Optimize DT[,...:=lapply(.SD,...)] to for(...)set(...)

[mattdowle]

So the user doesn't need to know to use for(...)set(...) to reduce memory usage, detailed here:
http://stackoverflow.com/a/33000778/403310

Also for combining with by= i.e. optimize DT[,...:=lapply(.SD,...), by=...] which is harder to achieve with manual for(...)set(...).

Optimizing :=lapply could solve the symbol repetition issue too raised in the question linked above.

[mattdowle]

Also see here: http://stackoverflow.com/questions/32940580/convert-some-column-classes-in-data-table/32942319?noredirect=1#comment55531299_32942319

[jangorecki]

this is a duplicate of #4225, isn't it? @MichaelChirico

[MichaelChirico]

yes i'd say so. nice to see some SO references in this one