delta-phi in one line

Calculating the delta-phi correctly can be annoying, as you would need to ensure the result is always in the range of [-π, π]. Or you could just use a simple trigonometric function:

double dphi = std::acos(std::cos(phi1 - phi2));

However, the above result is always positive. If you care about the sign, you could work with cos θ instead:

double cos_dphi = std::cos(phi1 - phi2);

The one-liner can come in handy when you want to plot delta-phi from the ROOT command prompt.

Remove a package after git cms-addpkg

You can checkout a particular package in CMSSW by calling:

git cms-addpkg L1Trigger/L1TMuonEndCap

(see CMSSW FAQ).

But if you happen to remove the package later, the next time you call git status, all the files in the package will show up as “deleted”. To remedy this, notice that what git cms-addpkg really does is a git sparse-checkout operation. So, you can edit the internal file .git/info/sparse-checkout to remove the package name from the file, then call:

git read-tree -mu HEAD

Now, your local git working area should return to the state before you had called git cms-addpkg.

Merge arrays from multiple npz files

When splitting your task into multiple jobs, you’ll need a way to merge the output files. ROOT allows you to merge files using the hadd program. It finds the histograms and trees from multiple ROOT files and merges them, resulting in a single ROOT file.

As I started to move away from ROOT into the Python environment, I kind of missed hadd. So I wrote a script that emulates it to merge numpy arrays from multiple .npz files. It can be used in the following way (I named the script as

python out_add.npz out_*.npz

The script (

#!/usr/bin/env python

from __future__ import absolute_import
from __future__ import division
from __future__ import print_function

import numpy as np
import os
import six

class Hadd(object):
  """Merge arrays from multiple .npz files.

  It emulates the interface of the utility 'hadd' in ROOT
  (see <>).
  def __init__(self):
    self.d = {}
    self.dout = {}

  def process(self, target, source, force=False):
    print('hadd Target file: {}'.format(target))

    if not force:
      if os.path.isfile(target):
        msg = 'hadd error opening target file (does {} exist?).\n'.format(target)
        msg += 'Pass "-f" argument to force re-creation of output file.'
        raise ValueError(msg)

    # Loop over the source files
    for i, s in enumerate(source):
      print('hadd Source file {}: {}'.format(i + 1, s))
      with np.load(s) as loaded:
        # Insert the keys
        if i == 0:
          for k in loaded.files:
            self.d[k] = []
        # Keep the arrays
        for k in loaded.files:

    # Merge arrays via np.hstack() or np.vstack()
    for k, v in six.iteritems(self.d):
      print('array: {}'.format(k))
      if v[0].ndim == 0:
        vv = np.array(v)
      elif v[0].ndim == 1:
        vv = np.hstack(v)
      elif v[0].ndim == 2:
        vv = np.vstack(v)
      elif v[0].ndim == 3:
        vv = np.dstack(v)
        vv = np.concatenate(v, axis=-1)
      self.dout[k] = vv

    # Write to the target file
    np.savez_compressed(target, **self.dout)

# Main
if __name__ == '__main__':
  import argparse
  parser = argparse.ArgumentParser(description='hadd for npz files.')
  parser.add_argument('-f', '--force', action='store_true', help='Force write the target file')
  parser.add_argument('target', help='target file')
  parser.add_argument('source', nargs='+', help='source files')
  args = parser.parse_args()

  hadd = Hadd()
  hadd.process(, args.source, force=args.force)


HTCondor or Condor is the job scheduling system widely used in CMS. Some helpful resources and guides are listed below:


CMSConnect is a more powerful alternative to condor that allows users to submit jobs to multiple sites on the CMS computing grid.

In particular, this page shows how to add the public SSH key to your account:

When using the CMSConnect, one has to specify the “project” name in the submit script. For UF, it is:


To submit jobs to SLC7 machines, one has to specify:


I’m using the CMSConnet “client” on UF HiPerGator as it is the only way to submit condor jobs from there.