data.table drop key rows and summarize

Question

I\'m looking for an elegant way to iterate over the key of data.table, drop the rows that have that key, then take a summary over the remaining rows. For example:

Answer 1

I think this does it, using more traditional data.table code:

setkey(mydt,cat)
mydt[, list(means=mean(mydt[!.BY,vals])), by=cat]

# or without needing to key first
mydt[, list(means=mean(mydt[cat != .BY,vals])), by=cat]

#   cat means
#1:   a   5.0
#2:   b   4.2
#3:   c   2.5