From bcb6fa0a41025870eccc203f5713ab9bd44d78cc Mon Sep 17 00:00:00 2001
From: ggkogkou <ggkogkou@ggkogkou.gr>
Date: Wed, 20 Oct 2021 15:58:56 +0300
Subject: [PATCH] Corrections

---
 .../midpoint_integral_method.cpp              | 154 ++++++++++++++++++
 .../midpoint_integral_method.hpp              |   3 -
 2 files changed, 154 insertions(+), 3 deletions(-)
 create mode 100644 numerical_methods/midpoint_integral_method.cpp
 delete mode 100644 numerical_methods/midpoint_integral_method.hpp

diff --git a/numerical_methods/midpoint_integral_method.cpp b/numerical_methods/midpoint_integral_method.cpp
new file mode 100644
index 000000000..568444181
--- /dev/null
+++ b/numerical_methods/midpoint_integral_method.cpp
@@ -0,0 +1,154 @@
+#include <iostream>
+#include <cmath>
+#include <cassert>
+#include <cstdlib>
+#include <functional>
+#include <map>
+
+/*!
+ * @title Calculate definite integrals with midpoint method
+ * @see https://en.wikipedia.org/wiki/Midpoint_method
+ * @brief A numerical method for easy approximation of integrals
+ * @details The idea is to split the interval into N of intervals and use as interpolation points the xi
+ * for which it applies that xi = x0 + i*h, where h is a step defined as h = (b-a)/N where a and b are the
+ * first and last points of the interval of the integration [a, b].
+ *
+ * We create a table of the xi and their corresponding f(xi) values and we evaluate the integral by the formula:
+ * I = h * {f(x0+h/2) + f(x1+h/2) + ... + f(xN-1+h/2)}
+ *
+ * In this program there are 4 sample test functions f, g, k, l that are evaluated in the same interval [1, 3.
+ *
+ * Arguments can be passed as parameters from the command line argv[1] = N, argv[2] = a, argv[3] = b.
+ * In this case if the default values N=16, a=1, b=3 are changed then the tests/assert are disabled.
+ *
+ * In the end of the main() and if and only if N, a, b are on their default values,
+ * i compare the program's result with the one from mathematical software with 2 decimal points margin.
+ *
+ * Add your own sample function by replacing one of the f, g, k, l and the corresponding assert
+ *
+ * @author ggkogkou
+*/
+
+/**
+ * @namespace midpoint_rule
+ * @brief Contains the function of the midpoint method implementation
+*/
+namespace midpoint_rule{
+    /*!
+     * @fn double midpoint(const int N, const double h, const double a, const std::function<double (double)>& func)
+     * @brief Implement midpoint method
+     * @param N number of intervals
+     * @param h step
+     * @param a x0
+     * @param func The function that will be evaluated
+     * @returns the result of the integration
+    */
+    double midpoint(const int N, const double h, const double a, const std::function<double (double)>& func){
+        std::map<int, double> data_table; /// Contains the data points, key: i, value: f(xi)
+        double xi = a; /// Initialize xi to the starting point x0 = a
+
+        // Create the data table
+        /// Loop from x0 to xN-1
+        double temp;
+        for(int i=0; i<N; i++){
+            temp = func(xi + h/2); /// find f(xi+h/2)
+            data_table.insert(std::pair<int ,double>(i, temp)); /// add i and f(xi)
+            xi += h; /// Get the next point xi for the next iteration
+        }
+
+        /// Evaluate the integral.
+        // Remember: {f(x0+h/2) + f(x1+h/2) + ... + f(xN-1+h/2)}
+        double evaluate_integral = 0;
+        for(int i=0; i<N; i++) evaluate_integral += data_table.at(i);
+
+        /// Multiply by the coefficient h
+        evaluate_integral *= h;
+
+        /// If the result calculated is nan, then the user has given wrong input interval.
