[R] how to include 'NA's in xtabs?

[Sundar Dorai-Raj]

Kosenkov Kirill wrote:
> Hello!
> 
> I have a dataset with NA's in some variables (factors), for example:
> $ P67 : Factor w/ 2 levels "-","+": NA 2 1 NA NA 2 1 1 2 NA ...
> 

The key is here. Should be 3 levels if you want NAs included.

> I need to use 'xtabs' like
> xtabs(~x$P67)
> It works well and produces something like this:
> x$P67
>    -    +
>  779 1318
> but i want to compute NA's too, like this:
> x$P67
>    -    +   NA
>  779 1318  137
> 
> I am trying xtabs(~x$P67, exclude=NULL) but xtabs does not compute 'NA's 
> in this case too.
> 
> I do not want to transform my data (do not want to do any substitution 
> on NA's).
> 
> How i can say 'xtabs' to compute NA-values?
> 
> and second question: how to use argument 'na.action' in 'xtabs'?
> 'xtabs' help page does not explain this.
> 
> Thanks!
> 


Try this:

x$P67 = factor(x$P67, exclude = NULL)
xtabs(~x$P67)


Here's an example:
R> x = factor(c(1:2, NA), exclude = NULL, labels = c("+", "-", "0"))
R> xtabs(~ x)
x
+ - 0
1 1 1

Sundar