Working with a Sample Driver

Drivers are for processing events one-by-one and doing some work on the data. This may include performing some physics analysis or creating plots. A driver can contain additional, "child" drivers, so the complexity of the processing algorithm can be segmented into several different logical functions.

Opening the LCIO File

Start JAS3 and open the file from the LCSim Event Browser Tutorial using the File -> Open command.

You should now see the record loop commands and the name of the file displayed in the record source drop-down box.

Opening the Sample Driver

Open a sample driver by going to Help -> Examples and navigating to org.lcsim Examples. Click on Analysis101 to load the sample driver in a new window.

Compiling

To compile the sample Driver into code that will be executed when given events from the LCIO record, right-click on the window and click Compile.

You should see a message similar to the following.

4:09:28 PM ----------- compile successful

This indicates that the code was successfully compiled into a binary class file.

Loading

To load the Driver so that it will process events, select Load from the right-click menu.

Subsequent compiles of this file will cause the Driver to be automatically reloaded.

Processing Events

To process some events using the loaded Driver, click on the Go button on the toolbar or press F5. This will run the Driver's process method over all the records in the LCIO file.

Viewing Histograms

This particular example produces some histograms, which should be visible.

Explanation of Driver Code

Below is the complete text of the Analysis101 Driver, stripped of comments.

import org.lcsim.util.aida.AIDA;
import hep.physics.vec.VecOp;
import java.util.List;
import org.lcsim.event.EventHeader;
import org.lcsim.event.MCParticle;
import org.lcsim.util.Driver;

public class Analysis101 extends Driver
{
   private AIDA aida = AIDA.defaultInstance();
   
   protected void process(EventHeader event)
   {
      List<MCParticle> particles = event.get(MCParticle.class,event.MC_PARTICLES);

      aida.cloud1D("nTracks").fill(particles.size());
      
      for (MCParticle particle : particles)
      {
         aida.cloud1D("energy").fill(particle.getEnergy());
         aida.cloud1D("cosTheta").fill(VecOp.cosTheta(particle.getMomentum()));
         aida.cloud1D("phi").fill(VecOp.phi(particle.getMomentum()));
      }
   }
}

First, as in all Java programs, there are the import statements.

import org.lcsim.util.aida.AIDA;
import hep.physics.vec.VecOp;
import java.util.List;
import org.lcsim.event.EventHeader;
import org.lcsim.event.MCParticle;
import org.lcsim.util.Driver;

This includes code libraries into the Driver that are needed to do the analysis.

For instance, methods of org.lcsim.util.aida.AIDA are used to book and fill the plots.

The hep.physics.vec.VecOp package is for doing math operations on vectors.

Next is the declaration of the Driver class.

public class Analysis101 extends Driver

The Analysis101 class extends Driver, which means that it can override some or all of Driver's public or protected methods to do some useful work, like filling histograms.

The class stores a reference to the default AIDA object.

private AIDA aida = AIDA.defaultInstance();

This is convenient for creating and filling histograms "on-the-fly".

The single function in this class is called process. Analysis101 inherits this method from Driver.

protected void process(EventHeader event)

Its single argument is the EventHeader of the current LCIO event. All collections in this event are accessible through the EventHeader interface.