[datatable-help] Over-writing factors

[Matthew Dowle]

It's intended. data.table requires sorted factor levels. If the levels
somehow become unsorted then binary search joins don't work.  We might
be able to allow it and make data.table maintain things (with a speed
penalty due to the resort of levels and re-write of the entire integer
column), but, allowing character columns is (hopefully) not far away
which should be much better solution.

Or, in the meantime, you can go 'under the hood'. Add "something
different" to the end of the factor levels using levels()<-, then the
assignment to the column should work. If that column is part of a key
then make sure to recall setkey() which will check and resort the levels
for you (with a warning message). If an existing level is changing name,
then it's faster to assign (once) directly to the levels(). But again,
careful to maintain sorted levels if that column is to be used in joins.

Matthew


On Thu, 2011-09-08 at 14:27 -0500, Damian Betebenner wrote:
> Not sure whether the following is an intended behavior with
> data.table. Perhaps it is something idiosyncratic with factors.
> 
> It is different than what one gets with data.frame
> 
>  
> 
> test.df <- data.frame(X=letters[1:10], Y=rnorm(10))
> 
> test.dt <- data.table(X=letters[1:10], Y=rnorm(10))
> 
>  
> 
> test.df$X <- "Something Different"
> 
> test.dt$X <- "Something Different"
> 
> Error in `[<-.data.table`(x, j = name, value = value) : 
> 
>   Some or all RHS not present in factor column levels
> 
>  
> 
>  
> 
> test.df <- data.frame(X=letters[1:10], Y=rnorm(10))
> 
> test.dt <- data.table(X=letters[1:10], Y=rnorm(10))
> 
>  
> 
> test.df[["X"]] <- "Something Different"
> 
> test.dt[["X"]] <- "Something Different"
> 
>  
> 
>  
> 
>  
> 
>  
> 
> Damian Betebenner
> 
> Center for Assessment
> 
> PO Box 351
> 
> Dover, NH   03821-0351
> 
>  
> 
> Phone (office): (603) 516-7900
> 
> Phone (cell): (857) 234-2474
> 
> Fax: (603) 516-7910
> 
>  
> 
> dbetebenner at nciea.org
> 
> www.nciea.org
> 
>  
> 
>  
> 
>  
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> 
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