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

[Romain Francois]

Le 17 févr. 2014 à 12:17, Alessandro Mammana <mammana at 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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