How to get R list column names from C code

c++, r

Solution

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`.

Problem

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?

Original source