I need to write a C/C++ function that will retrieve R list column names.
From RI 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?
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
.
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>
The attr
member function can also be used for this (although @gagolews' answer is clearly simpler):
require(inline)
l <- list(a=1:3, b=runif(10))
f <- cxxfunction(signature(x_ = "List"), plugin = "Rcpp", body = '
Rcpp::List x(x_);
CharacterVector names = x.attr("names");
return names;
')
f(l)
# [1] "a" "b"
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