RE: [ROOT] Reading 2D arrays from a Tree

From: Philippe Canal (pcanal@fnal.gov)
Date: Tue Nov 13 2001 - 17:21:23 MET 

Hi Ben,

I do not know if the following are typos or actual slight errors, so 
I'll just go over them to make sure.

Your original code says:

   TFile file("adc.root");
   TTree *tree = new TTree("tree");

This does NOT load the tree in the file.  What this 2 lines of code
do is to open the file "adc.root" and create a new empty TTree name
tree.  So your second attempt is better

   TFile file("adc.root");
   TTree *tree = (TTree*)file->Get("tree");

Except that in compiled C++ (as opposed to interpreted CINT), you have
to pay attention to whether your variable are object or pointer to 
objects.  When a variable is an object (as in your line: TFile file("adc.root");)
to access the object you need to use the dot ('.') notation.  Thus the 
proper syntax is:

   TFile file("adc.root");
   TTree *tree = (TTree*)file.Get("tree");

When a variable is a pointer to an object, to access the object you need to 
use the arrow ('->') notation.  Thus a proper alternative is:

   TFile *file = new TFile("adc.root");
   TTree *tree = (TTree*)file->Get("tree");

Note that declaring your file object as 
	TFile file("adc.root");
or
      TFile *file = new TFile("adc.root");
as strong implication on the lifetime of the object.  In the first case,
the object will be deleted as soon as the function it is declared in ends.
In the second case, the object will be deleted only when you explicitly 
call: "delete file".

Cheers,
Philippe.


-----Original Message-----
From: Ben Morgan [mailto:B.Morgan@sheffield.ac.uk]
Sent: Tuesday, November 13, 2001 9:59 AM
To: Philippe Canal
Cc: roottalk
Subject: RE: [ROOT] Reading 2D arrays from a Tree


Thanks Philippe,
                I should have remembered, I'm using ROOT v3.01/06 on an
Intel PC running Red Hat 6.2.
Yes, Glen's codes run fine when run as macros in an interactive session,
but simply cutting and pasting them into a very simplistic C++ code like:

#include<iostream.h>
#include<iomanip.h>
#include<fstream.h>
#include<stdio.h>
#include <stdlib.h>
#include<math.h>

//ROOT Headers
#include "TROOT.h"
#include "TFile.h"
#include "TNetFile.h"
#include "TRandom.h"
#include "TTree.h"
#include "TBranch.h"
#include "TClonesArray.h"
#include "TStopwatch.h"

int main(int argc, char **argv)
{


Float_t na[10][10];
TFile file("adc.root");
TTree *tree = new TTree("tree");

TBranch *b = tree->GetBranch("adc");
TLeaf *l = b->GetLeaf("adc");
tree->SetBranchAddress("adc",&na);

for (Int_t i=0; i<150; i++) {
  b->GetEvent(i);
  printf("na[1][1] = %f, ",na[1][1]);
}

 return 0;
}

This doesn't compile. The problem seems to be with the line:
TTree *tree = new TTree("tree");
This doesn't appear in Glen's code, but something like this appears to be
neccessary in a C++ program otherwise the compiler complains about 'tree'
being undefined in the next line, i.e. leaving this line out produces:

twodread.C: In function `int main(int, char **)':
twodread.C:25: `tree' undeclared (first use this function)

on compilation. Equally, using a line like

TTree *tree = (TTree*)file->Get("tree")

as used in the examples of reading trees in the manual doesn't compile
properly either. Hope this makes things clearer,

Thanks,

Ben Morgan.


-- 
-------------------------------------------------------------------------------
Mr. Ben Morgan
Postgraduate Student
University of Sheffield
Department of Physics & Astronomy
Hicks Building
Hounsfield Road
Sheffield  S3 7RH
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