[R] Problem adding a class to a POSIX vector

[Frank E Harrell Jr]

Under 

platform i686-pc-linux-gnu
arch     i686             
os       linux-gnu        
system   i686, linux-gnu  
status                    
major    1                
minor    5.1              
year     2002             
month    06               
day      17               
language R                

I am having the following problem.

d <- Sys.time()
w <- data.frame(d)
class(w[['d']]) <- c('labelled',class(w[['d']]))
class(w[['d']])
[1] "POSIXt"  "POSIXct"

whereas the $ method works:

class(w$d) <- c('labelled',class(w$d))
class(w[['d']])
[1] "labelled" "POSIXt"   "POSIXct" 

Is this a bug?  Does anyone know of a workaround?  Thanks.
-- 
Frank E Harrell Jr              Prof. of Biostatistics & Statistics
Div. of Biostatistics & Epidem. Dept. of Health Evaluation Sciences
U. Virginia School of Medicine  http://hesweb1.med.virginia.edu/biostat
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