Accessing the value of pi in ROOT

ROOT
1

Hi

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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2

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

Cheers,
Stefano

1 Like
3

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

4

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?

5

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

6

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

7

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

8

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

9

image
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.

10

This was apparently taken from
https://root.cern.ch/doc/master/classTFormula.html
, 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.

11

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.

12

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

13

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

14

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()
15 
root [0] auto f1 = new TF2("f2", "2*pi*sqrt(x/y)", 1, 5, 1, 5);
root [1] f2->Draw("surf2")
16

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