R devel - Jul 2012 - Understanding tracemem

Hi all,

I've been trying to get a better handle on what manipulations lead R
to duplicate a vector, creating small experiments and using tracemem
to observe what happens (all in 2.15.1). That's lead me to a few
questions, illustrated using the snippet below.

x <- 1:10
tracemem(x)
# [1] "<0x1058f8238>"
x[5] <- 5
# tracemem[0x1058f8238 -> 0x105994ab0]:
x[11] <- 11

Why does x[5] <- 5 create a copy, when x[11] (which should be
extending a vector does not) ?  I can understand that maybe x[5] <- 5
hasn't yet been optimised to not make a copy, but if that's the case
then why doesn't x[11] <- 11 make one? I thought it might be because
somehow tracemem loses track, but adding an additional tracemem(x)
after x[5] <- 5 doesn't change the output.

Hadley

-- 
Assistant Professor / Dobelman Family Junior Chair
Department of Statistics / Rice University
http://had.co.nz/

Hadley Wickham <hadley <at> rice.edu> writes:
> Why does x[5] <- 5 create a copy
That assigns 5 not 5L. x is being coerced from integer to double.

x[5] <- 5L doesn't copy.
> , when x[11] (which should be
> extending a vector does not) ?  I can understand that maybe x[5] <- 5
> hasn't yet been optimised to not make a copy, but if that's the
case
> then why doesn't x[11] <- 11 make one?
Extending a vector is creating a new (longer) vector and copying the old 
(shorter) one in.  That's different to duplicate().  tracemem only reports 
calls to duplicate().

Matthew

Read the help carefully as to what 'copy' means:

      When an object is traced any copying of the object by the C
      function ?duplicate? produces a message to standard output, as
      does type coercion and copying when passing arguments to ?.C? or
      ?.Fortran?.

If you want to understand when 'duplicate' is called, you need to read 
the source code.  File src/main/subassign.c will explain the different 
paths taken by your two cases.  But isn't it rather obvious that 
duplicating x is not useful when a new longer vector needs to be created?

(BTW, in earlier versions of R tracemem reported some transformations of 
x to objects of the same length, but not at all consistently.]

On 12/07/2012 17:15, Hadley Wickham wrote:
> Hi all,
>
> I've been trying to get a better handle on what manipulations lead R
> to duplicate a vector, creating small experiments and using tracemem
> to observe what happens (all in 2.15.1). That's lead me to a few
> questions, illustrated using the snippet below.
>
> x <- 1:10
> tracemem(x)
> # [1] "<0x1058f8238>"
> x[5] <- 5
> # tracemem[0x1058f8238 -> 0x105994ab0]:
> x[11] <- 11
>
> Why does x[5] <- 5 create a copy, when x[11] (which should be
> extending a vector does not) ?  I can understand that maybe x[5] <- 5
> hasn't yet been optimised to not make a copy, but if that's the
case
> then why doesn't x[11] <- 11 make one? I thought it might be because
> somehow tracemem loses track, but adding an additional tracemem(x)
> after x[5] <- 5 doesn't change the output.
>
> Hadley
>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595