Re: [eigen] how to create a "virtual" array or matrix?

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Hi David, I am not sure if I am understanding but you want to do. But, if you have your vectors already allocated somewhere you can use the Map function of Eigen to map that memory into an Eigen matrix without duplicating it so you can do something like:

Map<MatrixXf> vectors( startingPointer, numRows, numCols );

float totalSum = vectors.array().square().colwise().sum();

(I haven't tested this code but this is the idea)

You can also map a single vector since the solution above would be mapping several of them. Also take into account if you are using row- or column-major alignment.

I hope it helps,

On Mon, Jun 7, 2010 at 9:20 PM, David Roundy <roundyd@xxxxxxxxxxxxxxxxxxxxxxx> wrote:
Hi all,

I'd like to create an object that describes an easily-computable
array.  In particular, I want to work with some vectors describing
functions of position.  In order to easily manipulate these, I'd like
to create an object containing the value $|\vec{r}|^2$.  I could, of
course, fill a vector with this value, but that would be a waste of
memory, since it's really cheap to compute.  And these vectors will
eventually end up being quite large (maybe a GB or so), so I'd prefer
not to allocate the memory.  In the past, I'd have just written a loop
to sum over all positions, but that leads to ugly and hard-to-maintain
code, since a non-trivial loop then shows up many times in the code
(there's a bit of trickiness involving finding the Wigner-Seitz cell).
 And it seems that template expressions provide almost precisely what
I want.

Is there a class that does this, e.g. something like

class VirtualMatrix<...? {
  VirtualMatrix(double (*func)(int, int));

which could just take a function of indices and turns it into a matrix?

If there isn't something like this built into eigen, would it be easy
to create? I've tried (briefly) to create a new class deriving
MatrixBase, but immediately ran into trouble.  Perhaps I just know too
little about C++ templates to use eigen effectively.  :(

David Roundy

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