[R] Q: replacing elements in an array

[Laurent Gautier]

Many thanks for the details... however I could find much about
the function 'memory.size' you use in your example (see below)...
is it a new function only in R-devel ?


L.

PS: In case it matters:
platform i686-pc-linux-gnu
arch     i686             
os       linux-gnu        
system   i686, linux-gnu  
status                    
major    1                
minor    5.1              
year     2002             
month    06               
day      17               
language R 



On Tue, Aug 13, 2002 at 04:05:36PM +0100, ripley at stats.ox.ac.uk wrote:
> On Tue, 13 Aug 2002, Laurent Gautier wrote:
> 
> > Dear all,
> >
> >
> > I have a question about the way the replacement of elements in an array
> > works in R (having a slight idea of the answer), and a question about
> > the way to achieve my goals.
> >
> > If I have a 3 ways-array called 'a', the command
> >
> > a[1, , ] <- rnorm(10)  # assumes the size matches
> >
> > will result in having the whole object 'a' copied (since arrays are
> > just 'vectors with dimensions' in R) ... and at some point
> > two objects like 'a' will be in memory, right ? (this was the first
> > question).
> 
> I don't think so.  See do_subassign and descendants.  It contains
> 
>     /* If there are multiple references to an object we must */
>     /* duplicate it so that only the local version is mutated. */
>     /* This will duplicate more often than necessary, but saves */
>     /* over always duplicating. */
>     /* FIXME: shouldn't x be protected? */
> 
>     if (NAMED(CAR(args)) == 2)
> 	x = SETCAR(args, duplicate(CAR(args)));
> 
> Now coercion can force a copy, but starting from a numeric array I think
> your example does not duplicate.  And I can prove it:
> 
> > A <- array(0, dim=c(500, 500, 10))
> > object.size(A)
> [1] 20000128
> > memory.size(T)
> [1] 29028352
> > A[1,,] <- 1
> > memory.size(T)
> [1] 29028352
> 
> so there was never a second copy.
> 
> > I have rather large arrays, and I would like to modify some of their
> > elements in-situ (to save memory). Is there a way to do that ?
> 
> See above.  Do your own experiments / reading of code?
> 
> -- 
> 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 272860 (secr)
> Oxford OX1 3TG, UK                Fax:  +44 1865 272595
> 

-- 
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Laurent Gautier			CBS, Building 208, DTU
PhD. Student			DK-2800 Lyngby,Denmark	
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