R devel - Apr 2008 - returning vectors of unknown size from C (with .C)

Dear All,

In a package, I am using ".C" to call some C functions. In one case,
the number of elements of the return vectors are not known in R before
the C call. (Two of the vectors are integers, the third is vector of
character strings).

Passing from R a vector of the maximum possible size would be a huge
waste. I understand one alternative is to use ".Call", but I'd
rather
avoid it if I can (all of the code seems working except for the return
of values into R). Another would be to write to a file from C and then
read that into R, but this looks very ugly. Are there any other
reasonable alternatives, or should I just use .Call?

Thanks,

R.



-- 
Ramon Diaz-Uriarte
Statistical Computing Team
Structural Biology and Biocomputing Programme
Spanish National Cancer Centre (CNIO)
http://ligarto.org/rdiaz

Ramon Diaz-Uriarte wrote:
> Dear All,
>
> In a package, I am using ".C" to call some C functions. In one
case,
> the number of elements of the return vectors are not known in R before
> the C call. (Two of the vectors are integers, the third is vector of
> character strings).
>
> Passing from R a vector of the maximum possible size would be a huge
> waste. I understand one alternative is to use ".Call", but
I'd rather
> avoid it if I can (all of the code seems working except for the return
> of values into R). Another would be to write to a file from C and then
> read that into R, but this looks very ugly. Are there any other
> reasonable alternatives, or should I just use .Call?
>   
.Call is usually easiest, but another possibility is to have two entry 
points:  one to calculate how much space you need, a second to pass in a 
vector that's the right size to hold the result.

Duncan Murdoch
> Thanks,
>
> R.
>
>
>
>