While profiling some C code, I rolled my own nchar function which
appears to be much faster than base R's (25 times faster for a 10M
length vector). Obviously base::nchar provides significantly more
features than my barebones function (C snippet below); however, for
argument type = "bytes" it seems that the R_nchar and do_nchar
functions do not actually do anything more than this function.
My suspicion is that I have overlooked some subtlety in the base R
code, or that my benchmarks are not representative. Alternatively,
the action in `do_nchar` of preparing the potential error message
before being passed to `R_nchar` may be quite costly indeed. Or the
function cannot be unswitched from the more complex width and chars
arguments by the compiler.
If I haven't missed something, would a patch be warranted?
SEXP Cnchar(SEXP x) {
R_xlen_t N = xlength(x);
SEXP ans = PROTECT(allocVector(INTSXP, N));
int * restrict ansp = INTEGER(ans);
// Ignoring NA to avoid the branch has a very small
// impact on performance.
for (R_xlen_t i = 0; i < N; ++i) {
SEXP sxi = STRING_ELT(x, i);
if (sxi == NA_STRING) {
ansp[i] = NA_INTEGER;
continue;
}
ansp[i] = length(sxi);
}
UNPROTECT(1);
return ans;
}
x <- rep_len(c(as.character(c(5L, 1:1e6)), NA_character_, 1e6:15e5), 1e7)
Cnchar(x)
90ms
nchar(x, type = "bytes")
2500 ms
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