Transforming boolean subsetting into index subsetting

One of the techniques to subset, a vector for instance, is the following:

V <- c(18, 8, 5, 41, 8, 7)

## [1] 18 8 5 41 8 7
( I <- abs(V - 9) <= 3 )
## [1] FALSE TRUE FALSE FALSE TRUE TRUE
V[I]
## [1] 8 8 7

However, sometimes we are interested in the indexes of the elements where the
condition holds. Is there an easy way to transform the I vector into an
indexes vector similar to: I == c(2,5,6) ?

I know that I can traverse the I vector with a for() loop collecting the
indexes, I just wonder if such an operation can be avoided.

Thanks,

-Sergio.