Re: [eigen] Recursion and block matrices

[Norman Goldstein <normvcr@xxxxxxxxx>]

Thank you for the pointers.  It is reassuring to know that it was a bug 
that stumped me.
I coded up an override Block::block() method to avoid the infinite 
recursion:

////////////////////// Plugin Code //////////////////////////////////////
// EIGEN_BLOCK_PLUGIN

inline
Block<XprType, BlockRows, BlockCols, InnerPanel, HasDirectAccess>
block( Index startRow2,
       Index startCol2,
       Index blockRows2,
       Index blockCols2 )
{
  return
Block<XprType, BlockRows, BlockCols, InnerPanel, HasDirectAccess>
    ( derived(),
      startRow2 + startRow(),
      startCol2 + startCol(),
      blockRows2,
      blockCols2 );
}
/////////////////////////////////////////////////////////////////////////////

There is no Block plugin hook, so I put one in right before the 
protected section of Block.h .
Also, I slightly modified the test driver:

//////////////////////////////// Code ////////////////////////////////////
#include <Eigen/Core>
using namespace Eigen;

#include <iostream>
using namespace std;

template< class Derived >
void incr( const Block< Derived >& mat )     // <-- Using Block< Derived >
{
  Block< Derived >& matV =
    const_cast< Block< Derived >& >( mat );

  ++matV(0,0);

  if( ( 1 == mat.rows() ) ||
      ( 1 == mat.cols() ) )
  {
    return;
  }
  else
  {
    incr( matV.block( 1,                                      // <-- 
Using dynamic blocks, only
              1,
              mat.rows() - 1,
              mat.cols() - 1 ) );
  }
}// incr


int main( int argc, char* argv[] )
{
  MatrixXd mat( 2, 3 );
  mat << 1, 2, 3, 4, 5, 6;
  cout << "Before: " << mat << endl;

  incr( mat.block( 0, 0, mat.rows(), mat.cols() ) );   // <-- Start off 
with a block

  cout << "After: " << mat << endl;
  return 0;
}// main
////////////////////////////////////////////////////////////////////////////////////////////////////

But there is still infinite recursion, even though the Block::block
method returns the same type of Block.  What am I missing?



On 08/27/2012 12:13 PM, Christoph Hertzberg wrote:
> On 27.08.2012 19:33, Norman Goldstein wrote:
> > I haven't been able to figure out how to write a routine that calls
> > itself recursively
> > with successively smaller sub-blocks of the original matrix. Here is a
> > simple example,
>
> With the current stable branch this is not possible, because this 
> would possibly generate infinitely many template instanciations. See 
> also:
> http://eigen.tuxfamily.org/bz/show_bug.cgi?id=408
> There is an experimental solution here:
> http://eigen.tuxfamily.org/bz/show_bug.cgi?id=481.
>
>
> > [...]
> >
> > template< class Derived >
> > void incr( const DenseBase< Derived >& mat )
> > {
> >    DenseBase< Derived >& matV =
> >      const_cast< DenseBase< Derived >& >( mat );
> > [...]
> >    {
> >      incr( mat.bottomRightCorner( mat.rows() - 1,
> >                                                       mat.cols() - 1 
> > ) );
>
> Your problem here, however, is that mat is const, so 
> mat.bottomRightCorner(...) is a Block<const ...> and that inner const 
> is not removed by a const_cast. If you do matV.bottomRightCorner(...) 
> instead, you should get to the problem above ^^.
>
> Btw:
> If you just do tail recursion, you don't need to do recursion at all, 
> just do sth like this:
>
> template<class Derived>
> void incr(const DenseBase<Derived>& mat){
>
>     for(int i=mat.rows(); i>0; --i) {
>         ++mat.bottomRightCorner(i,i)(0,0);
>     }
> }
>
> This version only works for square matrices, I guess you get the idea, 
> though.
>
>
> Christoph