Hi there, I was badly bitten (and it took LOOONG before I got the idea to check it) by this code -- it creates elastic stiffness matrix, which is 6x6 with zeros everywhere except for the 3rd rank major submatrix (upper left corner) and the main diagonal.

I initialized with comma initializer, giving 4 3x3 blocks:

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Real E=1e6, nu=0; // for instance
Matrix6r C; // the "r" stands for Real, which is usually typedef for double
C<<
  (Matrix3r()<<1,-nu,-nu,-nu,1,-nu,-nu,-nu,1).finished(),
  Matrix3r::Zero(),
  Matrix3r::Zero(),
  Vector3r(2*(1+nu),2*(1+nu),2*(1+nu)).asDiagonal();
C*=E/((1+nu)*(1-2*nu));


with this result:

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     1388.89     -277.778     -277.778            0            0            0
    -277.778      1388.89     -277.778            0            0            0
    -277.778     -277.778      1388.89            0            0            0
           0            0            0 3.54736e-310            0 3.54752e-310
           0            0            0  4.08345e-79 7.39258e-181 2.11972e+229
           0            0            0 3.50662e-310 1.68824e-310 1.33285e-303


hinting at wrong interpretation of the .asDiagonal() expression; I changed the last block initializer to

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  Matrix3r(Vector3r(2*(1+nu),2*(1+nu),2*(1+nu)).asDiagonal());


getting then the correct result

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1388.89 -277.778 -277.778        0        0        0
-277.778  1388.89 -277.778        0        0        0
-277.778 -277.778  1388.89        0        0        0
       0        0        0  3333.33        0        0
       0        0        0        0  3333.33        0
       0        0        0        0        0  3333.33


I would think it is a bug, but having been badly hurt, I think I would not be impartial now. Using Eigen 3.0.1.

well, the bug is that the bad version should not compile. Nevertheless, here it does fail at runtime with the following assertion:

Eigen/src/Core/CommaInitializer.h:106: Eigen::CommaInitializer<MatrixType>::~CommaInitializer() [with XprType = Eigen::Matrix<double, 6, 6>]: Assertion `(m_row+m_currentBlockRows) == m_xpr.rows() && m_col == m_xpr.cols() && "Too few coefficients passed to comma initializer (operator<<)"' failed.

assuming you don't define NDEBUG