Hi
I am not sure if it solves your problem
tab1<-xtabs(value ~ r07 + r08, df1)
tab1[xtabs( ~ r07 + r08, df1)==0]<-NA
tab1
r08
r07 A B C D
A 1 2 4
B 5 6 7 8
C 9 10 11 12
D 13 0 15 16
>
however this is slightly different from tapply output. As I understand
help page if you put value at the left hand side you get result summed for
all levels. Combination AC does not exist however A and C levels are
present so the result tells you correctly that the sum of AC combination
is zero.
Regards
Petr
r-help-boun...@r-project.org napsal dne 13.12.2009 01:21:48:
> Dear list,
>
> I am trying to make a contingency table with xtabs but I am getting
> a 0 where I expect a 'NA'. Here is a simple example:
>
> options(stringsAsFactors = FALSE)
> rn <- LETTERS[1:4]
> df1 <- data.frame(r07 = rep(rn, each=4),
> r08 = rep(rn, 4), value = 1:16)
> xtabs(value ~ r07 + r08, df1)
>
> # Delete the combination [A, C]
> df1 <- df1[-3,]
>
> # Set 'value' for this combination to 0
> df1[13, 3] <- 0
>
> # This is the output I want
> tapply(df1[, "value"], df1[, c("r07", "r08")], c)
>
> # but using 'xtabs' I get a 0 for [A, C]
> xtabs(value ~ r07 + r08, df1)
>
> Hmm, what have I missed...
>
> Thanks for any help!
>
> Best,
> Patrick
>
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