How to combine TMatrixD?


Just wondering if there’s an easy way to combine matrices in ROOT?

For example, if I have given number of matrices similar to this one (but not always 2x2):

and want to combine them to give a new matrix, M

Is there a function that will easily do this?


Hi Sam,

Suppose you want to combine matrix x1 and x2 as blocks
as you indicated in your example:

const Int_t nrow1 = x1.GetNrows();
const Int_t nrow2 = x2.GetNrows();
const Int_t ncol1 = x1.GetNcols();
const Int_t ncol2 = x2.GetNcols();

const Int_t nrow3 = nrow1+nrow2;
const Int_t ncol3 = ncol1+ncol2;

TMatrixD x3(nrow3,ncol3);
TMatrixDSub(x3,       0,nrow1-1,     0,ncol1-1) = x1;
TMatrixDSub(x3,nrow1,nrow3-1,ncol1,ncol3-1) = x2;

Thanks, that’s really helpful. My “combine” function needs to be able to handle any number of matrices, so I guess I’ll have to have a think about that; this is a really good starting point though! Thanks :slight_smile:

If anyone is interested, I’ve written a function to combine matrices:

TMatrixDSym PrepInputs::CombineMatrices(std::vector<TMatrixDSym> input_matrices){

  // Get the size of the combined matrix:
  int total_rows = 0;
  for(int m = 0; m < (int) input_matrices.size(); m++){
    total_rows += input_matrices[m].GetNrows();

  // Combine the input matrices together
  TMatrixDSym combined_matrix(total_rows);
  int sum_nrows = 0;
  for(int m = 0; m < (int) input_matrices.size(); m++){
    int nrows = input_matrices[m].GetNrows();
    int lower_bound = sum_nrows;
    int upper_bound = sum_nrows + nrows - 1;
    TMatrixDSub(combined_matrix, lower_bound, upper_bound, lower_bound, upper_bound) = input_matrices[m];
    sum_nrows += nrows;

  return combined_matrix;