Another post to update: http://stackoverflow.com/questions/27755518/data-table-sd-lapply-multiple-columns-in-argument

Bit late, but adding this question of mine to the pile

I didn't even think about it as a bug, usually I provide additional required fields to .SDcols and later in j I use .SD[, !"total", with=FALSE] to exclude unwanted column.

That's another good workaround, I wonder the performance difference vis-a-vis using dt$total. And yes, this sort of dances the line between FR and bug, IMO.

Bumping this up again. Looks like this could be a very important fix. this question seem to be related to and could be potentially solved via DT[, (deltaColsNewNames) := lapply(.SD, normalDelta, price), .SDcols = deltaColsNames]

Here's another simple case where this would be useful: http://stackoverflow.com/a/32498711/1191259

Here's another simple case that suffers: http://stackoverflow.com/questions/32944060/using-data-table-to-calculate-new-columns/32944519#32944519

Another to update when fixed: http://stackoverflow.com/q/32915770/1191259

Yay! we can now do this:

require(data.table)
dt = data.table(grp=c(2,3,3,1,1,2,3), v1=1:7, v2=7:1, v3=10:16)
dt.out = dt[, c(v1 = sum(v1),  lapply(.SD,mean)), by = grp, .SDcols = v2:v3]
  #  grp v1  v2   v3
# 1:   2  7 4.5 12.5
# 2:   3 12 4.0 13.0
# 3:   1  9 3.5 13.5

Updated all SO posts linked here. Thanks to all.

Thanks, @arunsrinivasan. I was waiting for this fix for couple of years.

Awesome! Thank you