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c++matrixeigendiagonal
Nick
Nick

Reputation: 10499

Modify Eigen matrix diagonal

I have an Eigen::MatrixXd and I need to modify the value of the elements in its diagonal. In particular I have another Eigen::MatrixXd with one single column and the same number of rows of the first matrix.

I need to subtract to the diagonal of the first matrix the value of the elements of the second matrix.

Example:

A
 1 2 3
 4 5 6
 7 8 9

B
 1
 1
 1


A'
 0 2 3
 4 4 6
 7 8 8

How can I do?

Upvotes: 7

Views: 7411

Answers (4)

ggael
ggael

Reputation: 29265

The simplest and fastest way to do achieve this is:

Eigen::MatrixXd A1(3,3), B(3,1), A2;
...
A2 = A1;
A2.diagonal() -= B;

of course, better use the VectorXd type for vectors (here for B), and finally if B is constant, then you can use array facilities:

A2.diagonal().array() -= 1;

Upvotes: 8

otah007
otah007

Reputation: 525

for(int i = 0; i < matrix1.rows(); ++i)
    matrix1(i, i) -= matrix2(i, 0);

This code iterates over each row of the matrix (matrix1.rows()) and subtracts the corresponding value of matrix2 (matrix2(i, 0)) from the diagonals in matrix1 (matrix1(i, i)).

Upvotes: 0

Marvin
Marvin

Reputation: 305

Matrix manipulation in Eigen works very similar to that of arrays. The indexing starts from zero and it is row major. The documentation (Eigen: The Matrix Class is well written and may help you to solve future problems.

For your stated problem the solution is given below.

#include <iostream>
#include <eigen3/Eigen/Dense>
using Eigen::MatrixXd;
int main()
{
  MatrixXd matA(3,3), matB(3,1);
  matA<<1,2,3,
    4,5,6,
    7,8,9;
  matB<<1,1,1;
  for(int i=0; i<3;i++)
    matA(i,i) -= matB(i);
  std::cout<<matA<<std::endl;
  return 0;
}

However, I would use Matrix3d and Vector3d for the first and second matrix, respectively.

Upvotes: 0

Ash
Ash

Reputation: 4728

This works for me:

A_2=A-B.asDiagonal();

Upvotes: 7

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