import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.math.BigDecimal;

public class Test01 {

    public static void main(String[] args) {

        try {

            int precision, i;
            BufferedReader str = new BufferedReader(new InputStreamReader(
                    System.in));
            System.out.println("输入整数：");
            precision = Integer.parseInt(str.readLine());
            str.close();

            System.out.println("the e is :" + getE(precision+1).setScale(precision, BigDecimal.ROUND_FLOOR).toPlainString());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    /**
     * exp(1) = 1 + 1/1! + 1/2! + ... + 1/n!
     * @param precision
     * @return
     */
    public static BigDecimal getE(int precision) {
        BigDecimal one = BigDecimal.ONE;
        BigDecimal value = BigDecimal.ONE;
        BigDecimal n = BigDecimal.ONE;
        BigDecimal factorial = BigDecimal.ONE;
        BigDecimal lastValue = BigDecimal.ZERO;
        
        while (true){
            value = value.add(one.divide(factorial, precision, BigDecimal.ROUND_HALF_UP));
            if (value.compareTo(lastValue) == 0) {
                break;
            } else if (value.compareTo(lastValue) < 0) {
                System.out.println("???");
                break;
            }
            n = n.add(one);
            factorial = factorial.multiply(n);
            lastValue = value;
        }
        
        return value;
    }

    
}

/**
 * 
 */
package houlei.test;

import java.math.BigDecimal;

/**
 * 
 *
 * 该类创建于 2008-9-22 下午01:31:44
 * @version 1.0.0
 * @author 侯磊
 */
public class H {

    public static void main(String[] args) {
        System.out.println(exp(20).toString());
    }
    
    
    /**
     * 该方法返回自然对数e的精确值。参数即为精确度（小数点后几位）,
     * 速度比较慢。
     * @param scale 精确度
     * @return 返回自然对数e的精确值
     */
    public static BigDecimal exp(int scale){
        final BigDecimal a = new BigDecimal(1);
        BigDecimal t = new BigDecimal(1);
        BigDecimal sum = new BigDecimal(1);
        
        for(int i=1;t.toString().length()<scale;i++){
             t = factorial(i);
             BigDecimal sub = a.divide(t,scale, BigDecimal.ROUND_HALF_UP);
            sum=sum.add(sub);
        }
        return sum;
    }
    
    private static BigDecimal factorial(int n){
        if(n<1)throw new IllegalArgumentException("参数必须为正整数");
        BigDecimal tmp = new BigDecimal(1);
        for(int i=2;i<=n;i++){
            tmp=tmp.multiply(new BigDecimal(i));
        }
        return tmp;
    }


}