Plotting a formula

[TheScientist]

Hello there,

I am trying to plot the given formula. I have uploaded the file that has formula on it. Please let me know where I am making errors in my code.

Thank you very much.

{
	
	double_t E;
	double_t L;
		
	f1 = new TF1("f1", "TMath::Exp(-6.4*TMath::Power(10,-2)*(L*TMath::Sqrt(4*TMath::Power({E},2)+TMath::Power(0.14,2)/E))", -3, 3); 
	f1->SetLineColor(kBlue+2); 
	f1->SetTitle("Title; X-Axis; Y-Axis");
	f1->Draw(); 

}

[yus]

Hi Aayush,

there are three major problems:

1. TMath::Power({E},2) instead of TMath::Power(E,2)
2. E in the denominator should be outside of the square root
3. You never defined the type of f1

There are also a few minor problems:

1. Instead of writing -6.4*TMath::Power(10,-2) you can simply write -6.4e-2 in C++
2. When taking the square of something, you don’t need to do TMath::Power(x,2), this is very slow. You can do instead TMath::Sq(x) or even x*x.

Having said all this, I would change you formula into the following:

TF1* f1 = new TF1("f1", "TMath::Exp(-6.4e-2*L*TMath::Sqrt(4*TMath::Sq(E)+TMath::Sq(0.14))/E)", -3, 3); 

[TheScientist]

Thank you for your reply, Yus.

I changed my formula the way you suggested. It’s still showing some errors.

Please help me with it.

[yus]

You seem to have braces around L and E - why?

[TheScientist]

It just shows that for some reason. I don’t have braces around L and E on code though.
Also, how do I define E in terms of x?

I tried this way. double_t E = x;

But it does not seem to work.

[yus]

You should just replace E with x in the TF1 ctor. Also, you should put [0] instead of L, and then set correspondence between this parameter [0] and L like this:

f1->SetParameter(0, L);

But you should define L first to do this.

[TheScientist]

It looks like I am really close now. But still I am missing something as I do not have all the basic understanding.

Where should I change my code?

Thank you.

[yus]

I don’t know what to make of your error message, I don’t have it after running your code. Probably has something to do with your ROOT installation. Regarding the plot, as I said earlier, you should initialize L before using it. For example:

double_t L = 5;

Then you should get a nice plot I think (I know I do)

[TheScientist]

Alrighty!!! Finally got the plot. Thank you very much for your help!

This is what I got for x limit (-20, 20) for L equals to 50.

I know I have bugged you enough today, but can you please make sure you have a similar plot for these values?

I want to be sure before I send it to my mentor.

Thank you again.

[yus]

Yep, mine is exactly the same!

[TheScientist]

Thank you, sir. Have a good day!