The Matrix::replicate(int) is very useful for writing compact formulas. I am also using (through the plugin mechanism) a similar expression, called blockDiag(int): Eigen::Matrix A; Eigen::Matrix B = A.blockDiag(3); produces the matrix exrpression: [ A 0 0 ] [ 0 A 0 ] [ 0 0 A ] The implementation is very similar to the replicate. This can be used eg. in the context of elasticity finite element matrices, if A are derivatives of scalar basis functions and the block diagonal expression provides derivatives of (3D) displacement vector. Maybe a better name could be Matrix::diagonalReplicate(int). Posting it here just in case that it is of interest for other purposes as well.
This feature would be more powerful with a true concept of block-diagonal for which diagonalReplicate would only be a special case. Otherwise the special structure is lost.
Sure, that would be nice, then something like std::vector<Eigen::Matrix<double> > mVec(3); Eigen::BlockDiagonal<double> M(mVec); or Eigen::BlockDiagonal<double> M; M.append(A); M.append(B); M.append(C); to get [ A 0 0 ] [ 0 B 0 ] [ 0 0 C ] so that Eigen::BlockDiagonal<double> M(A,3); yields diagonalReplicate.
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