df026_AsNumpyArrays.py

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## \file
## \ingroup tutorial_dataframe
## \notebook
## This tutorial shows how read data of a RDataFrame into Numpy arrays.
##
## \macro_code
## \macro_output
##
## \date December 2018
## \author Stefan Wunsch

import ROOT
from sys import exit

# Let's create a simple dataframe with ten rows and two columns
df = ROOT.RDataFrame(10) \
         .Define("x", "(int)rdfentry_") \
         .Define("y", "1.f/(1.f+rdfentry_)")

# Next, we want to access the data from Python as Numpy arrays. To do so, the
# content of the dataframe is converted using the AsNumpy method. The returned
# object is a dictionary with the column names as keys and 1D numpy arrays with
# the content as values.
npy = df.AsNumpy()
print("Read-out of the full RDataFrame:\n{}\n".format(npy))

# Since reading out data to memory is expensive, always try to read-out only what
# is needed for your analysis. You can use all RDataFrame features to reduce your
# dataset, e.g., the Filter transformation. Furthermore, you can can pass to the
# AsNumpy method a whitelist of column names with the option `columns` or a blacklist
# with column names with the option `exclude`.
df2 = df.Filter("x>5")
npy2 = df2.AsNumpy()
print("Read-out of the filtered RDataFrame:\n{}\n".format(npy2))

npy3 = df2.AsNumpy(columns=["x"])
print("Read-out of the filtered RDataFrame with the columns option:\n{}\n".format(npy3))

npy4 = df2.AsNumpy(exclude=["x"])
print("Read-out of the filtered RDataFrame with the exclude option:\n{}\n".format(npy4))

# You can read-out all objects from ROOT files since these are wrapped by PyROOT
# in the Python world. However, be aware that objects other than fundamental types,
# such as complex C++ objects and not int or float, are costly to read-out.
ROOT.gInterpreter.Declare("""
// Inject the C++ class CustomObject in the C++ runtime.
class CustomObject {
public:
    int x = 42;
};
// Create a function that returns such an object. This is called to fill the dataframe.
CustomObject fill_object() { return CustomObject(); }
""")

df3 = df.Define("custom_object", "fill_object()")
npy5 = df3.AsNumpy()
print("Read-out of C++ objects:\n{}\n".format(npy5["custom_object"]))
print("Access to all methods and data members of the C++ object:\nObject: {}\nAccess data member: custom_object.x = {}\n".format(
    repr(npy5["custom_object"][0]), npy5["custom_object"][0].x))

# Note that you can pass the object returned by AsNumpy directly to pandas.DataFrame
# including any complex C++ object that may be read-out.
try:
    import pandas
except:
    print("Failed to import pandas.")
    exit()

df = pandas.DataFrame(npy5)
print("Content of the ROOT.RDataFrame as pandas.DataFrame:\n{}\n".format(df))