bf2c4d2586
* Revert most of the changes to examples, because ugly macros don't belong in examples. Just use constexpr as appropriate. * Revert Lambda changes as they're wrong and Boost.Lambda doesn't support constexpr. * Remove include of broken math_fwd.hpp.
101 lines
3.2 KiB
C++
101 lines
3.2 KiB
C++
// Boost.Units - A C++ library for zero-overhead dimensional analysis and
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// unit/quantity manipulation and conversion
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//
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// Copyright (C) 2014 Erik Erlandson
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//
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// Distributed under the Boost Software License, Version 1.0. (See
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// accompanying file LICENSE_1_0.txt or copy at
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// http://www.boost.org/LICENSE_1_0.txt)
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//#include <boost/units/systems/information.hpp>
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/**
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\file
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\brief information.cpp
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\details
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Demonstrate information unit system.
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Output:
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@verbatim
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bytes= 1.25e+08 B
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bits= 8e+06 b
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nats= 4605.17 nat
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1024 bytes in a kibi-byte
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8.38861e+06 bits in a mebi-byte
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0.000434294 hartleys in a milli-nat
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entropy in bits= 1 b
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entropy in nats= 0.693147 nat
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entropy in hartleys= 0.30103 Hart
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entropy in shannons= 1 Sh
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entropy in bytes= 0.125 B
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@endverbatim
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**/
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#include <cmath>
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#include <iostream>
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using std::cout;
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using std::endl;
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using std::log;
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#include <boost/units/quantity.hpp>
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#include <boost/units/io.hpp>
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#include <boost/units/conversion.hpp>
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namespace bu = boost::units;
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using bu::quantity;
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using bu::conversion_factor;
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// SI prefixes
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#include <boost/units/systems/si/prefixes.hpp>
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namespace si = boost::units::si;
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// information unit system
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#include <boost/units/systems/information.hpp>
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using namespace bu::information;
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// Define a function for the entropy of a bernoulli trial.
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// The formula is computed using natural log, so the units are in nats.
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// The user provides the desired return unit, the only restriction being that it
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// must be a unit of information. Conversion to the requested return unit is
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// accomplished automatically by the boost::units library.
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template <typename Sys>
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constexpr
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quantity<bu::unit<bu::information_dimension, Sys> >
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bernoulli_entropy(double p, const bu::unit<bu::information_dimension, Sys>&) {
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typedef bu::unit<bu::information_dimension, Sys> requested_unit;
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return quantity<requested_unit>((-(p*log(p) + (1-p)*log(1-p)))*nats);
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}
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int main(int argc, char** argv) {
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// a quantity of information (default in units of bytes)
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quantity<info> nbytes(1 * si::giga * bit);
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cout << "bytes= " << nbytes << endl;
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// a quantity of information, stored as bits
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quantity<hu::bit::info> nbits(1 * si::mega * byte);
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cout << "bits= " << nbits << endl;
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// a quantity of information, stored as nats
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quantity<hu::nat::info> nnats(2 * si::kilo * hartleys);
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cout << "nats= " << nnats << endl;
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// how many bytes are in a kibi-byte?
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cout << conversion_factor(kibi * byte, byte) << " bytes in a kibi-byte" << endl;
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// how many bits are in a mebi-byte?
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cout << conversion_factor(mebi * byte, bit) << " bits in a mebi-byte" << endl;
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// how many hartleys are in a milli-nat?
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cout << conversion_factor(si::milli * nat, hartley) << " hartleys in a milli-nat" << endl;
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// compute the entropy of a fair coin flip, in various units of information:
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cout << "entropy in bits= " << bernoulli_entropy(0.5, bits) << endl;
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cout << "entropy in nats= " << bernoulli_entropy(0.5, nats) << endl;
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cout << "entropy in hartleys= " << bernoulli_entropy(0.5, hartleys) << endl;
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cout << "entropy in shannons= " << bernoulli_entropy(0.5, shannons) << endl;
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cout << "entropy in bytes= " << bernoulli_entropy(0.5, bytes) << endl;
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return 0;
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}
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