When writing the TClonesArray, use the option kSingleKey,
otherwise each entry in the TClonesArray becomes a separate key.
See your file modified below
Rene Brun
void write_lv() {
TFile tf("test_lv.root","recreate");
TClonesArray clarr("TLorentzVector",5);
TLorentzVector* lv = new (clarr[0]) TLorentzVector();
lv->SetPxPyPzE(1,2,3,4);
cout << "storing a lorentz vec with a mas of "<<lv->M() << endl;
cout << "momentum: ("<<lv->Px()<< ", "<< lv->Py() << ", "
<< lv->Pz() << " )"<<endl;
cout << "array has "<< clarr.GetEntries()<<" entries"<<endl;
clarr->Write("an_array",TObject::kSingleKey);
tf.Close();
}
void read_lv() {
TFile tf("test_lv.root");
TClonesArray* arr = (TClonesArray*)tf.Get("an_array");
printf("entries = %d\n",arr->GetEntries());
}
void lv() {
gSystem->Load("libPhysics");
write_lv();
read_lv();
}
On Wed, 13 Nov 2002, Paul Balm wrote:
> Hello,
>
> I'm trying to write a TClonesArray containing one TLorentzVector to a
> file and read it back. To write the file, I use this function:
>
> void write_lv() {
>
> TFile tf("test_lv.root","recreate");
> TClonesArray clarr("TLorentzVector",5);
> TLorentzVector* lv = new (clarr[0]) TLorentzVector();
> lv->SetPxPyPzE(1,2,3,4);
>
> cout << "storing a lorentz vec with a mas of "<<lv->M() << endl;
> cout << "momentum: ("<<lv->Px()<< ", "<< lv->Py() << ", "
> << lv->Pz() << " )"<<endl;
> cout << "array has "<< clarr.GetEntries()<<" entries"<<endl;
>
> clarr->Write("an_array");
> tf.Close();
> }
>
> Then I start new root session, open the file, and try to read the
> TClonesArray:
>
> root [0] TFile tf("test_lv.root")
> root [1] tf.ls()
> TFile** test_lv.root
> TFile* test_lv.root
> KEY: TLorentzVector an_array;1 A four vector with (-,-,-,+) metric
> root [2] TClonesArray* arr = tf.Get("an_array")
> root [3] arr->GetEntries()
>
> *** Break *** segmentation violation
> Root >
> root [4] arr
> (class TClonesArray*)0x8979720
>
> So I didn't get a null pointer, but using it as a TClonesArray, e.g. by
> calling GetEntries() on it, doesn't work. Also, from the TFile::ls()
> output, you could already tell that what has been written is a
> TLorentzVector, and not a TClonesArray. And indeed I can use the pointer
> I get from the file if I interpret it as a TLorentzVector:
>
> root [7] TLorentzVector* lv2 = tf.Get("an_array")
> root [8] lv2->M()
> (const Double_t)1.41421356237309515e+00
>
> This number matches what I expect from what I wrote (sqrt(2)).
>
> This is root 3.03/09 on Linux RH6. Does anyone have any idea what is
> going wrong?
> Thanks!
>
> Paul Balm
>
>
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