Loop over several cuts using TCut

Hi rooters,

I want to plot a histogram and apply different cuts inside a loop, then get the mean of the histogram after each cut. What I did:

TCut icut[] = {“x<1 && x>0” , “x<2 && x>1”} ;

for (Int_t i =0 ; i<2; i++){
tree->Draw(“w>>his”, icut[i]);
Double_t y = his->GetMean();

However, the icut is correct only for i=0, the following values are zero.

Can anyone help me with that?



You did not provide a full example, so it is a little difficult to help. Here are some tips based on the for loop:

for (Int_t i =0 ; i<2; i++){
   tree->Draw("w>>his", icut[i]);
   Double_t y = his->GetMean();

The Draw method will repeatedly replace the histogram with the name “his”. If you want access to it later it will be not be accessible. The declaration of y is scoped to the for loop and will not be accessible after the loop ends as it will have gone out of scope.

Here is an example that “works”:

   //Make a tree and fill it with some random values.
   TTree *tree = new TTree("tree", "Tree");
   double value;
   for (int i=0;i<5000;i++) {
      value = gRandom->Uniform(0,2);

   //Create some TCuts
   std::vector< TCut > cuts = {"x<1 && x>0", "x<2 && x>1"};

   //Create a vector to store the histograms.
   std::vector< TH1D* > hists;

   //Create a canvas
   TCanvas *c = new TCanvas("c","");

   //Loop over the cuts and draw a histogram for each. Out put the mean.
   for (int i=0;i<cuts.size();i++) {
      hists.push_back(new TH1D(Form("h%d",i),Form("Hist %d",i), 100, 0, 2));
      tree->Draw(Form("x>>h%d", i), cuts.at(i));
      std::cout << "Cut " << i << " Mean: " << hists.at(i)->GetMean() << "\n";


root [0]
Processing TCutLoop.C...
Cut 0 Mean: 0.499663
Cut 1 Mean: 1.50661


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