[R-Forge #4957] .N not set when inside in ... of lapply(.SD, function(x) {...})

Submitted by: Matt Dowle; Assigned to: Arun ; [R-Forge link](https://r-forge.r-project.org/tracker/index.php?func=detail&aid=4957&group_id=240&atid=975)

As highlighted here :

http://stackoverflow.com/a/19095083/403310
