R: selecting subset without copying

Question

Is there a way to select a subset from objects (data frames, matrices, vectors) without making a copy of selected data?

I work wit

Answer 1

Try package ref. Specifically, its refdata class.

What you might be missing about data.table is that when grouping (by= parameter) the subsets of data are not copied, so that's fast. [Well technically they are but into a shared area of memory which is reused for each group, and copied using memcpy which is much faster than R's for loops in C.]

:= in data.table is one way to modify a data.table in place. data.table departs from usual R programming style in that it is not copied-on-write. User has to call copy() explicitly to copy a (potentially very large) table, even within a function.

You're right that there isn't a mechanism like refdata built into data.table. I see what you mean and it would be a nice feature. refdata should work on a data.table, though, and you might be fine with data.frame (but be sure to monitor copies with tracemem(DF)).

There is also idata.frame (immutable data.frame) in package plyr you could try.