Gettin correct values of histograms

I wrote a code that outputs a list of values corresponding to the maxima of histograms for a separate branch for all files from the folder. But I noticed that not all values are correct. As an example, I attach two histograms. For a wide one, the correct value is obtained. For a narrow one, no. How to solve this problem?

def ch7_939(y):
    c = ROOT.TCanvas()
    myfile = ROOT.TFile.Open("/home/erg/2022.12/2022.12.01/"+y)
    tree = myfile.Get("FADCData 939")
    h = ROOT.TH1D("h", "histogram with channel 7", 100, 0., 0.)
    tree.Draw("Channel7 >> h")
    return h.GetMaximum(maxval=100000.0)

second = []
chan7_939 =[]
m = os.listdir("/home/erg/2022.12/2022.12.01")
for i in range(len(m)):
for j in range(len(m)):

_ROOT Version: 6.26/10
_Platform: Ubuntu
_Compiler: pycharm community

Well, I guess you get the “x” around 15915 for the first and 15910.5 for the second histogram (btw. remove “maxval=100000.0” from your function).

I mean maximum of Y axis.

I guess you get the “y” around 9800 for the first and around 1540 for the second.

Maybe you want to use:
h = ROOT.TH1D("h", "histogram with channel 7", 10000, 10000., 20000.) # fix bin size = 1

But code output 1541 and 1533
I guess it relate with the line h = ROOT.TH1D(“h”, “histogram with channel 7”, 100, 0., 0.)

Well, after the “Draw”, try to add: ROOT.gPad.Update()

Unfortunately, it doesn’t help.

Well, after the “Draw”, try to add: h.BufferEmpty(1) # 1 or -1 ?

It is also useless.

It seems we need a “reproducer” (i.e., the macro plus data files).

Dec01031235.root (120.3 KB)
Dec01031201.root (118.1 KB)

The problem is that your "h" histogram with automatic bins uses the default “buffer size” of 1000 (and then the automatically computed x-axis limits may be “narrow” when the first 1000 entries are near the maximum).

import ROOT
f = ROOT.TFile("Dec01031235.root")
t = f.Get("FADCData 939")
# ROOT.TH1.SetDefaultBufferSize(100000) # either before creating any histogram
h = ROOT.TH1D("h", "histogram with channel 7", 100, 0., 0.)
# h.SetBuffer(100000) # or after creating this histogram
t.Draw("Channel7 >> h")

It didn’t work.

Did you try to uncomment one of the additional lines?

Which lines exactly do you mean?

One of these which begin with a “#” in the small test macro in my previous post, of course.

I did not uncomment it.

Is there a way to set borders automatically?

I’d say, in order to have meaningful results, you should create a histogram with a “fix bin size” = 1, e.g.: