1001 Exponentiation

在学校做了杭电的几道oj题，本想着回家后再继续做下去的，可惜了，不知道出什么问题了，提交后看到是在排队，整个页面的人都是在排队，而在我提交的记录里却找不到我提交的记录，烦！所以我转战poj了，一上来第二道题 1001 的不会做，尴尬，哈哈哈

百度了才知道，用java做，很简单的，简称水题，学不到什么，但是对于我来说，能多认识几个方法还是不错的。

Description

Problems involving the computation of exact values of very large magnitude and precision are common. For example, the computation of the national debt is a taxing experience for many computer systems. 

This problem requires that you write a program to compute the exact value of Rⁿ where R is a real number ( 0.0 < R < 99.999 ) and n is an integer such that 0 < n <= 25.

Input

The input will consist of a set of pairs of values for R and n. The R value will occupy columns 1 through 6, and the n value will be in columns 8 and 9.

Output

The output will consist of one line for each line of input giving the exact value of R^n. Leading zeros should be suppressed in the output. Insignificant trailing zeros must not be printed. Don't print the decimal point if the result is an integer.

Sample Input

95.123 12
0.4321 20
5.1234 15
6.7592  9
98.999 10
1.0100 12

Sample Output

548815620517731830194541.899025343415715973535967221869852721
.00000005148554641076956121994511276767154838481760200726351203835429763013462401
43992025569.928573701266488041146654993318703707511666295476720493953024
29448126.764121021618164430206909037173276672
90429072743629540498.107596019456651774561044010001
1.126825030131969720661201

代码如下:

import java.math.BigDecimal;
import java.util.*;
public class Main{	public static void main(String[] args)	{	
	Scanner scan = new Scanner(System.in);	
	while (scan.hasNext()){		
		BigDecimal a = scan.nextBigDecimal();	
		int b = scan.nextInt();		
		String ans = a.pow(b).stripTrailingZeros().toPlainString();	//stripTrailingZeros去掉尾部的零，                                  //   toPlantString 将科学计数法的数平铺开来	
		if (ans.startsWith("0"))              //判断首个字符
			ans = ans.substring(1);		//返回1到末尾的字符
		System.out.println(ans);		}	
	scan.close();	
	}
}

而使用BigDecimal的这个包时：

1.它是商业的计算，为了追求精度。

2.使用时，尽量使用参数类型String的构造函数，结果可预知

3.BigDecimal都是不可变（immutable）的，在进行每一步运算的时候，都会产生一个新的对象，所以在做加减法的时候要保存操作的对象。