Confusion regarding allocating Matrices.

Sorry, I made a mistake while writing the code. The declaration of Data
should have been first.

I still have some doubts:

When you say calloc and realloc are you talking about R's C interface Calloc
and Realloc or the regular calloc and realloc?

I want to feed data directly into a R matrix and grow it as required. So at
one time I might have 100 rows coming in from a data source. The next time I
might have 200 rows coming in from a data source. I want to be able to
expand the R-matrix instead of creating a regular C float matrix and then
make an R-matrix based on the new size. I just want to have one R object and
be able to expand it's size dynamically.

I was reading the language specs. It says that one could declare an object
in R like this:

m=matrix(nrows=10,ncols=10)

and then one could assign

m[101]=1.00

to expand the object.

but this has one problem when I do a

dim(m)

I get

NULL instead of 10 10

So what is happening here?


I am aware that R matrices are stored in column major order.

Thanks for the tip on using float *dat= REAL(Data);

Regards

Abhijit Bera



On Fri, Oct 23, 2009 at 7:27 PM, Douglas Bates <bates at stat.wisc.edu> wrote:

On Fri, Oct 23, 2009 at 9:23 AM, Douglas Bates <bates at stat.wisc.edu>
wrote:

On Fri, Oct 23, 2009 at 8:39 AM, Abhijit Bera <abhibera at gmail.com>
wrote:

Hi

I'm having slight confusion.

Indeed.

I plan to grow/realloc a matrix depending on the data available in a C
program.

Here is what I'm tried to do:

Data=allocMatrix(REALSXP,3,4);
SEXP Data;

Those lines should be in the other order, shouldn't they?

Also, you need to PROTECT Data or bad things will happen.

REAL(Data)[8]=0.001123;
REAL(Data)[200000]=0.001125;
printf("%f %f\n\n\n\n",REAL(Data)[8],REAL(Data)[200000]);

And I forgot to mention, it is not a good idea to write REAL(Data)
many times like this. ?REAL is a function, not a macro and you are
calling the same function over and over again unnecessarily. ?It is
better to write

double *dat = REAL(Data);

and use the dat pointer instead of REAL(Data).

Here is my confusion:

Do I always require to allocate the exact number of data elements in a
R
Matrix?

Yes.

In the above code segment I have clearly exceeded the number of
elements that have been allocated but my program doesn't crash.

Remember that when programming in C you have a lot of rope with which
to hang yourself. ? You have corrupted a memory location beyond that
allocated to the array but nothing bad has happened ?- yet.

I don't find any specific R functions for reallocation incase my data
set
grows. How do I reallocate?

You allocate a new matrix, copy the contents of the current matrix to
the new matrix, then release the old one. ?It gets tricky in that you
should unprotect the old one and protect the new one but you need to
watch the order of those operations.

This approach is not a very good one. ?If you really need to grow an
array it is better to allocate and reallocate the memory within your C
code using calloc and realloc then, at the end of the calculations,
allocate an R matrix and copy the results over.

Also, you haven't said whether you are growing the matrix by row or by
column or both. ?If you are adding rows then you can't just reallocate
storage because R stores matrices in column-major order. The positions
of the elements in a matrix with n+1 rows are different from those in
a matrix with n rows.

Is it necessary to reallocate or is R handling
the memory management for the matrix that I have allocated?

Regards

Abhijit Bera

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