[R] array of variable length vectors

[Prof Brian Ripley]

On Mon, 2 Feb 2004, Giampiero Salvi wrote:

> Hi,
> I'd like to store N vectors of different lengths, and to be able to
> access them with an index, and eventually free the memory for one
> of them without modifying the indexes to the others.
> 
> In C this would be a vector of N pointers that point to memory cells
> independently allocated.
> 
> For example
> 
> int *pv[3];
> 
> pv[0] = (int *) malloc(13 * sizeof(int));
> pv[1] = (int *) malloc(7 * sizeof(int));
> pv[2] = (int *) malloc(110 * sizeof(int));
> 
> free(pv[1])
> ...
> 
> What is the best data type (or class) in R to do such a thing?

Sounds like an R list.  However, in R you cannot free memory, but what 
you can do (carefully) is to change the list element to NULL and then 
memory will be salvaged at a future garbage collection.

z <- vector("list", 3)
z[[1]] <- integer(13)
z[[2]] <- integer(7)
z[[3]] <- integer(110)

then

z[1] <- list(NULL)  # and not z[[1]] <- NULL

will potentially release the memory allocated for the first element.

-- 
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