Dear Rooters,
this might be a newbie question, but it should be nevertheless interesting
as an illustration how non-intuitive (and thus non-trivial) some of
ROOT's features are.
I'm trying to simultaneously fill an ntuple and read from it (in
another section of the code). One can argue that is perfectly
reasonable to collect a bunch of events, pause the run, analyze them,
and then keep on filling the ntuple.
However, it seams that SetBranchAddress() interferes with the Fill()
method for the variables for which SetBranchAddress() was issued.
These variables end up being 0.
The example below (which is a slightly modified root tutorial)
illustrates the point:
{
FILE *fp = fopen("foo.txt","r");
Float_t x,y,z;
Float_t x1;
Int_t ncols;
Int_t nlines = 0;
TNtuple *ntuple = new TNtuple("ntuple","data from ascii file","x:y:z");
// ******************************************************************
ntuple->SetBranchAddress("x",x1); // <----- why is this dangerous???
// ******************************************************************
while (1) {
ncols = fscanf(fp,"%f %f %f",&x, &y, &z);
if (ncols < 0) break;
if (nlines < 5) printf("x=%8f, y=%8f, z=%8f\n",x,y,z);
ntuple->Fill(x,y,z);
nlines++;
}
fclose(fp);
}
Where the ntuple is read from the file
% cat foo.txt
1 2 3
1.1 2.2 3.3
-1.1 -2.2 -3.3
-1 -2 -3
1.5 1.5 1.5
Here, if the SetBranchAddress() is commented out, everything is fine.
But once it's uncommented, all "x" entries are 0.0.
My questions:
1. It's obvious that I'm doing something silly, but for a non-expert
like me it's hard to pin down the location of this silliness.
2. If this behavior of TTree is a feature (which it may well be), then what
is the correct way of doing this?
Many thanks!
Petar
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