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namespace	df019_Cache

Detailed Description

This tutorial shows how the content of a data frame can be cached in memory in form of a data frame.

The content of the columns is stored in memory in contiguous slabs of memory and is "ready to use", i.e. no ROOT IO operation is performed.

Creating a cached data frame storing all of its content deserialised and uncompressed in memory is particularly useful when dealing with datasets of a moderate size (small enough to fit the RAM) over which several explorative loops need to be performed at as fast as possible. In addition, caching can be useful when no file on disk needs to be created as a side effect of checkpointing part of the analysis.

All steps in the caching are lazy, i.e. the cached data frame is actually filled only when the event loop is triggered on it.

 
import ROOT
RDataFrame = ROOT.ROOT.RDataFrame
import os
 
# We create a data frame on top of the hsimple example
hsimplePath = os.path.join(str(ROOT.gROOT.GetTutorialDir().Data()), "hsimple.root")
df = RDataFrame("ntuple", hsimplePath)
 
#We apply a simple cut and define a new column
df_cut = df.Filter("py > 0.f")\
           .Define("px_plus_py", "px + py")
 
# We cache the content of the dataset. Nothing has happened yet: the work to accomplish
# has been described.
df_cached = df_cut.Cache()
 
h = df_cached.Histo1D("px_plus_py")
 
# Now the event loop on the cached dataset is triggered. This event triggers the loop
# on the `df` data frame lazily.
h.Draw()

Date: June 2018

Author: Danilo Piparo

Definition in file df019_Cache.py.

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Detailed Description