|Re: [eigen] How to squeeze in place a vector matrix|
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- To: eigen@xxxxxxxxxxxxxxxxxxx
- Subject: Re: [eigen] How to squeeze in place a vector matrix
- From: Benoit Jacob <jacob.benoit.1@xxxxxxxxx>
- Date: Wed, 25 Aug 2010 10:54:06 -0400
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2010/8/25 Bastien ROUCARIES <roucaries.bastien@xxxxxxxxx>:
> On Wed, Aug 25, 2010 at 4:18 PM, Benoit Jacob <jacob.benoit.1@xxxxxxxxx> wrote:
>> 2010/8/25 Bastien ROUCARIES <roucaries.bastien@xxxxxxxxx>:
>>> I want to squeeze some line and column of matrix in place, in order to
>>> remove the null space.
>>> How can I do that?
>>> I have tryied block(i,j,k,l).eval() but i think it is suboptimal
>> Ah, I see, trying to avoid aliasing issues.
>> In fact, in assignments, we are always traversing matrices
>> left-to-right and top-to-bottom.
>> So you can remove the eval() if you are a little careful. For example,
>> this works:
>> matrix.block(i,j,k,l) = matrix.block(i+1,j,k,l); // squeeze row i
>> Of course this is relying on undocumented behavior; but I really don't
>> see why we'd ever change this.
> Because it is really really useful removing null space on eigenvector
> decomposition** ?
I wasn't questioning the interest of removing null parts of matrices :-)
I was saying that the behavior on why my trick above is relying,
though undocumented, is unlikely to change.
> In fact I need to punch out a column/line pair, and it will be really
> nice to have a function to punch out in place.
> As you should know null space is evil* :)
> * from a physical point of view :) In fact I am working on the
> solution of Ax=lambda B x, and null space does not carry physical
> signification and the devil from a numerical point of view
> ** computing the resonnance of some physical system. Null space is
> due overdetermined limit conidtion