倫倫的小blog

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script: http://goo.gl/oRtqYF

simple.root : http://goo.gl/rKkHt0

/*
*    In this tutorial,
*    We will expore the TEventList, TEntryList
*    
*
*******************************************************************/
{

    TFile* f = new TFile("simple.root");
    TTree* t;
    f->GetObject("t",t);

    /* review: setup selection */
    t->Draw("money");
    t->Draw("money","money<2500");

    t->Scan("money", "", "col=15.0F", 10,0);
    t->Scan("money", "money<3000", "col=15.0F", 10,0);

    /*
    |   But the selection may take long time
    |   especially when you have a complicated selection rule.
    |   and each time, Draw(), Scan(), will take long time.
    |
    |   We can make a list that store the sorted data
    |   and then just plot the list,
    |   hence we just need to sort once.
    */

    /*
    |   You can use:
    |   TEventList, TEntryList
    |   Their operations are very similar
    |   Let's start from "TEventList"
    */

    ////////////////////////////////////////////////////////////////    
    //
    //      TEventList section
    //


    t->Draw(">>myEvent","money<2500");
    /*
    |   using Draw() method to store selected data
    |   to generate the TEventList
    |   >>  redirect to TEventList obj "myEvent"
    |   >>+ redirect to TEventList obj "myEvent" in append mode
    |   
    |   you can specify the range as well.
    |   t->Draw(">>myEvent","money<3000","",100,0);
    */


    TEventList* myEvent;    
    gDirectory->GetObject("myEvent",myEvent);    

    t->SetEventList(myEvent);
    /*
    |   use TTree::SetEventList() method
    |   to restrict the tree within the selection rule.
    |   from the "myEvent".
    */


    t->Draw("money");
    t->Scan("money", "", "col=15.0F", 10,0);



    for( Int_t i = 0; i< 10; i++) {

        cout << i <<"\t"<< myEvent->GetEntry(i) << endl;    
    }
    /*
    |   TEventList::GetEntry() method is different from
    |   TTree::GetEntry() method.
    |
    |   TEventList::GetEntry() only return the index with which
    |   the entries satify the selection rules.
    */


    /* looping */

    Int_t n_List = myEvent->GetN();  // total run in myEvent
   
    Float_t money; t->SetBranchAddress("money", &money);

    Long64_t index_good;
    /*
    |   in case we have a long run
    |   we set up Long64_t type
    |   below we show 10 runs, for full run use i<n_List
    */

    for( Int_t i=0; i<10; i++) {

        index_good = myEvent->GetEntry(i);
        
        t->GetEntry( index_good );  

        cout<<" money = "<< fixed << setprecision(0) << money << endl;

    }
    
    t->SetEventList(0); // to reset

    /////////////////////////////////////////////////////////////////
    //
    //
    //      TEntryList section
    //   use considerably less memory for storage



    t->Draw(">>myEntry","money<2500","entrylist");
    /*
    |   using Draw() method, and add "entrylist" flag.
    */



 
    TEntryList* myEntry;
    gDirectory->GetObject("myEntry",myEntry);



    t->SetEntryList(myEntry);

    t->Draw("money");
    t->Scan("money", "", "col=15.0F", 10,0);

    for( Int_t i = 0; i< 10; i++) {

        cout << i <<"\t"<< myEntry->GetEntry(i) << endl;    
    }

    /* looping */

    Int_t n_List = myEntry->GetN();  // total run in myEntry
   

    for( Int_t i=0; i<10; i++) {

        index_good = myEntry->GetEntry(i);
        
        t->GetEntry( index_good );  

        cout<<" money = "<< fixed << setprecision(0) << money << endl;

    }

    /*
    |
    | or Next() method
    | it is quicker than TEntryList::GetEntry() method.
    |
    */
    
    index_good = myEntry->GetEntry(0); // setup the starting point

    for( Int_t i=0; i<10; i++) {

        t->GetEntry( index_good );

        index_good = myEntry->Next();

        cout<<" money = "<< fixed << setprecision(0) << money << endl;

    }

    TFile g("list.root","recreate");
    myEntry->Write();
    myEvent->Write();
    g->Close();
    f->Close();

}