Adding all hists for many files with weights?

ROOT
1

Is it possible to use hadd command but giving different weights to different files?
If not is there oter ways to do that?
I mean a simple way, not to get all hists from files and add them separetly.

2

Note that this makes sense only for files containing histograms.
The only suggestion that I can make is to modify the hadd program to use a weight for each file.

Rene

3

Hi Rene,

I would like to try what you suggest: modify hadd such that it accepts a vector of weights, and in case the objects to be merged are histograms, apply the weight.

I am a bit uncertain how to do that given that ::Merge(TCollection*list) is used … how would I best do that? TH1::Scale the histogram with its weight before registering it in the list? Calling Sumw2() beforehand? I would appreciate your recommendation on how to use Merge with weights, thanks! Or maybe Andrey did it already?

Cheers,
Philipp

4

Quick follow-up (related, but if it works, I’d still like to know how to do what I asked for before;):

I have n files with trees, which I process to write n files with histograms; these n files with histograms I would like to merge to one file with merged histograms, each of the n files weighted properly.

Would it work if I set TTree:SetWeight() for all n trees before creating the histograms? Well I guess it is up to me to take the weight of the tree into account when filling the histograms, unless I use TTree::Draw() or TTree::Project()? But then it should yield the desired result, right?

Thanks,
Philipp

5

Hi Philipp,

TTree::Draw and Project will automatically use the value of TTree::GetWeight to ‘weight’ the resulting histogram for that TTree.

Cheers,
Philippe.