Area under curve problem

[faca87]

Hello, I’m trying to calculate the area under curve of a TH1F. I wrote
Double_t integral= htempkinensp->Integral(0,45);
but I noticed that I get different values of the area depending of the bin number that I set. For exampl, you can see in the stat box, here I setted

ts->Draw(Form("SecondaryParticleKinEn >> htemp(45000, 0., 45000.)"));

here I setted
ts->Draw(Form("SecondaryParticleKinEn >> htemp(1000, 0., 45000.)"));

you see in the stat box I get different Integral values…

I think the right value should be 34725

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[dastudillo]

Read how to use Integral:

ROOT: TH1 Class Reference

[faca87]

Thank you @dastudillo, then, should I write

Double_t integral= htempkinensp->Integral(0,45, option="width");

?

[dastudillo]

From that page:
“TH1::Integral() returns the integral of bin contents in a given bin range”
(not “x-axis value range”)

[eguiraud]

To elaborate: given a 1-D histogram with 10 equidistant bins in the range 0-5, the full bin range (without the overflow/underflow bins) is 1-10 while the axis range is 0-5.

You can find the bin number corresponding to a value on the x axis with FindBin, e.g. histo.FindBin(4.9) would return 10 (bin number 10) for my example above.

Cheers,
Enrico