function on characters with recycling

gistfile1.r

SEXP _find_subst_expressions_list(SEXP strs, SEXP begin, SEXP end) {
  int ns, nb, ne;
  assert_type(strs, STRSXP);
  assert_type(begin, STRSXP);
  assert_type(end, STRSXP);
  ns = LENGTH(strs);
  nb = LENGTH(begin);
  ne = LENGTH(end);

  int nout;
  if (MIN3(ns, nb, ne) == 0) {
    nout = 0;
  } else {
    nout = MAX3(ns, nb, ne);
    if ((nout % ns > 0)||(nout % nb > 0)||(nout % ne > 0)) {
      warning("longer object length is not a multiple"
              " of shorter object length");
    }
  }

  SEXP out = PROTECT(allocVector(VECSXP, nout));
  if (ns == nout)
    setAttrib(out, R_NamesSymbol, getAttrib(strs, R_NamesSymbol));
  for (int i = 0; i < nout; i++) {
    SET_VECTOR_ELT(out, i,
                   find_subst_expressions(STRING_ELT(strs, i%ns),
                                          STRING_ELT(begin, i%nb),
                                          STRING_ELT(end, i%ne)));
  }

  UNPROTECT(1);
  return out;
}