# Df 0 0 7_Snapshot¶

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

Author: Danilo Piparo
This notebook tutorial was automatically generated with ROOTBOOK-izer from the macro found in the ROOT repository on Monday, July 06, 2020 at 11:32 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.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()


Draw all canvases

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