[R] local sequence function

[smu]

hey,

I can not find a function for the following problem, hopefully you can
help me.

I have a vactor like this one

v = c(NA,NA,TRUE,TRUE,NA,TRUE,NA,TRUE,TRUE,TRUE)

and I would like to the TRUE values by the their "local sequence
number".

This means, the result should look thike this:

c(NA,NA,1,2,NA,1,NA,1,2,3)

Of course I could solve the problems using a loop, but this would be
much to slow, because the real vector is much larger.
Can you point me in the right direction?

thank you!

regards,
 Stefan