Splitting THnSparse

Newbie
,
1

I have a 7-dimensional THnSparse histogram (hist_7d) and want to be able to split it up into a 2d vector of 5-dimensional THnSparse (hist_5d[i][j]) where the i and j indices correspond to specific bins in the first two axes of hist_7d. To my knowledge I cannot simply project into the other 5 dimensions as it would integrate over all other dimensions, which I don’t want.

I’ve tried brute force looping over all the bins and effectively building brand new THnSparse histograms from this, but I’ve found results to be suspicious and I would love an internal way of splitting up THnSparse histograms like this. Is there a built in functionality for this?

ROOT Version: 6.26.04
Platform: macOS Ventura 13.0
Compiler: g++

2

See the “Projections” paragraph in the THnSparse class reference.

3

I’ve looked at the Projections paragraph and the documentation for it for months and if one can use Projections to do what I’m talking about, then I don’t see it. Could you be a bit more specific as to how one could use Projections to do this?

4

I think you’re referring to the part where it says:

To only project parts of the histogram, call

THnSparse::GetAxis(12)->SetRange(from_bin, to_bin);

right?

It’s not clear to me how to use that or where I would incorporate it.
What I’m familiar with would be

int bins = 5;
int which_bins[5] = {2,3,4,5,6};
for(int j=0; j<hist_7d->GetAxis(0)->GetNbins(); j++){
    for(int k=0; k<hist_7d->GetAxis(1)->GetNbins(); k++){
         hist_5d[j][k]=hist_7d->Projection(bins, which_bins,"E"); 
    }
}

So it’s not clear to me how I would use the prior? Would I do something like

int bins = 5;
int which_bins[5] = {2,3,4,5,6};
for(int j=0; j<hist_7d->GetAxis(0)->GetNbins(); j++){
    for(int k=0; k<hist_7d->GetAxis(1)->GetNbins(); k++){
         hist_6d[k]=hist_7d->GetAxis(1)->SetRange(k+1,k+1); 
         hist_5d[j][k] = hist_6d[k]->GetAxis(0)->SetRange(j+1,j+1);
    }
}

or am I misunderstanding?

5

Try:

hist_7d->GetAxis(0)->SetRange(j + 1, j + 1);
hist_7d->GetAxis(1)->SetRange(k + 1, k + 1);
hist_5d[j][k] = hist_7d->Projection(bins, which_bins, "E");
6

I need to run a couple more tests to make sure it’s behaving as intended, but so far it looks like it’s working! To check comprehension: calling this hist_7d->GetAxis(n)->SetRange(j,k) for different n’s, j’s, and k’s would all appropriately switch it to the designated bins and the underlying data and error in hist_7d remains unchanged?
So if I were to do

hist_7d->GetAxis(0)->SetRange(1, 1);
hist_7d->GetAxis(1)->SetRange(2, 2);
hist_5d[1][2] = hist_7d->Projection(bins, which_bins, "E");
hist_7d->GetAxis(0)->SetRange(3,3);
hist_7d->GetAxis(1)->SetRange(4,4);
hist_5d[3][4] = hist_7d->Projection(bins, which_bins, "E");

Then even though they’re sequential, the GetAxis and SetRange resets the expressed range in that axis for hist_7d? And to get it back to “normal” I’d need to call

hist_7d->GetAxis(0)->SetRange(1,hist_7d->GetAxis(0)->GetNbins());
hist_7d->GetAxis(1)->SetRange(1, hist_7d->GetAxis(0)->GetNbins());

yes?

7 
hist_7d->GetAxis(0)->SetRange(); // () or (0, 0)
hist_7d->GetAxis(1)->SetRange(); // () or (0, 0);

BTW. See: TAxis::SetRange