THStack::Add members of histogram vector

ROOT
1

Hello - I’m trying to loop through vector of TH1D histograms and add each to a THStack, but I receive an error. I’m learning c++ as I go, so this may be a problem of syntax. Here is the code:

// This works fine
vector<TH1D> hThresh_particlesCPV1(11);
   for (unsigned int i=0; i<11; i++) {
      hThresh_particlesCPV1[i]=TH1D(hThresh_names[i], hThresh_titles[i], bins,0,5);
   }  
// The Add below is causing the error  
THStack *hsCombined = new THStack("hStacked_threshold", "Charged particles above each energy threshold");
for (unsigned int i=0;i<11;i++){
    hsCombined->Add((TH1)hThresh_particlesCPV1[i]);
  }

And here is the error:

2

Hi,

in

hsCombined->Add((TH1)hThresh_particlesCPV1[i]);

you should pass a pointer to the histogram and not the histogram itself. I believe something like

hsCombined->Add((TH1*)&hThresh_particlesCPV1[i]);

may yield better results.

Cheers,
Benedikt

3

Benedikt - Yes! That’s it! I was thinking I had to pass a pointer, but I couldn’t figure out how. I understand your syntax clearly. Thank you so much!

4

Sure. The difference between

and

is that in the first case you deal with an object directly, while in the second case you deal with a pointer to a TH1. That’s the function of the *.

Now coming to the &. The value contained within a pointer is actually the physical address of the object you point at. And the C++ syntax to retrieve the address of an object is &theObject.

So in combination you do the following. you have an object and you need to pass in a pointer to the object. You do &hThresh_particlesCPV1[i] to retrieve the address and with (TH1*) you let the compiler know that the address you got points actually to an object of the kind TH1. It may even be that you can remove the (TH1*) in your statement if the compiler is smart enough to guess what you mean.

Hope that helps,
Benedikt