For cycle with strings

Newbie
1

Hello again,
I am trying to make a for loop that would help me shorten my code. I have several histograms that have almost same name, but differ in suffix. I would like the loop to take the histogram, set all the things like color, scaling, filling etc. So far I have managed to make a piece of code that might do it (hopefully… I am new to programming and struggle a lot), but I get error: no member named ‘SetFillStyle’ in ‘std::__cxx11::basic_string’ name.SetFillStyle(1001);.

 std::vector<string> names {"ttbar1_pt", "ttbar1_eta", "ttbar1_phi"};
  for (const auto& name: names){
   name->SetFillStyle(1001);
//...color, scaling...
}

I could make that pt, eta, phi a number (ttbar1_1, ttbar1_2,…) and know how to do it, but I prefer having letters there, because they make clear what does the variable stay for.
Any ideas how to do that so that it works?
Thank you!

2

Hi Marta,
“where” are the histograms? Are they stored in a ROOT file or are they objects that you have created earlier in your script?

The error you get is because name, within the for loop, is a std::string, so you are trying to call SetFillStyle on a std::string and that obviously doesn’t work. You need something like

for (const auto &name: names) {
   TH1* histo = nullptr; 
   gDirectory->GetObject(name.c_str(), histo);
   histo->SetFillStyle(1001);
   // ...color, scaling...
}

i.e. you need to retrieve the histogram object from ROOT using the name before you can do anything with it.

Cheers,
Enrico

3

Thank you for your response! I get what is wrong there now. Unfortunatelly, your idea would not work well for me, because each of the histograms is getting something else from the file, like this:

TFile *ttbar = TFile::Open("ttbar_out.root");
     TH1D *ttbar1_pt = (TH1D*)ttbar->Get("h_muon1_pt");
     TH1D *ttbar1_eta = (TH1D*)ttbar->Get("h_muon1_eta");
     TH1D *ttbar1_phi = (TH1D*)ttbar->Get("h_muon1_phi");

But do you think I could get something like std::vector<TH1D>? I mean, to put there histograms instead of strings.

4

If your histograms are all of the same type (TH1D) and they are all in the same file, I don’t see why my code above shouldn’t work:

std::vector<string> names({"ttbar1_pt", "ttbar1_eta", "ttbar1_phi"});
TFile ttbar("ttbar_out.root");
for (const auto &name: names) {
   TH1D* histo = nullptr; 
   ttbar.GetObject(name.c_str(), histo);
   histo->SetFillStyle(1001);
   // ...color, scaling...
}

You can indeed put them in a std::vector<TH1D*> (note the *, it’s a vector of pointers to histograms, not a vector of histograms):

std::vector<string> names({"ttbar1_pt", "ttbar1_eta", "ttbar1_phi"});
TFile ttbar("ttbar_out.root");
std::vector<TH1D*> histos;
for (const auto &name: names)
   histos.push_back(static_cast<TH1D*>(ttbar.Get(name.c_str())));

for (auto histo: histos) {
   histo->SetFillStyle(1001);
   // ...color, scaling...
}

I haven’t tested the snippets, but they should give you an idea.
Cheers,
Enrico

5

When I try to compile this, I get error: no matching constructor for initialization of 'std::vector<TH1D *>' std::vector<TH1D*> histos;. So I try to include TH1::TH1D, but that gives me error, too, saying no member named ‘TH1D’ in 'TH1. (Sorry if it is stupid question, I am a beginner learning as I go)

6

You might need to #include <TH1D.h>

If that doesn’t work, please post the minimal, complete program that reproduces the error together with the full error message.

7

I tried it, doesn’t help.
So, here is the code:
mwe.C (932 Bytes)

And here is the error message:
error.txt (2.7 KB)
(I put it in a separate text file, because it’s longer then the code…)

8

Hi Marta,
thank you for the minimal reproducer, very helpful!

Here’s the main issue (the code also contains typos, e.g. histo.push_back instead of histos.push_back, which the compiler will point out):

for (const auto& name: names){
    histo.push_back(static_cast<TH1D*>(name))

names is a vector<string>, so in this loop name is a string.
You can’t convert the string "ttbar1_pt" to the variable ttbar1_pt!

Here’s one way to do what you want. Make sure you understand what’s happening, C++ can be tricky – if not, just ask:

void mwe(){
  TFile *ttbar = TFile::Open("ttbar_out.root");
  std::vector<std::string> names{"h_muon1_pt", "h_muon1_eta", "h_muon1_phi"};
  std::vector<TH1D*> histos; // vector of pointers to histograms
 
  // fill the vector of TH1D pointers and do things that you want to do for each histogram
  for (const auto& name: names) {
    TH1D* h_ptr = static_cast<TH1D*>(ttbar->Get(name.c_str()));
    histos.push_back(h_ptr);
    h_ptr->SetFillStyle(1001);
    // ...
  }

  TCanvas* Canvas1_pt = new TCanvas("Canvas1_pt", "muon1 pt", 1000, 800);
  histos[0]->Draw("same hist"); // I chose one of them
}

Hope this helps!
Enrico

9

It works! Hooray!
Thank you very much!

1 Like