[Rd] Shrinking a List

Prof Brian Ripley ripley at stats.ox.ac.uk
Thu Mar 11 07:27:11 CET 2010

And how would you do this at R level?

I would use length(A) <- M, and the corresponding C function is 

Of course, the wasteful part is to create a far-too-long vector in the 
first place, rather than growing it as needed.

On Wed, 10 Mar 2010, Saptarshi Guha wrote:

> Hello,
> I create a VECSXP(call it A) with size N(~ 5000), i then proceed to
> fill the elements and find out I don't need to fill more than
> M (M<< N).
> Thus if i return A to the user's R code, he/she will see a list of
> length 5K of which N-M are NULLs.
> To avoid this, I create a new VECSXP B of length M and /duplicate/ the
> elements of A.
> Since I do this often, it appears to be wasteful, so can I
> a) is there a resize function for a VECSXP?
> if not
> b) can i just do something like
> SET_VECTOR_ELT(B,i, VECTOR_ELT(A,i))  instead of wrapping
> VECTOR_ELT(A,i) inside a call to Rf_duplicate
> (i will be UNPROTECTING A and B, though I will return B)
> I suppose I can do (b), since B (since it is being returned) is
> automatically protected and therefore all its elements will also be
> protected, correct?
> Thank you
> Saptarshi
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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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