If rate is not specified, it assumes the default value of
1.
The Exponential distribution with rate \(\lambda\) has density
$$
f(x) = \lambda e^{-\lambda x}
$$
for \(x \ge 0\).
The generation algorithm uses fast numerical inversion. The parameters
lb and ub can be used to generate variates from
the Exponential distribution truncated to the interval (lb,ub).