So I have a root file named histfile.root that has two histograms named myhist1 and myhist2. So what I want to do now is loop over the bins of first histogram. Then, loop over the bins of the second histogram in the same order and add these two numbers on a bin-by-bin basis. I know the easier way is to probably just add these two histograms and then look at the bin contents. However, this is a practice exercise for me to understand how to loop over the bins of one histogram and then move on to the second histogram and loop over its bins again.
if the histograms have identical binning, it’s as easy as:
for (auto x=0; ...) {
for (auto y=0; ...) {
histo1->GetBinContent(x, y);
histo2->GetBinContent(x, y);
}
}
Note that bin #0 is the underflow, bin #nBins+1 is the overflow bin (see TH1 docs I link below). Maybe you want to skip those.
If the binning is not identical, you need to use histo...->GetXaxis()->FindBin(coordinate)link to find the bin index on the x or y axis that corresponds to a certain x or y coordinate.
Hi @StephanH, thank you for your reply. In terms of the things inside the nested loop, I don’t think this is what I want. Perhaps, I phrased the question poorly. So let me ask here what I actually wanted to ask. Let’s say I have 4 histograms with identical binning. Let’s call them hist1, hist2, hist3, hist4. What I want to do is take the first hist, grab all of its bin contents using the
GetBinContent()
method. Then, let’s stay do some operations on those numbers. And only then, move on to the next hist until all the hists are completed. So my rough idea is to have a vector where those four hists are stored and then have a triple nested loop. But I am not sure if what I am thinking of makes sense.
With identical binning, it’s much easier, because you can use single-value bin numbers. The code would look something like:
std::vector<double> bins(histo1->GetNcells());
for (auto histo : std::initializer_list<TH2*>{histo1, histo2, histo3, histo4}) {
for (int bin = 0; bin < histo->GetNcells(); ++i) {
const double binContent = histo->GetBinContent(i);
... do stuff with each bin, e.g.
bins[bin] += binContent;
}
}
Hi @StephanH, do you mind explaining what the part
histo1->GetNcells()
is supposed to do? Cause when I load the root file and try something like this:
Int_t x = histo1->GetNcells()
it gives me an error saying
Error: Can’t call TH2D::GetNcells() in current scope (tmpfile):1:
Possible candidates are…
(in TH2D)
(in TH2)
(in TH1)
*** Interpreter error recovered ***
The root file that I attached in the main question does have two histograms, and I even tried plotting them, which works. The GetNcells() method,however, gives the same error.
That’s probably more than 10 years old, so a lot might have changed in the mean time. You can probably try to do histo->GetNbinsX() * histo->GetNbinsY() * histo->GetNbinsZ() etc instead, but you have to manually keep track of how many dimensions the histogram has.
