Le 17 févr. 2014 à 12:17, Alessandro Mammana mamm...@molgen.mpg.de a écrit :
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
Ihnestraße 63-73
D-14195 Berlin, Germany
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