[Rd] reducing a too-large matrix obtained with allocMatrix()

Simon Urbanek simon.urbanek at r-project.org
Wed Nov 2 02:14:54 CET 2011

On Nov 1, 2011, at 4:29 PM, Steven Dirkse wrote:

> Hello,
> I have some C code (for a shared lib called via .External)  that uses
> PROTECT(w= allocMatrix(REALSXP, m, n));
> mostly successfully.  In rare cases, though, the row count m will be
> an overestimate.  Is there a way to reallocate the matrix in-place,
> something like
> reAllocMatrix (w,m-excess,n)    /* where excess is > 0 */
> to chop off the last excess rows of w?  I only find out about the
> excess rows during the single pass over the data made to copy it from
> its source to w.
> Short of this, all I know to do is allocate a matrix of the proper
> size and copy the values.  Any other ways to do this?


In theory you could use SETLENGTH, and then adjust the dimensions, but since you are talking about rows you'll need to re-arrange the whole matrix anyway so you would a) waste  memory in the result and b) still need to essentially copy in place in an additional pass.


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