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Bug 401

Summary: obtain the coefficients of the indices for a row/colwise call to maxCoeff
Product: Eigen Reporter: abachrach
Component: Core - generalAssignee: Nobody <eigen.nobody>
Status: DECISIONNEEDED ---    
Severity: Feature Request CC: abachrach, chtz, gael.guennebaud, jacob.benoit.1, rmlarsen
Priority: Normal    
Version: 3.4 (development)   
Hardware: All   
OS: All   
Whiteboard:
Bug Depends on: 984    
Bug Blocks:    

Description abachrach 2012-01-05 22:49:47 UTC 
Reminder for implementing the features requested in this thread:
http://forum.kde.org/viewtopic.php?f=74&t=98384&sid=c5804c453b7e843d4488e7cbb3a2da2f

Basically, it would be nice to be able to get the indices of the min/max in each row in addition to the value

ie: 

  ArrayXXd test= ArrayXXd::Random(5,5);
  ArrayXi inds;
  ArrayXd mtest = test.rowwise().maxCoeff(inds);
Comment 1 abachrach 2012-01-11 15:08:55 UTC 
Just looked into attempting this, and unfortunately think it's well above my pay grade (for now :-p)

As far as I can tell, it'll require fairly major changes to the VectorWiseOp class because it currently only allows Unary Expressions. I could be (probably am) wrong though...
Comment 2 Christoph Hertzberg 2013-11-18 14:52:47 UTC 
I'm afraid this is a bit complicated, due to lazy evaluation. At the moment
  test.rowwise().maxCoeff();
does nothing until it is assigned to a variable or individual coefficients are evaluated. That would be very non-intuitive for the requested variant.
Maybe an alternative would be to essentially return a pair of two arrays (essentially, this is what Matlab does) -- I don't know how well this can be implemented with lazy evaluation, though.
A simpler solution would be to simply implement a maxCoeffIdx() which only returns an index vector. Or maybe it's possible to disable lazy evaluation for this expression?
Comment 3 Rasmus Munk Larsen 2016-02-23 21:43:15 UTC 
+1 

Christoph, I think having something like the maxCoeffIdx() you suggest (and perhaps a companion maxCoeffVal()) would be very helpful.
Comment 4 Christoph Hertzberg 2016-02-23 22:36:58 UTC 
Another idea would be implementing the technique suggested in Bug 984, i.e. something like
  vals, idxs = test.rowwise().maxCoeff();
or
  (vals, idxs) = test.rowwise().maxCoeff();
Comment 5 Nobody 2019-12-04 11:22:12 UTC 
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