From c396b98fd7ae3e5aa4724a620fe71141b759664a Mon Sep 17 00:00:00 2001
From: ggkogkou <76820848+ggkogkou@users.noreply.github.com>
Date: Sun, 24 Oct 2021 10:24:30 +0300
Subject: [PATCH] Add Simpson's integration numerical method (#2681)

Co-authored-by: ggkogkou <ggkogkou@ggkogkou.gr>
---
 Maths/SimpsonIntegration.java | 92 +++++++++++++++++++++++++++++++++++
 1 file changed, 92 insertions(+)
 create mode 100644 Maths/SimpsonIntegration.java

diff --git a/Maths/SimpsonIntegration.java b/Maths/SimpsonIntegration.java
new file mode 100644
index 00000000..87a7e4d2
--- /dev/null
+++ b/Maths/SimpsonIntegration.java
@@ -0,0 +1,92 @@
+package Maths;
+
+import java.util.TreeMap;
+
+public class SimpsonIntegration{
+
+    /*
+     * Calculate definite integrals by using Composite Simpson's rule.
+     * Wiki: https://en.wikipedia.org/wiki/Simpson%27s_rule#Composite_Simpson's_rule
+     * Given f a function and an even number N of intervals that divide the integration interval e.g. [a, b],
+     * we calculate the step h = (b-a)/N and create a table that contains all the x points of
+     * the real axis xi = x0 + i*h and the value f(xi) that corresponds to these xi.
+     *
+     * To evaluate the integral i use the formula below:
+     * I = h/3 * {f(x0) + 4*f(x1) + 2*f(x2) + 4*f(x3) + ... + 2*f(xN-2) + 4*f(xN-1) + f(xN)}
+     *
+     */
+
+    public static void main(String[] args) {
+        SimpsonIntegration integration = new SimpsonIntegration();
+
+        // Give random data for the example purposes
+        int N = 16;
+        double a = 1;
+        double b = 3;
+
+        // Check so that N is even
+        if(N%2 != 0){
+            System.out.println("N must be even number for Simpsons method. Aborted");
+            System.exit(1);
+        }
+
+        // Calculate step h and evaluate the integral
+        double h = (b-a) / (double) N;
+        double integralEvaluation = integration.simpsonsMethod(N, h, a);
+        System.out.println("The integral is equal to: " + integralEvaluation);
+    }
+
+    /*
+     * @param N: Number of intervals (must be even number N=2*k)
+     * @param h: Step h = (b-a)/N
+     * @param a: Starting point of the interval
+     * @param b: Ending point of the interval
+     *
+     * The interpolation points xi = x0 + i*h are stored the treeMap data
+     *
+     * @return result of the integral evaluation
+     */
+    public double simpsonsMethod(int N, double h, double a){
+        TreeMap<Integer, Double> data = new TreeMap<>(); // Key: i, Value: f(xi)
+        double temp;
+        double xi = a; // Initialize the variable xi = x0 + 0*h
+
+        // Create the table of xi and yi points
+        for(int i=0; i<=N; i++){
+            temp = f(xi); // Get the value of the function at that point
+            data.put(i, temp);
+            xi += h; // Increase the xi to the next point
+        }
+
+        // Apply the formula
+        double integralEvaluation = 0;
+        for(int i=0; i<data.size(); i++){
+            if(i == 0 || i == data.size()-1) {
+                integralEvaluation += data.get(i);
+                System.out.println("Multiply f(x" + i + ") by 1");
+            }
+            else if(i%2 == 1) {
+                integralEvaluation += (double) 4 * data.get(i);
+                System.out.println("Multiply f(x" + i + ") by 4");
+            }
+            else {
+                integralEvaluation += (double) 2 * data.get(i);
+                System.out.println("Multiply f(x" + i + ") by 2");
+            }
+        }
+
+        // Multiply by h/3
+        integralEvaluation = h/3 * integralEvaluation;
+
+        // Return the result
+        return integralEvaluation;
+    }
+
+    // Sample function f
+    // Function f(x) = e^(-x) * (4 - x^2)
+    public double f(double x){
+        return Math.exp(-x) * (4 - Math.pow(x, 2));
+//        return Math.sqrt(x);
+    }
+
+}