[datatable-help] Copy on assign broken in some cases
Matthew Dowle
mdowle at mdowle.plus.com
Sat Oct 29 02:57:24 CEST 2011
On Fri, 2011-10-28 at 17:42 -0700, Muhammad Waliji wrote:
> On Fri, Oct 28, 2011 at 5:32 PM, Matthew Dowle
> <mdowle at mdowle.plus.com> wrote:
>
> On Fri, 2011-10-28 at 09:52 -0700, Muhammad Waliji wrote:
> > >From the user's perspective, DT2 <- DT should either be a
> new copy or
> > a new reference. Anything in between is confusing.
>
>
> Agreed. With picky caveat: even in base it's not at this point
> the copy
> is taken. It's later: copy-on-write. It's setkey and := that
> don't copy
> on write, not the (earlier) <-.
>
>
> Hmm, I would prefer for these to have the same behavior.
Not sure I follow, please expand.
>
>
> > How about this - add a new argument to data.table(), say
> max.cols.
> > max.cols defaults to a couple orders of magnitude above the
> initial
> > number of columns. data.table allocates enough memory for
> max.cols
> > column pointers. If you try to add more than max.cols
> columns, it is
> > either an error, or it creates a copy and produces a
> warning.
>
>
> Very nice idea. To over allocate by default so that := can add
> columns
> fully by reference most of the time seems good to me since
> there's a
> very low cost to over allocating the vector of column
> pointers. Create
> the (shallow copy) and issue a warning, I'm thinking, not
> error. The
> "max.cols" names seems a bit absolute, could it be
> "alloc.cols"? We
> could have alloc(DT,2,ncol) or rowalloc(DT,n) and
> colalloc(DT,n), or
> realloc(...) so users can over alloc themselves before a loop
> that adds
> columns or inserts rows. tables() could also report truenrow,
> and
> truencol as well as nrow and ncol. What should alloc.cols be,
> by
> default? How about: max(100,2*ncol)
>
>
> Fine with me.
>
> What about as.data.table.data.frame()? Should that
> over-allocate, too,
> or for speed just change the class attribute as it does now.
>
>
> Yeah, I think any method of creating a data table should
> over-allocate. If people want the speed gains, they can set
> explicitly set alloc.cols.
>
>
>
> Maybe checking NAMED would work, in addition. If NAMED was 0,
> no need to
> warn. Only when NAMED was 1 (or 2) - (not too hot on NAMED) -
> would the
> warning be necessary.
>
>
> >
> > On Fri, Oct 28, 2011 at 1:10 AM, Matthew Dowle
> > <mdowle at mdowle.plus.com> wrote:
> > Interesting one. Adding columns is a bit different
> to deleting
> > and
> > modifying columns. Here's how it works. Could make
> changes,
> > could
> > document it, or both, what do people think?
> >
> > Just like data.frame there is a list vector holding
> pointers
> > to the
> > column vectors. A delete column op is done with a
> memmove to
> > budge up
> > the column pointers above the column by one place.
> That leaves
> > a gap at
> > the end. The length attribute of that vector
> (ncol(DT)) is
> > then
> > decremented and the spare 4 bytes (or 8 on 64bit)
> are left
> > unused at the
> > end.
> >
> > An add column can't be fully by reference because
> the list
> > vector is
> > full. A new list vector has to be allocated, one
> slot larger,
> > the old
> > pointers memcpy'd over, and the last spot assigned
> the pointer
> > to the
> > new column vector. This copying is negligible
> because it's a
> > small list
> > of pointers fitting well within one page. [Unless,
> there are
> > many 1000's
> > of columns, which is why it's done as efficiently as
> possible
> > using
> > memcpy].
> >
> > Aside : There is little known (I guess) distinction
> between
> > length and
> > truelength in R internals. Base R doesn't use it,
> but we could
> > in
> > data.table. A delete column sets length but leaves
> truelength
> > one
> > larger. When the next add column comes along, it
> could just do
> > the budge
> > up and insert the column. That may not be so
> advantageous for
> > (a small
> > number) of columns, but the same logic could work
> for
> > insert() and
> > delete()ing rows. Of course, this would mean
> whether a
> > visible copy or
> > not is taken depends on what happened previously,
> rather than
> > the
> > syntax. That's something we've disliked before, in
> the same
> > way we
> > dislike drop=TRUE behaviour and so dropped drop. One
> way to
> > approach
> > this might be to advise ":= add *may* not copy. Best
> to assume
> > it
> > doesn't; use copy()". If you get in the habbit of
> > "DT2=copy(DT)" then
> > that'll take a deep copy at the time and you're
> safe.
> >
> > To illustrate the partial (maybe shallow copy is
> better word),
> > consider
> > the following :
> >
> > > DT = data.table(1:2,3:4)
> > > DT2=DT
> > > DT2[,y:=10L]
> > V1 V2 y
> > [1,] 1 3 10
> > [2,] 2 4 10
> > > DT
> > V1 V2
> > [1,] 1 3
> > [2,] 2 4
> > > DT2
> > V1 V2 y
> > [1,] 1 3 10
> > [2,] 2 4 10
> > > DT2[1,V1:=99L]
> > V1 V2 y
> > [1,] 99 3 10
> > [2,] 2 4 10
> > > DT
> > V1 V2
> > [1,] 99 3
> > [2,] 2 4
> > >
> >
> > Matthew
> >
> >
> > On Thu, 2011-10-27 at 11:46 -0700, Muhammad Waliji
> wrote:
> > > I think this is a bug. DT.2 <- DT.1 doesn't seem
> to make a
> > copy in
> > > all cases.
> > >
> > >
> > > > DT.1 <- data.table(x=1, y=1)
> > > > DT.2 <- DT.1
> > > >
> > > > # Both DT.1 and DT.2 are changed.
> > > > DT.2[, y := NULL]
> > > x
> > > [1,] 1
> > > > DT.1
> > > x
> > > [1,] 1
> > > > DT.2
> > > x
> > > [1,] 1
> > > >
> > > > # Only DT.2 is changed
> > > > DT.2[, y := x]
> > > x y
> > > [1,] 1 1
> > > > DT.1
> > > x
> > > [1,] 1
> > > > DT.2
> > > x y
> > > [1,] 1 1
> > >
> > >
> >
> > > _______________________________________________
> > > datatable-help mailing list
> > > datatable-help at lists.r-forge.r-project.org
> > >
> >
> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help
> >
> >
> >
> >
>
>
>
>
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