[R-sig-Geo] subset grid and neighbour list

[Ronnie Babigumira]

Dear listers
This may be obvious however I have thrown in the towel after numerous futile attempts. Here is my problem. I have a grid 
with 5 levels including NA. I would like to perform join count stats on it. Prof. Bivand advised me to either add NA as 
a factor, or subset the grid and eliminate the NA's (I feel the later makes more sense)

I think the steps I need to take are

1. Drop the NA's in mygrid (and save the ids of the cells dropped)

2. subset the neighbors list using information from step1

Is there anyone familiar with this especially step 1

Ronnie