[datatable-help] stratified weighted means

[David Freedman]

I apologize for the simple question, but something that worked in v 1.8.4
fails in v 1.8.6 - it involves getting stratified weighted means from a data
table.  The code is

library(data.table)
(dt <- data.table(id     = rep(c(1,2), each=9),
                 month  = rep(100L:108L, times=2),
                 value=rnorm(18), 
                 swt=sample(1:18,18) ))
             
dt[,lapply(.SD,mean),keyby=list(id)] #fine - stratified means
dt[,lapply(.SD,function(x)weighted.mean(x,swt))] #fine - overall weighted
mean
dt[,lapply(.SD,function(x)weighted.mean(x,swt)),keyby=list(id)] #fails
'object "swt" not found'

I'm using v 1.8.6 of data.table, and I am sure that this statement for
calculating stratified weight means worked in v 1.8.4.  

Output of sessionInfo is:
R version 2.15.2 (2012-10-26)
Platform: i386-w64-mingw32/i386 (32-bit)

locale:
[1] LC_COLLATE=English_United States.1252  LC_CTYPE=English_United
States.1252   
[3] LC_MONETARY=English_United States.1252 LC_NUMERIC=C                          
[5] LC_TIME=English_United States.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] data.table_1.8.6

loaded via a namespace (and not attached):
[1] tools_2.15.2

Any help would be greatly appreciated as this is something I do almost
daily.  

And I realize that I could always use something like
dt[,list(wm(month,swt),wm(value,swt)),keyby=list(id)]
But this becomes difficult if I want the stratified weighted means for a
large number of variables 

thanks, david freedman






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