Read and write
In cpp4r, when you declare a variable, it is read-only
by default as in cpp11 (Vaughan, Hester, and
Francois 2024). This is different from Rcpp, where
variables are read-write by default (Eddelbuettel
et al. 2024).
Following from the package created in the previous vignette, try to
compile and run the following code by adding it to a new file
src/04_square_coordinates.h:
[[cpp4r::register]]
integers square_coordinates_(integers x) {
integers out = x;
for (int i = 0; i < x.size(); ++i) {
out[i] = x[i] * x[i];
}
return out;
}devtools::load_all() and
devtools::install() will fail with the following error:
This is because the argument x is a read-only object,
you need to explicitly declare a writable variable:
[[cpp4r::register]]
integers square_coordinates(integers x) {
writable::integers out = x;
for (int i = 0; i < x.size(); ++i) {
out[i] = x[i] * x[i];
}
return out;
}Alternatively, you can declare the argument as writable:
[[cpp4r::register]]
integers square_coordinates_2(writable::integers x) {
for (int i = 0; i < x.size(); ++i) {
x[i] = x[i] * x[i];
}
return x;
}Do not forget to include the new header file in
main.cpp:
Document these functions with roxygen and compare their outputs after loading the updated package:
Compare square_coordinates(1:3),
square_coordinates(c(1, 2, 3)), and
square_coordinates(c(1L, 2L, 3L)). Why do the outputs
differ? Are integers and doubles
interchangeable as double and int in C++ for
reasonable cases?