[Rd] Understanding tracemem

[Prof Brian Ripley]

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)
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