[R] Collapse factor levels

[Kevin E. Thorpe]

Peter Dalgaard wrote:
> Kevin E. Thorpe wrote:
>> I'm sure this is simple enough, but an R site search on my subject
>> terms did suggest a solution.  I have a numeric vector with many
>> values that I wish to create a factor from having only a few levels.
>> Here is a toy example.
>>
>>  > x <- 1:10
>>  > x <- 
>> factor(x,levels=1:10,labels=c("A","A","A","B","B","B","C","C","C","C"))
>>  > x
>>  [1] A A A B B B C C C C
>> Levels: A A A B B B C C C C
>>  > summary(x)
>> A A A B B B C C C C
>> 3 0 0 3 0 0 4 0 0 0
>>
>> So, there are clearly still 10 underlying levels.  The results I would
>> like to see from printing the value and summary(x) are:
>>
>>  > x
>>  [1] A A A B B B C C C C
>> Levels: A B C
>>  > summary(x)
>> A B C
>> 3 3 4
>>
>> Hopefully this makes sense.
>>
>> Thanks,
>>
>> Kevin
>>
> 
> It's an anomaly inherited frokm S-PLUS (or so I have been told). 
> Actually, with the current R, you should get a warning:
> 
>  > x <- 1:10
>  > x <- 
> factor(x,levels=1:10,labels=c("A","A","A","B","B","B","C","C","C","C"))
> Warning message:
> In `levels<-`(`*tmp*`, value = c("A", "A", "A", "B", "B", "B", "C",  :
>   duplicated levels will not be allowed in factors anymore
> 
> This works (as documented on the help page for levels!):
> 
>  > x <- 1:10
>  > x <- factor(x,levels=1:10)
>  > levels(x) <- c("A","A","A","B","B","B","C","C","C","C")
>  > table(x)
> x
> A B C
> 3 3 4
> 
> 

Thanks.  That's exactly what I need.  I knew it was simple.
I've even used levels() before, but it just didn't occur to
me this time.  I'm clearly not on current R. :-)
When I have some time, I'll upgrade.

Kevin

-- 
Kevin E. Thorpe
Biostatistician/Trialist, Knowledge Translation Program
Assistant Professor, Dalla Lana School of Public Health
University of Toronto
email: kevin.thorpe at utoronto.ca  Tel: 416.864.5776  Fax: 416.864.3016