[R] Excluding levels in table and xtabs

[Michael Friendly]

I'm trying to form contingincy tables among a set of character variables
which were read from a .csv file and 
have missing represented as "".  I want to exclude the missing levels
from the table.

> levels(CPIC)
[1] ""  "N" "Y"
> levels(Manix)
[1] ""  "N" "Y"
> xtabs(~CPIC + Manix, exclude=c("",NA))
    Manix
CPIC        N    Y
     272    4   15
   N 154 2812 1472
   Y 158  466 4870

> table(CPIC, Manix, exclude=c("",NA))

    Manix
CPIC        N    Y
     272    4   15
   N 154 2812 1472
   Y 158  466 4870

The only way I can exclude them is by

t <- table(CPIC, Manix)
t <- t[-1,-1]

that's not to hard in this case, but my application is to a much
larger table where this gets unweildly.

-- 
Michael Friendly              friendly at yorku.ca
York University               http://www.math.yorku.ca/SCS/friendly.html
Psychology Department
4700 Keele Street             Tel:  (416) 736-5115 x66249
Toronto, Ontario, M3J 1P3     Fax:  (416) 736-5814