Getting NaN when using R::runif in Rcpp - r

Question

To generate standard uniform random numbers with Rcpp, I always used
Rcpp::runif(1, 0, 1)[0]
The [0] is due to Rcpp::runif returning vectors. I recently found that you can also use the R API and use R::runif() instead if you only want a scalar so I can avoid using this [0]. I tried this out, but I always get nan's. Here is a small example, the .cpp file:
#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::export]]
double unif_R() {
const double u = R::runif(1, 0);
Rcout << "u: " << u << "\n";
return u;
}
// [[Rcpp::export]]
double unif_Rcpp() {
const double u = Rcpp::runif(1, 0, 1)[0];
Rcout << "u: " << u << "\n";
return u;
}
The unif_R() function uses R::runif(a,b) that returns a scalar, while unif_Rcpp() uses Rcpp::runif(n, a, b), which returns a vector. However, when I call these in R using:
sourceCpp('runif_test.cpp')
set.seed(21)
unif_R()
set.seed(21)
unif_Rcpp()
set.seed(21)
runif(1)
I get the following output:
> set.seed(21)
> unif_R()
u: nan
[1] NaN
> set.seed(21)
> unif_Rcpp()
u: 0.786115
[1] 0.7861149
> set.seed(21)
> runif(1)
[1] 0.7861149
Clearly unif_Rcpp\ works, but why does unif_R\ give me nan's?

Answer 1

As noted by #user20650, I made a silly mistake and that I need b >= a. So the correct code should've been:
#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::export]]
double unif_R() {
const double u = R::runif(0, 1);
Rcout << "u: " << u << "\n";
return u;
}
// [[Rcpp::export]]
double unif_Rcpp() {
const double u = Rcpp::runif(1, 0, 1)[0];
Rcout << "u: " << u << "\n";
return u;
}
And this is fine.