June 2018 Update

Task/Polynomial-regression/Java/polynomial-regression.java

@@ -0,0 +1,50 @@
+import java.util.Arrays;
+import java.util.function.IntToDoubleFunction;
+import java.util.stream.IntStream;
+
+public class PolynomialRegression {
+    private static void polyRegression(int[] x, int[] y) {
+        int n = x.length;
+        int[] r = IntStream.range(0, n).toArray();
+        double xm = Arrays.stream(x).average().orElse(Double.NaN);
+        double ym = Arrays.stream(y).average().orElse(Double.NaN);
+        double x2m = Arrays.stream(r).map(a -> a * a).average().orElse(Double.NaN);
+        double x3m = Arrays.stream(r).map(a -> a * a * a).average().orElse(Double.NaN);
+        double x4m = Arrays.stream(r).map(a -> a * a * a * a).average().orElse(Double.NaN);
+        double xym = 0.0;
+        for (int i = 0; i < x.length && i < y.length; ++i) {
+            xym += x[i] * y[i];
+        }
+        xym /= Math.min(x.length, y.length);
+        double x2ym = 0.0;
+        for (int i = 0; i < x.length && i < y.length; ++i) {
+            x2ym += x[i] * x[i] * y[i];
+        }
+        x2ym /= Math.min(x.length, y.length);
+
+        double sxx = x2m - xm * xm;
+        double sxy = xym - xm * ym;
+        double sxx2 = x3m - xm * x2m;
+        double sx2x2 = x4m - x2m * x2m;
+        double sx2y = x2ym - x2m * ym;
+
+        double b = (sxy * sx2x2 - sx2y * sxx2) / (sxx * sx2x2 - sxx2 * sxx2);
+        double c = (sx2y * sxx - sxy * sxx2) / (sxx * sx2x2 - sxx2 * sxx2);
+        double a = ym - b * xm - c * x2m;
+
+        IntToDoubleFunction abc = (int xx) -> a + b * xx + c * xx * xx;
+
+        System.out.println("y = " + a + " + " + b + "x + " + c + "x^2");
+        System.out.println(" Input  Approximation");
+        System.out.println(" x   y     y1");
+        for (int i = 0; i < n; ++i) {
+            System.out.printf("%2d %3d  %5.1f\n", x[i], y[i], abc.applyAsDouble(x[i]));
+        }
+    }
+
+    public static void main(String[] args) {
+        int[] x = IntStream.range(0, 11).toArray();
+        int[] y = new int[]{1, 6, 17, 34, 57, 86, 121, 162, 209, 262, 321};
+        polyRegression(x, y);
+    }
+}