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Image of singular matrix in C++ Eigen using FullPivLU

yo2xia (20)
here is the code I run in C++ using Eigen library FullPivLU decomposition, and I want to get the image of singular matrix including the first column of constant one rather than the other 3 columns.But the result dose not meet expectation.

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#include <Eigen/LU>
#include <Eigen/QR>
#include <Eigen/Dense>  
#include <iostream>  
using namespace std;  
using namespace Eigen;
//*---*---*---*---*---*---*---*---*---*---*---*---*---*---*---*---*---*
//*---*---*---*---*--rank and image in LU Decomposition---*---*---*---*
//*---*---*---*---*---*---*---*---*---*---*---*---*---*---*---*---*---*

main()
{    
  MatrixXd X(20,4);
  MatrixXd Y(20,1);
  X <<1,1,0,0
     ,1,1,0,0
     ,1,1,0,0
     ,1,1,0,0
     ,1,1,0,0
     ,1,0,2,0
     ,1,0,2,0
     ,1,0,2,0
     ,1,0,2,0
     ,1,0,2,0
     ,1,0,0,3
     ,1,0,0,3
     ,1,0,0,3
     ,1,0,0,3
     ,1,0,0,3
     ,1,0,0,3
     ,1,0,0,3
     ,1,0,0,3
     ,1,0,0,3
     ,1,0,0,3.000001;
  Y <<50,51,52,54,53,60,59,65,67,70,70,73,74,78,82,80,87,84,88,92;
  FullPivLU<MatrixXd> luA(X);
  luA.setThreshold(1e-6); 
  int rank = luA.rank();
  MatrixXd image = luA.image(X);
  std::cout << "rank"<< std::endl <<rank << std::endl << std::endl;  
  std::cout << "image"<< std::endl << image << std::endl << std::endl;
  system("pause");   
}


In fact, the image of singular matrix X does not include the first column of constant one. How do I set up the luA.image(X) or is there other decomposition (like QR decomposition) can do this? Thanks in advance.
Last edited on
Ispil (788)
Well, you don't need to create luA as a FullPivLU object. When I re-wrote the code to directly set rank to X.fullPivLu().rank() and image to X.fullPivLu().image(X), it worked fine. Not sure why it doesn't work the same with the actual luA object, though.
yo2xia (20)
@Ispil how did you set the luA.setThreshold(1e-6); option ?
Ispil (788)
X.fullPivLu().setThreshold(1e-6) should work.
yo2xia (20)
@Ispil Well,actually, an error in X.fullPivLu().setThreshold(1e-6); occurs.
Last edited on
Ispil (788)
...huh. I have no idea, then. Either you lose a column, or you lose the right threshold...
yo2xia (20)
@Ispil Anyway, thanks for your help.
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