Day 5 Normal Distribution I.java

import java.io.*;
import java.math.*;
import java.security.*;
import java.text.*;
import java.util.*;
import java.util.concurrent.*;
import java.util.function.*;
import java.util.regex.*;
import java.util.stream.*;
import static java.util.stream.Collectors.joining;
import static java.util.stream.Collectors.toList;

public class Solution {
    public static void main(String[] args) {
        double mean = 20;
        double std  = 2;
        System.out.format("%.3f%n", cumulative(mean, std, 19.5));
        System.out.format("%.3f%n", cumulative(mean, std, 22) - cumulative(mean, std, 20));
    }
    
    /* Calculates cumulative probability */
    public static double cumulative(double mean, double std, double x) {
        double parameter = (x - mean) / (std * Math.sqrt(2));
        return (0.5) * (1 + erf(parameter));
    }
        
    public static double erf(double z) {
        double t = 1.0 / (1.0 + 0.5 * Math.abs(z));

        // use Horner's method
        double ans = 1 - t * Math.exp( -z*z   -   1.26551223 +
                                            t * ( 1.00002368 +
                                            t * ( 0.37409196 + 
                                            t * ( 0.09678418 + 
                                            t * (-0.18628806 + 
                                            t * ( 0.27886807 + 
                                            t * (-1.13520398 + 
                                            t * ( 1.48851587 + 
                                            t * (-0.82215223 + 
                                            t * ( 0.17087277))))))))));
        if (z >= 0) return  ans;
        else        return -ans;
    }
}