Df 0 0 7_Snapshot

Write ROOT data with RDataFrame.

This tutorial shows how to write out datasets in ROOT format using the RDataFrame

Author: Danilo Piparo (CERN)
This notebook tutorial was automatically generated with ROOTBOOK-izer from the macro found in the ROOT repository on Thursday, June 24, 2021 at 07:12 AM.

In [ ]:
import ROOT

A simple helper function to fill a test tree: this makes the example stand-alone.

In [ ]:
def fill_tree(treeName, fileName):
    df = ROOT.RDataFrame(10000)
    df.Define("b1", "(int) rdfentry_")\
      .Define("b2", "(float) rdfentry_ * rdfentry_").Snapshot(treeName, fileName)

We prepare an input tree to run on

In [ ]:
fileName = "df007_snapshot_py.root"
outFileName = "df007_snapshot_output_py.root"
outFileNameAllColumns = "df007_snapshot_output_allColumns_py.root"
treeName = "myTree"
fill_tree(treeName, fileName)

We read the tree from the file and create a RDataFrame.

In [ ]:
d = ROOT.RDataFrame(treeName, fileName)

Select entries

We now select some entries in the dataset

In [ ]:
d_cut = d.Filter("b1 % 2 == 0")

Enrich the dataset

Build some temporary columns: we'll write them out

In [ ]:
getVector_code ='''
std::vector<float> getVector (float b2)
{
   std::vector<float> v;
   for (int i = 0; i < 3; i++) v.push_back(b2*i);
   return v;
}
'''
ROOT.gInterpreter.Declare(getVector_code)

d2 = d_cut.Define("b1_square", "b1 * b1") \
          .Define("b2_vector", "getVector( b2 )")

Write it to disk in ROOT format

We now write to disk a new dataset with one of the variables originally present in the tree and the new variables. The user can explicitly specify the types of the columns as template arguments of the Snapshot method, otherwise they will be automatically inferred.

In [ ]:
branchList = ROOT.vector('string')()
for branchName in ["b1", "b1_square", "b2_vector"]:
    branchList.push_back(branchName)
d2.Snapshot(treeName, outFileName, branchList)

Open the new file and list the columns of the tree

In [ ]:
f1 = ROOT.TFile(outFileName)
t = f1.myTree
print("These are the columns b1, b1_square and b2_vector:")
for branch in t.GetListOfBranches():
    print("Branch: %s" %branch.GetName())

f1.Close()

We are not forced to write the full set of column names. We can also specify a regular expression for that. In case nothing is specified, all columns are persistified.

In [ ]:
d2.Snapshot(treeName, outFileNameAllColumns)

Open the new file and list the columns of the tree

In [ ]:
f2 = ROOT.TFile(outFileNameAllColumns)
t = f2.myTree
print("These are all the columns available to this dataframe:")
for branch in t.GetListOfBranches():
    print("Branch: %s" %branch.GetName())

f2.Close()

We can also get a fresh RDataFrame out of the snapshot and restart the analysis chain from it.

In [ ]:
branchList.clear()
branchList.push_back("b1_square")
snapshot_df = d2.Snapshot(treeName, outFileName, branchList);
h = snapshot_df.Histo1D("b1_square")

c = ROOT.TCanvas()
h.Draw()
c.SaveAs("df007_snapshot.png")

print("Saved figure to df007_snapshot.png")

Draw all canvases

In [ ]:
from ROOT import gROOT 
gROOT.GetListOfCanvases().Draw()