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

[Alessandro Mammana]

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.

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.

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;
}

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;
}

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:

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