Accessing the value of pi in ROOT


I can’t access the value of pi in root though many tutorials/examples show its active use. I have also tried using TMath::pi

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in root you should use TMath::Pi(),


I saw in many examples which used the constant simply by writing ‘pi’. Can you explain that in case?

In that screenshot of yours you do not seem to be in a ROOT session, it looks more like a cmd.exe session.

Can you maybe point us to such example?

yes I had accidentally uploaded the wrong pic, I have corrected it now

Can you point one of this example ? In such examples the variable pi should defined somewhere. Have a close look at the code.

The same point is discussed in documentation of Formula that how some names have their const values defined and can not be changed

Again can you point one of the examples where “pi” is used ? thanks.

This is a snip from the same as mentioned
Please mind the last line
However I cannot yet find the use of pi in any of the examples.

This was apparently taken from
, and it says the following below:

Formula may contain constants, eg:

  • sqrt2
  • e
  • pi
  • ln10
  • infinity

, where by “Formula” the authors mean “a TFormula class object”. However, if you simply right pi in the ROOT prompt, this is not considered a TFormula class object.

yeah, apparently that would mean that TFormula:: pi would give us the value, right?

I should mention that one way of doing your work is define
auto pi = TMath::Pi(), but that is a different matter altogether.

No, the pi is not a member function of TFormula.

Then how does the page suggest we use it?
There are no clear examples

You can e.g. construct a TF1 object using operations/functions (including the pi) defined in TFormula, for example:

root [0] TF1 *f1 = new TF1("f1", "x*pi" , 0, 5) // pi comes from TFormula
root [1] f1->Draw()
root [0] auto f1 = new TF2("f2", "2*pi*sqrt(x/y)", 1, 5, 1, 5);
root [1] f2->Draw("surf2")

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