Converting a TH2D to a Matrix for Inversion

[jalopezg]

Hi @fstocker,

This forum post looks similar to what you are trying to attain.

TMatrixD Inversion Issue

Hi all- I have a covariance matrix that I need to invert to use for a Chi-2 calculation (X^2 = M^(T) * C^(-1) * M). I first convert the covariance matrix from a TH2 to a TMatrixD. Then I invert it and assigned the inverted matrix back to another TH2. I then attempt to multiply the two TH2’s (doing standard matrix multiplication) to get the identity. What I get is nonsense. The TH2’s I’m multiplying are hCovMatrixTotal_alt and hCovMatrixTotalInv. I keep in mind that TH2’s are enumerated from th…

However, the following was suggested in RootTalk here:

TH2D histo(...);
// ...
TMatrix m(nbinsx+2, nbinsy+2, histo.GetArray(), "F")

Maybe @couet and/or @moneta can confirm the validity of the last method?

Cheers,
J.