Re: [eigen] Blocks and EvalBeforeAssign

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 19.05.2013 00:12, Michael Tao wrote:
> Currently assigning sub-blocks of variables to themselves like
> x = x.head(1);

If you want the head of a vector or a top-left corner of your matrix, 
you can use conservativeResize():
http://eigen.tuxfamily.org/dox-devel/classEigen_1_1PlainObjectBase.html#a4ac24a195e3e35ce2d8946cee5873e51

> its Flags.  Is there a particular reason why EvalBeforeAssigningBit
> isn't set in Block?

Block methods are usually not intended to assign any kind of overlapping 
sub matrices of the same matrix. Maybe a warning for that should be put 
into the docu somewhere.

> A.data().  Does anyone know why that might be happening?

I think first a new data area is allocated and the old one is 
deallocated. Then during the assignment, the data pointer of the block 
is already the new one, so the actual assignment is essentially a NOP.
I guess it's not guaranteed that it happens exactly that way, because 
the compiler might be allowed to do certain optimizations which might 
cause your program to accidentally work in rare cases.


Christoph

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Dipl.-Inf., Dipl.-Math. Christoph Hertzberg
Cartesium 0.049
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