和歌山サイエンスカフェ「インフィニティ」

public class LagrangeInterpolation {

	public static void main(String... args) {

		int n_points = 4;  

	

		double p = 20.; // x-value for f(p)

		double ans=0.;



		double L_bunshi1;

		double L_bunbo1;

		double L_bunshi2;

		double L_bunbo2;



		double lj;

		

		double[] x = new double[n_points];

		double[] f = new double[n_points];

		//double[] l = new double[n_points];

		

		x[0]=19.9466;

		f[0]=2.712;

		x[1]=19.9907;

		f[1]=2.714;

		x[2]=20.0349;

		f[2]=2.716;

		x[3]=20.0793;

		f[3]=2.718;

		

		for (int j = 0; j < n_points; j++) {

			L_bunshi1=1.;

			L_bunbo1=1.;

			L_bunshi2=1.;

			L_bunbo2=1.;

		

			for (int jj = 0; jj <= j - 1; jj++) {

				L_bunshi1=L_bunshi1*(p-x[jj]);

				L_bunbo1=L_bunbo1*(x[j]-x[jj]);

			}

			for (int jj = j+1; jj < n_points; jj++) {

				L_bunshi2=L_bunshi2*(p-x[jj]);

				L_bunbo2=L_bunbo2*(x[j]-x[jj]);

			}

			lj=(L_bunshi1*L_bunshi2)/(L_bunbo1*L_bunbo2);

			ans=ans+f[j]*lj;

		}

		System.out.printf("x = %8.6f, f(x) = %11.9f" , p,ans);  //True f(20) = 2.7144176

	}

}