2D arrays as a leaf

ROOT
1

Hello,
I’m trying to do the following:

const Int_t nbins=2;
const Int_t nbins2=3;

struct mystruct
{
  Double_t N[nbins][nbins2];
};

void func()
{
   struct mystruct myone;

   TTree tr("abc","tree");
   tr.Branch("a",&nbins,"nbins/I");
   tr.Branch("b",&nbins2,"nbins2/I");
   tr.Branch("c",myone.N,"N[nbins][nbins2]/D");

   myone.N[0][1]=4;
   tr.Fill();
   tr.Scan("N");
}

If N is 1D array such code works well. It also works well If I explicit write a dimensions of array:

tr.Branch("c",myone.N,"N[2][3]/D");  //instead of N[nbins][nbins2]

Is there a way to have 2D arrays and number of dimensions declared as a const variable?

2

TTree’s leaflist mechanism only support one variable dimension (and several fixed dimensions).

Cheers,
Philippe.

3

To bad :cry:

4

Hi,

If you really need a double variable array, you should use a higher level structure (for example a vector<vector >)

Cheers,
Philippe.

5

Thanks for hint.
What about some workaround:

const Int_t nbins=2;
const Int_t nbins2=3;
const Int_t dim=nbins*nbins2;

struct mystruct
{
  Double_t N[nbins][nbins2];
};

void wiocha()
{
  
  struct mystruct myone;
  
  TTree tr("abc","tree");
  
  tr.Branch("a",&dim,"dim/I");
  tr.Branch("b",myone.N,"N[dim]/D");
  
  myone.N[0][0]=1;
  myone.N[0][1]=2;
  myone.N[0][2]=3; 
  myone.N[1][0]=4;
  myone.N[1][1]=5;
  myone.N[1][2]=6;
 
  tr.Fill();
  tr.Scan("N");
}

It seems to works ok.
May I expect some unexpected behavior?

6

this is fine (but TTree::Draw will only know about one dimension)

Philippe

7

Hi,

If you say

tr.Branch(“c”,myone.N,“N[2][3]/D”);
works and
tr.Branch(“c”,myone.N,“N[nbins][nbins2]/D”);
doesn’t

try:

TString bvar(“N[”);
bvar += nbins;
bvar += “][”;
bvar += nbins2;
bvar += “]/D”;
tr.Branch(“c”,myone.N,bvar.Data());

I use this technique a lot.

A nice day to all,

Rui