[R] how to do randomly sampling in raster layer

[Mao Jianfeng]

Dear r-helpers,

I want to randomly sample n points from regions of a raster layers,
the cells denoted as "NA" is not
included in this sampling process. And, I want to got the longitude
and latitude of the sampled points.

I checked the manual of raster package, I found several functions is
relative to my purpose. I tried them all, but I failed.

Can it can be done by raster functionalities. Could you please refer
me to the right direction?

I expect to hearing from you. Your helps are very valuable for a
Chinese who can not reach helps nearby.

Best,

Sincerely,
Mao Jian-Feng