ROOT
v6-22
Reference Guide
df012_DefinesAndFiltersAsStrings.C
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/// \file
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/// \ingroup tutorial_dataframe
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/// \notebook -nodraw
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///
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/// This tutorial illustrates how to save some typing when using RDataFrame
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/// by invoking functions that perform jit-compiling at runtime.
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///
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/// \macro_code
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/// \macro_output
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///
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/// \date October 2017
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/// \author Guilherme Amadio
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void
df012_DefinesAndFiltersAsStrings
()
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{
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// We will inefficiently calculate an approximation of pi by generating
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// some data and doing very simple filtering and analysis on it
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// We start by creating an empty dataframe where we will insert 10 million
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// random points in a square of side 2.0 (that is, with an inscribed circle
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// of radius 1.0)
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size_t
npoints = 10000000;
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ROOT::RDataFrame
df(npoints);
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// Define what we want inside the dataframe. We do not need to define p as an array,
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// but we do it here to demonstrate how to use jitting with RDataFrame
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// NOTE: Although it's possible to use "for (auto&& x : p)" below, it will
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// shadow the name of the data column "x", and may cause compilation failures
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// if the local variable and the data column are of different types or the
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// local x variable is declared in the global scope of the lambda function
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auto
pidf = df.Define(
"x"
,
"gRandom->Uniform(-1.0, 1.0)"
)
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.Define(
"y"
,
"gRandom->Uniform(-1.0, 1.0)"
)
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.Define(
"p"
,
"std::array<double, 2> v{x, y}; return v;"
)
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.Define(
"r"
,
"double r2 = 0.0; for (auto&& x : p) r2 += x*x; return sqrt(r2);"
);
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// Now we have a dataframe with columns x, y, p (which is a point based on x
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// and y), and the radius r = sqrt(x*x + y*y). In order to approximate pi, we
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// need to know how many of our data points fall inside the unit circle compared
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// with the total number of points. The ratio of the areas is
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//
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// A_circle / A_square = pi r*r / l * l, where r = 1.0, and l = 2.0
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//
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// Therefore, we can approximate pi with 4 times the number of points inside the
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// unit circle over the total number of points in our dataframe:
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auto
incircle
= *(pidf.Filter(
"r <= 1.0"
).Count());
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double
pi_approx = 4.0 *
incircle
/ npoints;
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std::cout <<
"pi is approximately equal to "
<< pi_approx << std::endl;
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}
ROOT::RDataFrame
ROOT's RDataFrame offers a high level interface for analyses of data stored in TTrees,...
Definition:
RDataFrame.hxx:42
df012_DefinesAndFiltersAsStrings
Definition:
df012_DefinesAndFiltersAsStrings.py:1
incircle
REAL incircle(struct mesh *m, struct behavior *b, vertex pa, vertex pb, vertex pc, vertex pd)
Definition:
triangle.c:5863
tutorials
dataframe
df012_DefinesAndFiltersAsStrings.C
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