would be to handle this on the R side. The attribute parser is not a fully capable C++ parser, stick to defaults for only very simple things, like int or double or strings. 

For anything else, you?re better of with dealing with the parameter logic on the R side.
NumericVector empty(0);

// [[Rcpp::export]]
int nilVec(NumericVector v = empty){
   int sum = 0;
   for (int i = 0; i < v.length(); ++i){
       sum += v[i];
   }
   return sum;
}

Suggestions? I would be super happy if in the future the special value
R_NilValue could be converted to an empty vector, it would make things
very easy and coherent with R's behaviour.

Thx in advance,
Ale

-- 
Alessandro Mammana, PhD Student
Max Planck Institute for Molecular Genetics
Ihnestra?e 63-73
D-14195 Berlin, Germany
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[Rcpp-devel] Default empty values for vector and matrices

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