[R] Still can't find missing data - How do I get NA in xtabs with factors?

[hadley wickham]

>> Let's see if I understand this.  Do I iterate through
>>    x <- factor(x, levels(c(levels(x), NA), exclude=NULL)
>> for each of the few hundred variables (x) in my data frame?
>
>
> Yes, for all being factors.

Wouldn't addNA() be the preferred method?

To do it for all variables is pretty simple:
cat <- sapply(df, is.factor)
df[cat] <- lapply(df[cat], addNA, ifany = TRUE)

Hadley

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