[R] redundant factor levels after subsetting a dataset

[David Winsemius]

On Nov 11, 2009, at 9:00 PM, Daniel Malter wrote:

> #I have a data frame with a numeric and a character variable.
>
> x=c(1,2,3,2,0,2,-1,-2,-4)
> md=c(rep("Miller",3), rep("Richard",3),rep("Smith",3))
> data1=data.frame(x,md)
>
> #I subset this data.frame in a way such that one level of the  
> character
> variable does not appear in the new dataset.
>
> data2=data1[x>0,]
> data3=subset(data1,x>0)

I thought this was asked and answered yesterday ((???)):

 > data2 <- as.data.frame(lapply(data2, function(x) x[,drop=TRUE]))
 > data2
   x      md
1 1  Miller
2 2  Miller
3 3  Miller
4 2 Richard
5 2 Richard
 > data3 <- as.data.frame(lapply(data3, function(x) x[,drop=TRUE]))
 > data3
   x      md
1 1  Miller
2 2  Miller
3 3  Miller
4 2 Richard
5 2 Richard

 > unique(data2$md)
[1] Miller  Richard
Levels: Miller Richard
 > unique(data3$md)
[1] Miller  Richard
Levels: Miller Richard

-- 
David

>
> #However, when I check the levels of the factor variable in the  
> subset data
> frame, it still shows the levels that are now unused.
>
> unique(data2$md)
> unique(data3$md)
>
> #This leads to complications in table and tapply that I want to avoid.
>
> table(data2$md)
> tapply(data2$x,data2$md,mean)
>
> table(data3$md)
> tapply(data3$x,data3$md,mean)
>
> #Basically, I want to completely remove "Smith" from data frame  
> data2 or
> data3 so that it would not show up in table or tapply operations.
>
> Thanks for any pointers,
> Daniel
>
>
>
>
>
>
>
> -----------------------------------------------
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>
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David Winsemius, MD
Heritage Laboratories
West Hartford, CT