diff --git a/DIRECTORY.md b/DIRECTORY.md index 53e2090ab..d1822f7cf 100644 --- a/DIRECTORY.md +++ b/DIRECTORY.md @@ -182,6 +182,7 @@ * [Gcd Of N Numbers](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/math/gcd_of_n_numbers.cpp) * [Gcd Recursive Euclidean](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/math/gcd_recursive_euclidean.cpp) * [Integral Approximation](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/math/integral_approximation.cpp) + * [Integral Approximation2](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/math/integral_approximation2.cpp) * [Inv Sqrt](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/math/inv_sqrt.cpp) * [Large Factorial](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/math/large_factorial.cpp) * [Large Number](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/math/large_number.h) diff --git a/math/integral_approximation2.cpp b/math/integral_approximation2.cpp new file mode 100644 index 000000000..706672d12 --- /dev/null +++ b/math/integral_approximation2.cpp @@ -0,0 +1,196 @@ +/** + * @file + * @brief [Monte Carlo Integration](https://en.wikipedia.org/wiki/Monte_Carlo_integration) + * + * @details + * In mathematics, Monte Carlo integration is a technique for numerical integration using random numbers. + * It is a particular Monte Carlo method that numerically computes a definite integral. + * While other algorithms usually evaluate the integrand at a regular grid, Monte Carlo randomly chooses points at which the integrand is evaluated. + * This method is particularly useful for higher-dimensional integrals. + * + * This implementation supports arbitrary pdfs. + * These pdfs are sampled using the [Metropolis-Hastings algorithm](https://en.wikipedia.org/wiki/Metropolis–Hastings_algorithm). + * This can be swapped out by every other sampling techniques for example the inverse method. + * Metropolis-Hastings was chosen because it is the most general and can also be extended for a higher dimensional sampling space. + * + * @author [Domenic Zingsheim](https://github.com/DerAndereDomenic) + */ + +#define _USE_MATH_DEFINES /// for M_PI on windows +#include /// for math functions +#include /// for fixed size data types +#include /// for time to initialize rng +#include /// for function pointers +#include /// for std::cout +#include /// for random number generation +#include /// for std::vector + +/** + * @namespace math + * @brief Math algorithms + */ +namespace math { +/** + * @namespace monte_carlo + * @brief Functions for the [Monte Carlo Integration](https://en.wikipedia.org/wiki/Monte_Carlo_integration) implementation + */ +namespace monte_carlo { + +using Function = std::function; /// short-hand for std::functions used in this implementation + +/** + * @brief Generate samples according to some pdf + * @details This function uses Metropolis-Hastings to generate random numbers. It generates a sequence of random numbers by using a markov chain. + * Therefore, we need to define a start_point and the number of samples we want to generate. + * Because the first samples generated by the markov chain may not be distributed according to the given pdf, one can specify how many samples + * should be discarded before storing samples. + * @param start_point The starting point of the markov chain + * @param pdf The pdf to sample + * @param num_samples The number of samples to generate + * @param discard How many samples should be discarded at the start + * @returns A vector of size num_samples with samples distributed according to the pdf + */ +std::vector generate_samples(const double& start_point, const Function& pdf, const uint32_t& num_samples, const uint32_t& discard = 100000) { + std::vector samples; + samples.reserve(num_samples); + + double x_t = start_point; + + std::default_random_engine generator; + std::uniform_real_distribution uniform(0.0, 1.0); + std::normal_distribution normal(0.0, 1.0); + generator.seed(time(nullptr)); + + for(uint32_t t = 0; t < num_samples + discard; ++t) { + // Generate a new proposal according to some mutation strategy. + // This is arbitrary and can be swapped. + double x_dash = normal(generator) + x_t; + double acceptance_probability = std::min(pdf(x_dash)/pdf(x_t), 1.0); + double u = uniform(generator); + + // Accept "new state" according to the acceptance_probability + if(u <= acceptance_probability) { + x_t = x_dash; + } + + if(t >= discard) { + samples.push_back(x_t); + } + } + + return samples; +} + +/** + * @brief Compute an approximation of an integral using Monte Carlo integration + * @details The integration domain [a,b] is given by the pdf. + * The pdf has to fulfill the following conditions: + * 1) for all x \in [a,b] : p(x) > 0 + * 2) for all x \not\in [a,b] : p(x) = 0 + * 3) \int_a^b p(x) dx = 1 + * @param start_point The start point of the Markov Chain (see generate_samples) + * @param function The function to integrate + * @param pdf The pdf to sample + * @param num_samples The number of samples used to approximate the integral + * @returns The approximation of the integral according to 1/N \sum_{i}^N f(x_i) / p(x_i) + */ +double integral_monte_carlo(const double& start_point, const Function& function, const Function& pdf, const uint32_t& num_samples = 1000000) { + double integral = 0.0; + std::vector samples = generate_samples(start_point, pdf, num_samples); + + for(double sample : samples) { + integral += function(sample) / pdf(sample); + } + + return integral / static_cast(samples.size()); +} + +} // namespace monte_carlo +} // namespace math + +/** + * @brief Self-test implementations + * @returns void + */ +static void test() { + std::cout << "Disclaimer: Because this is a randomized algorithm," << std::endl; + std::cout << "it may happen that singular samples deviate from the true result." << std::endl << std::endl;; + + math::monte_carlo::Function f; + math::monte_carlo::Function pdf; + double integral = 0; + double lower_bound = 0, upper_bound = 0; + + /* \int_{-2}^{2} -x^2 + 4 dx */ + f = [&](double& x) { + return -x*x + 4.0; + }; + + lower_bound = -2.0; + upper_bound = 2.0; + pdf = [&](double& x) { + if(x >= lower_bound && x <= -1.0) { + return 0.1; + } + if(x <= upper_bound && x >= 1.0) { + return 0.1; + } + if(x > -1.0 && x < 1.0) { + return 0.4; + } + return 0.0; + }; + + integral = math::monte_carlo::integral_monte_carlo((upper_bound - lower_bound) / 2.0, f, pdf); + + std::cout << "This number should be close to 10.666666: " << integral << std::endl; + + /* \int_{0}^{1} e^x dx */ + f = [&](double& x) { + return std::exp(x); + }; + + lower_bound = 0.0; + upper_bound = 1.0; + pdf = [&](double& x) { + if(x >= lower_bound && x <= 0.2) { + return 0.1; + } + if(x > 0.2 && x <= 0.4) { + return 0.4; + } + if(x > 0.4 && x < upper_bound) { + return 1.5; + } + return 0.0; + }; + + integral = math::monte_carlo::integral_monte_carlo((upper_bound - lower_bound) / 2.0, f, pdf); + + std::cout << "This number should be close to 1.7182818: " << integral << std::endl; + + /* \int_{-\infty}^{\infty} sinc(x) dx, sinc(x) = sin(pi * x) / (pi * x) + This is a difficult integral because of its infinite domain. + Therefore, it may deviate largely from the expected result. + */ + f = [&](double& x) { + return std::sin(M_PI * x) / (M_PI * x); + }; + + pdf = [&](double& x) { + return 1.0 / std::sqrt(2.0 * M_PI) * std::exp(-x * x / 2.0); + }; + + integral = math::monte_carlo::integral_monte_carlo(0.0, f, pdf, 10000000); + + std::cout << "This number should be close to 1.0: " << integral << std::endl; +} + +/** + * @brief Main function + * @returns 0 on exit + */ +int main() { + test(); // run self-test implementations + return 0; +}