+        assert(!std::isnan(evaluate_integral) && "The definite integral can't be evaluated. Check the validity of your input.\n");
+        // Else return
+        return evaluate_integral;
+    }
+
+} // midpoint_rule ends here
+
+/**
+ * @fn double f(double x)
+ * @brief A function f(x) that will be used to test the method
+ * @param x The independent variable xi
+ * @returns the value of the dependent variable yi = f(xi)
+*/
+double f(double x);
+/**
+ * @brief Another test function
+*/
+double g(double x);
+/**
+ * @brief Another test function
+*/
+double k(double x);
+/**
+ * @brief Another test function
+*/
+double l(double x);
+
+int main(int argc, char** argv){
+    int N = 16; /// Number of intervals to divide the integration interval. MUST BE EVEN
+    double a = 1, b = 3; /// Starting and ending point of the integration in the real axis
+    double h; /// Step, calculated by a, b and N
+
+    bool used_argv_parameters = false; // If argv parameters are used then the assert must be omitted for the tst cases
+
+    // Get user input (by the command line parameters or the console after displaying messages)
+    if(argc == 4){
+        N = std::atoi(argv[1]);
+        a = (double) std::atof(argv[2]);
+        b = (double) std::atof(argv[3]);
+        // Check if a<b else abort
+        assert(a < b && "a has to be less than b");
+        assert(N > 0 && "N has to be > 0");
+        if(N<4 || a!=1 || b!=3) used_argv_parameters = true;
+        std::cout << "You selected N=" << N << ", a=" << a << ", b=" << b << std::endl;
+    }
+    else
+        std::cout << "Default N=" << N << ", a=" << a << ", b=" << b << std::endl;
+
+    // Find the step
+    h = (b-a)/N;
+
+    // Call midpoint() for each of the test functions f, g, k, l
+    // Assert with two decimal point precision
+    double result_f = midpoint_rule::midpoint(N, h, a, f);
+    assert((used_argv_parameters || (result_f >= 4.09 && result_f <= 4.10)) && "The result of f(x) is wrong");
+    std::cout << "The result of integral f(x) on interval [" << a << ", " << b << "] is equal to: " << result_f << std::endl;
+
+    double result_g = midpoint_rule::midpoint(N, h, a, g);
+    assert((used_argv_parameters || (result_g >= 0.27 && result_g <= 0.28)) && "The result of g(x) is wrong");
+    std::cout << "The result of integral g(x) on interval [" << a << ", " << b << "] is equal to: " << result_g << std::endl;
+
+    double result_k = midpoint_rule::midpoint(N, h, a, k);
+    assert((used_argv_parameters || (result_k >= 9.06 && result_k <= 9.07)) && "The result of k(x) is wrong");
+    std::cout << "The result of integral k(x) on interval [" << a << ", " << b << "] is equal to: " << result_k << std::endl;
+
+    double result_l = midpoint_rule::midpoint(N, h, a, l);
+    assert((used_argv_parameters || (result_l >= 7.16 && result_l <= 7.17)) && "The result of l(x) is wrong");
+    std::cout << "The result of integral l(x) on interval [" << a << ", " << b << "] is equal to: " << result_l << std::endl;
+
+    return 0;
+}
+
+double f(double x){
+    return std::sqrt(x) + std::log(x);
+}
+
+double g(double x){
+    return std::exp(-x) * (4 - std::pow(x, 2));
+}
+
+double k(double x){
+    return std::sqrt(2*std::pow(x, 3)+3);
+}
+
+double l(double x){
+    return x + std::log(2*x+1);
+}
diff --git a/numerical_methods/midpoint_integral_method.hpp b/numerical_methods/midpoint_integral_method.hpp
deleted file mode 100644
index a5800d2d5..000000000
--- a/numerical_methods/midpoint_integral_method.hpp
+++ /dev/null
@@ -1,3 +0,0 @@
-#pragma once
-
-class
\ No newline at end of file