Question

I have a routine that generates a series of data vectors, one iteration at a time. I would like to find a way to “grow” either a list or a matrix that holds these vectors. I tried to create a list,

PROTECT( myList = allocVector( VECSXP, 1 ) )

But is there a way to grow the list, by pushing a vector element in the end?

Also, I wouldn’t mind using a matrix, since the vectors I generate are of the same length.

Answer 1

Rf_lengthgets in Rinternals.h; implemented in builtin.c:lengthgets. The returned pointer needs to be PROTECTed, so one pattern is

SEXP myList;
PROTECT_INDEX ipx;
PROTECT_WITH_INDEX(myList = allocVector( VECSXP, 1 ), &ipx);
REPROTECT(mylist = Rf_lengthgets(mylist, 100), ipx);

If one were growing a list based on some unknown stopping condition, the approach might be like in R, with pre-allocate and fill followed by extension; the following is psuedo-code:

const int BUF_SIZE = 100;
PROTECT_INDEX ipx;
SEXP myList;
int i, someCondition = 1;

PROTECT_WITH_INDEX(myList=allocVector(VECSXP, BUF_SIZE), &ipx);
for (i = 0; some_condition; ++i) {
    if (Rf_length(myList) == i) {
        const int len = Rf_length(myList) + BUF_SIZE;
        REPROTECT(myList = Rf_lengthgets(mYlist, BUF_SIZE), &ipx);
    }
    PROTECT(result = some_calculation();
    SET_VECTOR_ELT(myList, i, result);           
    UNPROTECT(1);
    // set some_condition
}
Rf_lengthgets(myList, i); // no need to re-PROTECT; we're leaving C
UNPROTECT(1)
return myList;

This performs a deep copy of myList, so can become expensive and in some ways if ht emain objective to evaluate some_calculation, then it seems like it’s easier and not too much less efficient to do the pre-allocate and extend operations in an R loop, calling some_calculation and doing assignment inside the loop.