The plyr package uses a couple of small C functions to optimise a number of particularly bad bottlenecks. Recently, two functions were converted to C++. This was mostly stimulated by a segmentation fault caused by some large inputs to the split_indices()
function: rather than figuring out exactly what was going wrong with the complicated C code, it was easier to rewrite with simple, correct C++ code.
The job of split_indices()
is simple: given a vector of integers, x
, it returns a list where the i
th element of the list is an integer vector containing the positions of x
equal to i
. This is a useful building block for many of the functions in plyr.
It is fairly easy to see what is going on the in the C++ code:
#include
using namespace Rcpp;
// [[Rcpp::exports]]
std::vector<std::vector<int> > split_indices(IntegerVector x, int n = 0) {
if (n < 0) stop("n must be a positive integer");
std::vector<std::vector<int> > ids(n);
int nx = x.size();
for (int i = 0; i < nx; ++i) {
if (x[i] > n) {
ids.resize(x[i]);
}
ids[x[i]  1].push_back(i + 1);
}
return ids;
}

We create a
std::vector
of integers calledout
. This will grow efficiently as we add new values, and Rcpp will automatically convert to a list of integer vectors when returned to R. 
The loop iterates through each element of
x
, adding its index to the end ofout
. It also makes sure thatout
is long enough. (The plus and minus ones are needed because C++ uses 0 based indices and R uses 1 based indices.)
The code is simple, easy to understand (if one is a little familiar with the STL), and performant. Compare it to the original C code:
#include
#include
SEXP split_indices(SEXP group, SEXP n) {
SEXP vec;
int i, j, k;
int nlevs = INTEGER(n)[0];
int nobs = LENGTH(group);
int *pgroup = INTEGER(group);
// Count number of cases in each group
int counts[nlevs];
for (i = 0; i < nlevs; i++)
counts[i] = 0;
for (i = 0; i < nobs; i++) {
j = pgroup[i];
if (j > nlevs) error("n smaller than largest index");
counts[j  1]++;
}
// Allocate storage for results
PROTECT(vec = allocVector(VECSXP, nlevs));
for (i = 0; i < nlevs; i++) {
SET_VECTOR_ELT(vec, i, allocVector(INTSXP, counts[i]));
}
// Put indices in groups
for (i = 0; i < nlevs; i++) {
counts[i] = 0;
}
for (i = 0; i < nobs; i++) {
j = pgroup[i]  1;
k = counts[j];
INTEGER(VECTOR_ELT(vec, j))[k] = i + 1;
counts[j]++;
}
UNPROTECT(1);
return vec;
}
This function is almost three times as long, and has a bug in it. It is substantially more complicated because it:

has to take care of memory management with
PROTECT
andUNPROTECT
; Rcpp takes care of this for us 
needs an additional loop through the data to determine how long each vector should be; the
std::vector
grows efficiently and eliminates this problem
Conversion to C++ can make code shorter and easier to understand and maintain, while remaining just as performant.
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