RE: [eigen] Unexpected behavior with rowwise().reverse()

[Rob Conde <rob.conde@xxxxxxxxxxxxxxxx>]

Understandable, just wanted to make sure I wasn't missing some fundamental tenet of eigen.

Thanks,
Rob

-----Original Message-----
From: Christoph Hertzberg [mailto:chtz@xxxxxxxxxxxxxxxxxxxxxxxx] 
Sent: Friday, November 22, 2013 12:09 PM
To: eigen@xxxxxxxxxxxxxxxxxxx
Subject: Re: [eigen] Unexpected behavior with rowwise().reverse()

On 22.11.2013 17:52, Rob Conde wrote:
> Results:
> 1 2 3 4 5
> 5 4 3 4 5
> 5 4 3 2 1
>
> Isn't eigen supposed to detect these scenarios and call eval if necessary?

It is very hard to detect all possible aliasing issues. If we introduced a detection for this case, we would rather assert than calling eval -- at least we already do so for A = A.transpose();

There is a blah.reverseInPlace(); however this seems to be missing for
rowwise() and colwise().
As a workaround until we implement that, something like this should work:
   int width = blah.cols()/2;
   blah.leftCols(3).swap(blah.rightCols(3).rowwise().reverse());

Christoph

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