How to get R list column names from C code

Question

I need to write a C/C++ function that will retrieve R list column names.

From R I want to do the following.

> dyn.load("R_list.dll")
> x = list(param1="value1", param2="value2", param3="value3")
> .Call("func", x)

and as an output I want to see "param1" "param2" "param3" as names(x) function does from R

> names(x) [1] "param1" "param2" "param3"

In my cpp file I have the following

#include <R.h>
#include <Rinternals.h>
#include <Rdefines.h>

extern "C" __declspec( dllexport ) SEXP func(SEXP list)
{
    try
    {
        if (isNewList(list))
        {
            int n = length(list);
            printf("%d\n", n);
            for (int i=0; i<n; ++i)
                printf("%s\n", CHAR(STRING_ELT(VECTOR_ELT(list, i), 0)));
        }
        else
        {
            throw std::exception("'list' variable must be a list!");
        }       
    }
    catch(const std::exception& ex)
    {
        printf("Exception was thrown: %s\n", ex.what());
    }
    return R_NilValue;
}

How to get column names, not the values, from C/C++ code?

Answer 1

It's all there in Rf_getAttrib, R_NamesSymbol, see Writing R Extensions:

library('inline')
listnames <- cfunction(signature(x="list"), "
   return Rf_getAttrib(x, R_NamesSymbol);
")

listnames(list(param1="value1", param2="value2", param3="value3"))
## [1] "param1" "param2" "param3"

As you see, Rf_getAttrib returns an ordinary character vector here, which may be manipulated with STRING_ELT.

Answer 2

Even shorter -- one statement as Rcpp takes care of all the conversions.

First load Rcpp:

R> library(Rcpp)

The create our one-statement function

R> cppFunction('CharacterVector mynames(List l) { return l.attr("names"); }') 

and test it:

R> mynames(list(a=1:3, b=runif(10), c=LETTERS))
[1] "a" "b" "c"
R> 

Edit: As Romain reminded me, an even shorter variant is using names():

R> cppFunction('CharacterVector mynames(List l) { return l.names(); }') 
R> mynames(list(a=1:3, b=runif(10), c=LETTERS))
[1] "a" "b" "c"
R>