Hello Lille,
raw data of a data.frame (or more precisely a list, because data.frame
is just a list with "data.frame" class) is an array of R specific data
structures (SEXP), so a generic C function will not be able to work with
them.
As a per-processing step, you may allocate an array for the pointers to
the raw data of the columns yourself (there will be hopefully only a few
compared to the size of the columns themselves). For this you'll need
functions VECTOR_ELT to access the columns and DATAPTR to get their raw
data (eventually TYPEOF to find out their type). Note that this won't
work for a data frame that contains another list. If this memory layout
doesn't work for you, then you may need to copy the whole data frame.
If you want to update the data from C, then keep in mind that
1) R vectors have value semantics and you should not be altering raw
data of any vector unless you know that its not referenced from anywhere
else -- otherwise you should make a copy, alter that copy instead and
return it as the result from your C function.
2) R has generational garbage collector, so it *must* know about
references between R objects and so you should use SET_VECTOR_ELT to
update the data of a list (some would say that you can update the raw
data if you really understand how the GC and R internals work, I would
say: just don't)
Best,
Stepan
On 09. 01. 20 12:48, lille stor wrote:
Hello,
I would like for my C function to be able to manipulate some values stored in
an R data frame.
To achieve this, a need the (real) memory address where the R data frame stores
its data (hopefully in a contiguous way). Then, from R, I call the C function
and passing this memory address as a parameter.
The question: how can we get the memory address of the R data frame?
Thank you!
L.
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