HowTo: Set Sumw2 in TH1?

ROOT
1

Hi,

I cache plot entries in arrays: entries and erorr - and dump values into histogram with:

TH1::SetBinContent(…)
TH1::SetBinError(…)

methods. I’ve just realized, that Sumw2 should be also cached per bin and set in TH1.

How can it be set in TH1?

2

So, I see that TH1::GetSumw2() method will do the job for TH1. One more interesting observation:

Setting sumw2 per bin will automatically update the error, therefore only one function is sufficient to use, e.g. either:

TH1::SetBinError(bin, error);

or

TArrayD *array = TH1::GetSumw2();
array->SetAt(error * error, bin);

Is there any preference which one should be used?

It is more important now: how do I achieve the same effect with TH2? How bins are enumerated in TArrayD?

3

Hi,

For a TH2, you can use TH1::SetBinError(binx, biny, error);

Cheers,
Philippe.