Filling the histogram with Value/Sum

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1

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Dear All,
I have a data file, and I want to sum all the data then fill the value/Sum.

in.open(“test.dat”);
double x;
double Sum=0.;

TFile *file = new TFile("test.root","RECREATE");
TH1F *hist = new TH1F("test","test",10,0.,10);

while (!in.eof())
{
 in >>x;
    Sum=Sum+x;
  cout<<x<<"  "<<Sum<<"  "<<x/Sum<<endl;
    
    hist->Fill(x/Sum);
}

but this is wrong as I can see, because it fill before it complete the summing process.
can anyone help me in this issue please.

2

Hello,

I suggest using a vector to store the value of x, then fill the histogram by the vector and the sum:

	in.open(“test.dat”);
	double x;
	double Sum=0.;
	TFile *file = new TFile("test.root","RECREATE");
	TH1F *hist = new TH1F("test","test",10,0.,10);
	std::vector<double> x_vector;
	x_vector.clear( );
	while (!in.eof())
	{
		in >>x;
		Sum=Sum+x;
		cout<<x<<"  "<<Sum<<"  "<<x/Sum<<endl;
		x_vector . push_back( x );
	}
	for ( int i=0; i<x_vector.size( ); i++ )
		hist -> Fill( x_vector[i]/Sum );

On the other hand, if you want to scale your histogram by 1/Sum. You can fill the histogram with x, then use hist → Scale( 1/Sum );

	in.open(“test.dat”);
	double x;
	double Sum=0.;
	TFile *file = new TFile("test.root","RECREATE");
	TH1F *hist = new TH1F("test","test",10,0.,10);
	while (!in.eof())
	{
		in >>x;
		Sum=Sum+x;
		cout<<x<<"  "<<Sum<<"  "<<x/Sum<<endl;
		hist -> Fill( x );
	}
	hist -> Scale( 1/Sum );
3

Thanks for the answer,
I missed the scaling idea.

Well, I made a mistake my explaining my issue.
After plotting the original histogram , I need to divide the total input in one bin (Y value) over the (X) value rather than the scaling factor.

Is this possible?

4

Can you describe a bit more. I don’t really get your idea.
Do you mean, for example, you have a histogram of 4 bins, ranging from 0 to 4, and the bins are:

[0-1] [1-2] [2-3] [3-4]

And let assume that the 3rd bins, [2-4], has the content of 5 (y=5).
The what do you want to do with it? divide the content by 2 so you will get y = 2.5?

5

yes, i need to do what you described.

6

Then you may try the SetBinContent function (together with GetBinContent)
It looks like this:

// Get number of bins
int n_bins = hist -> GetNbinsX( );
for ( int i=1; i<=n_bins; i++ )
{
	// read the bin content, aka y value
	float bcont = hist->GetBinContent( i );
	// determine the x value of the bin, here I use the central x.
	// here x_max and x_min is the maximum/minimum value on the x axis.
	float bval = ( i-0.5 )( x_max-x_min ) / n_bins;
	// then modify the bin content, be careful that bval may be zero in some cases
	hist -> SetBinContent( i, bcont/bval );
}