# [R] Avoiding a memory copy by [[

Matthew Dowle mdowle at concordiafunds.com
Tue May 23 18:16:22 CEST 2006

```Thanks.

I looked some more and found that L\$b[1:10] doesn't seem to copy L\$b.  If
that's correct why does L[[2]][1:10] copy L[[2]] ?

> -----Original Message-----
> From: Prof Brian Ripley [mailto:ripley at stats.ox.ac.uk]
> Sent: 23 May 2006 16:23
> To: Matthew Dowle
> Cc: 'r-help at stat.math.ethz.ch'
> Subject: Re: [R] Avoiding a memory copy by [[
>
>
> On Tue, 23 May 2006, Matthew Dowle wrote:
>
> >
> > Hi,
> >
> > n = 10000000
> > L = list(a=integer(n), b=integer(n))
> >
> > L[[2]][1:10]  gives me the first 10 items of the 2nd vector in the
> > list L. It works fine.  However it appears to copy the
> entire L[[2]]
> > vector in memory first, before subsetting it.  It seems reasonable
> > that "[[" can't know that all that is to be done is to do [1:10] on
> > the result and therefore a copy in memory of the entire
> vector L[[2]]
> > is not required.  Only a new vector length 10 need be
> created.  I see
> > why [[ needs to make a copy in general.
> >
> > L[[c(2,1)]]  gives me the 1st item of the 2nd vector in the
> list L.
> > It works fine,  and does not appear to copy L[[2]] in
> memory first.
> > Its much faster as n grows large.
> >
> > But I need more than 1 element of the vector ....
> L[[c(2,1:10)]]   fails
> > with "Error: recursive indexing failed at level 2"
>
> Note that [[ ]] is documented to only ever return one
> element, so this is
> invalid.
>
> > Is there a way I can obtain the first 10 items of L[[2]] without a
> > memory copy of L[[2]]  ?
>
> Use .Call
>
> --
> 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
>

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