Comparing the contents of two TLists

Newbie
1

Hi!

I’d like to know if there is a way to compare the contents of two TLists in ROOT?

I’ve got two THStacks which each have a list of histograms(TH1Ds), and I would like to compare both of them to make sure they both have elements of the same name.

Thanks in advance!

2

The various ways to iterate on the TList are explained here:
https://root.cern/doc/master/classTList.html
You can then use GetName() on each elements of both lists and compare them

3

Hi @couet:

I’ve tried the first option, but ROOT crashes after the first line :frowning:

What I’ve did was the following:
for const auto&& stack_Ge: *GetListOfPrimitives())
and ROOT crashed. (stack_Ge) is my THStack with the TH1Ds.

4

the following example can be a good start :slight_smile:

{
   auto st1 = new THStack();
   auto h1st1 = new TH1F("h1", "h1", 10, 0 , 1); st1->Add(h1st1);
   auto h2st1 = new TH1F("h2", "h2", 10, 0 , 1); st1->Add(h2st1);
   auto h3st1 = new TH1F("h3", "h3", 10, 0 , 1); st1->Add(h3st1);

   auto st2 = new THStack();
   auto h1st2 = new TH1F("h1", "h1", 10, 0 , 1); st2->Add(h1st2);
   auto h2st2 = new TH1F("h4", "h4", 10, 0 , 1); st2->Add(h2st2);
   auto h3st2 = new TH1F("h3", "h3", 10, 0 , 1); st2->Add(h3st2);

   auto l1 = st1->GetHists();
   auto l2 = st2->GetHists();

   TIter n1(l1);
   TIter n2(l2);

   TH1F    *h1, *h2;
   TObject *o1, *o2;

   while ((o1 = n1())) {
      h1 = (TH1F*) o1;
      printf("%s \n",h1->GetName());
   }

   while ((o2 = n2())) {
      h2 = (TH1F*) o2;
      printf("%s \n",h2->GetName());
   }
}
1 Like
5

Hi @couet:

thanks for the reply, but I have a question or two, regarding your solution:

  • I don’t understand why there are two while loops, neither do I understand the reason for defining the new variables h1, h2.
  • I don’t understand what the TIter objects n1(l1) and n2(l2) does, and I’ve tried looking in the documentation but I can’t find the part which explains this to me. (my apologies if it is listed but I didn’t see it due to my inexperience)

Thanks again for your help!

6

As I said it was just an example showing how to loop on THStacks. You can now elaborate for this example. For instance the following version does a printout of the histograms names in each stack. In one loop:

{
   auto st1 = new THStack();
   auto h1st1 = new TH1F("h1", "h1", 10, 0 , 1); st1->Add(h1st1);
   auto h2st1 = new TH1F("h2", "h2", 10, 0 , 1); st1->Add(h2st1);
   auto h3st1 = new TH1F("h3", "h3", 10, 0 , 1); st1->Add(h3st1);

   auto st2 = new THStack();
   auto h1st2 = new TH1F("h1", "h1", 10, 0 , 1); st2->Add(h1st2);
   auto h2st2 = new TH1F("h4", "h4", 10, 0 , 1); st2->Add(h2st2);
   auto h3st2 = new TH1F("h3", "h3", 10, 0 , 1); st2->Add(h3st2);

   auto l1 = st1->GetHists();
   auto l2 = st2->GetHists();

   TIter n1(l1);
   TIter n2(l2);

   TH1F    *h1, *h2;
   TObject *o1, *o2;

   while ((o1 = n1())) {
      h1 = (TH1F*) o1;
      o2 = n2();
      h2 = (TH1F*) o2;
      printf(" %s - %s\n",h1->GetName(),h2->GetName());
   }
}
1 Like
7

@couet:

Thanks for those hints! I’m still trying out a few combinations to get what I want, but thanks a lot for pointing me in the right direction!