Excluding levels in table and xtabs

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.
I think this is a bug. The exclude= argument doesn't work for factors,
because the argument is passed to factor(), and its exclude argument has
a different format when the main argument is a factor.

	-thomas

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

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Thomas Lumley			Asst. Professor, Biostatistics
tlumley at u.washington.edu	University of Washington, Seattle
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Excluding levels in table and xtabs

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