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I have a function that extracts the index from an 

xts
object as an 
SEXP
object.

I’d like to create a pointer to it so that i can increment it with a condition.

I am able to do this via a deep copy of the index, however dirk’s advice on extracting indices from an xts was to prefer the 

SEXP
approach. one once i make that change i can’t get the pointer-stuff to work.

I can see that there’s no such thing as 

s.begin()
in the 
SEXP
code; so my question is how do i create the equivalent without the help of all that wonder 
Rccp-candy
?

This works:

cppFunction("double xtsIndex_deepPoint(NumericMatrix X) {
                DatetimeVector v = clone(NumericVector(X.attr("index")));
                double * pv = v.begin();
                pv++;
                return *pv; } ")

require(xts)
xx <- xts(1:10, order.by = seq.Date(Sys.Date(), by = "day", length.out = 10))

R> as.Date(xtsIndex_deepPoint(xx) %/% 86400) == index(xx)[2]
 [1] TRUE

However, the below does not …

cppFunction("double xtsIndex_point(NumericMatrix X) {
                SEXP s = X.attr("index");
                double * ps = s.begin();
                ps++;
                return *ps; } ")


    file4c783ed8.cpp: In function 'double xtsIndex_point(Rcpp::NumericMatrix)':
file4c783ed8.cpp:8:33: error: request for member 'begin' in 's', which is of pointer type 'SEXP' {aka 'SEXPREC*'} (maybe you meant to use '->' ?)
                 double * ps = s.begin();
                                 ^~~~~
make: *** [C:/PROGRA~1/R/R-40~1.5/etc/x64/Makeconf:229: file4c783ed8.o] Error 1
Error in sourceCpp(code = code, env = env, rebuild = rebuild, cacheDir = cacheDir,  : 
  Error 1 occurred building shared library.
Anonymous Asked question May 13, 2021