AliAnalysisTaskMyTask.cxx

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 * Author: The ALICE Off-line Project.                                    *
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 * provided "as is" without express or implied warranty.                  *
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/* AliAnaysisTaskMyTask
 *
 * empty task which can serve as a starting point for building an analysis
 * as an example, one histogram is filled
 */

#include "TChain.h"
#include "TH1F.h"
#include "TList.h"
#include "AliAnalysisTask.h"
#include "AliAnalysisManager.h"
#include "AliAODEvent.h"
#include "AliAODInputHandler.h"
#include "AliAnalysisTaskMyTask.h"

class AliAnalysisTaskMyTask;    // your analysis class

using namespace std;            // std namespace: so you can do things like 'cout'

ClassImp(AliAnalysisTaskMyTask) // classimp: necessary for root

AliAnalysisTaskMyTask::AliAnalysisTaskMyTask() : AliAnalysisTaskSE(), 
    fAOD(0), fOutputList(0), fHistPt(0)
{
    // default constructor, don't allocate memory here!
    // this is used by root for IO purposes, it needs to remain empty
}
//_____________________________________________________________________________
AliAnalysisTaskMyTask::AliAnalysisTaskMyTask(const char* name) : AliAnalysisTaskSE(name),
    fAOD(0), fOutputList(0), fHistPt(0)
{
    // constructor
    DefineInput(0, TChain::Class());    // define the input of the analysis: in this case we take a 'chain' of events
                                        // this chain is created by the analysis manager, so no need to worry about it, 
                                        // it does its work automatically
    DefineOutput(1, TList::Class());    // define the ouptut of the analysis: in this case it's a list of histograms 
                                        // you can add more output objects by calling DefineOutput(2, classname::Class())
                                        // if you add more output objects, make sure to call PostData for all of them, and to
                                        // make changes to your AddTask macro!
}
//_____________________________________________________________________________
AliAnalysisTaskMyTask::~AliAnalysisTaskMyTask()
{
    // destructor
    if(fOutputList) {
        delete fOutputList;     // at the end of your task, it is deleted from memory by calling this function
    }
}
//_____________________________________________________________________________
void AliAnalysisTaskMyTask::UserCreateOutputObjects()
{
    // create output objects
    //
    // this function is called ONCE at the start of your analysis (RUNTIME)
    // here you ceate the histograms that you want to use 
    //
    // the histograms are in this case added to a tlist, this list is in the end saved
    // to an output file
    //
    fOutputList = new TList();          // this is a list which will contain all of your histograms
                                        // at the end of the analysis, the contents of this list are written
                                        // to the output file
    fOutputList->SetOwner(kTRUE);       // memory stuff: the list is owner of all objects it contains and will delete them
                                        // if requested (dont worry about this now)

    // example of a histogram
    fHistPt = new TH1F("fHistPt", "fHistPt", 100, 0, 10);       // create your histogra
    fOutputList->Add(fHistPt);          // don't forget to add it to the list! the list will be written to file, so if you want
                                        // your histogram in the output file, add it to the list!
    
    PostData(1, fOutputList);           // postdata will notify the analysis manager of changes / updates to the 
                                        // fOutputList object. the manager will in the end take care of writing your output to file
                                        // so it needs to know what's in the output
}
//_____________________________________________________________________________
void AliAnalysisTaskMyTask::UserExec(Option_t *)
{
    // user exec
    // this function is called once for each event
    // the manager will take care of reading the events from file, and with the static function InputEvent() you 
    // have access to the current event. 
    // once you return from the UserExec function, the manager will retrieve the next event from the chain
    fAOD = dynamic_cast<AliAODEvent*>(InputEvent());    // get an event (called fAOD) from the input file
                                                        // there's another event format (ESD) which works in a similar wya
                                                        // but is more cpu/memory unfriendly. for now, we'll stick with aod's
    if(!fAOD) return;                                   // if the pointer to the event is empty (getting it failed) skip this event
        // example part: i'll show how to loop over the tracks in an event 
        // and extract some information from them which we'll store in a histogram
    Int_t iTracks(fAOD->GetNumberOfTracks());           // see how many tracks there are in the event
    for(Int_t i(0); i < iTracks; i++) {                 // loop ove rall these tracks
        AliAODTrack* track = static_cast<AliAODTrack*>(fAOD->GetTrack(i));         // get a track (type AliAODTrack) from the event
        if(!track) continue;                            // if we failed, skip this track
        fHistPt->Fill(track->Pt());                     // plot the pt value of the track in a histogram
    }                                                   // continue until all the tracks are processed
    PostData(1, fOutputList);                           // stream the results the analysis of this event to
                                                        // the output manager which will take care of writing
                                                        // it to a file
}
//_____________________________________________________________________________
void AliAnalysisTaskMyTask::Terminate(Option_t *)
{
    // terminate
    // called at the END of the analysis (when all events are processed)
}
//_____________________________________________________________________________