[R] Successive subsets from a vector?

Prof Brian Ripley ripley at stats.ox.ac.uk
Tue Aug 22 17:16:41 CEST 2006

On Tue, 22 Aug 2006, hadley wickham wrote:

> > The loop method took 195 secs.  Just assigning to an answer of the correct
> > length reduced this to 5 secs.  e.g. use
> >
> >     ADDRESSES <- character(length(VECTOR)-4)
> >
> > Moral: don't grow vectors repeatedly.
> Other languages (eg. Java) grow the size of the vector independently
> of the number of observations in it (I think Java doubles the size
> whenever the vector is filled), thus changing O(n) behaviour to O(log
> n).  I've always wondered why R doesn't do this.

At one point at least that was too expensive on memory/address space (and 
it may still be for 32-bit OSes). There is even a 'truelength' field in 
the vector header to allow for such a strategy, and the strategy is used 
in scan() and elsewhere.

In my experience it is relatively rare not to know the vector length in 
advance in R 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 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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