JAVA 處理 大數 POJ1001

由於種種原因最近一段時間一直情緒不佳，寫幾個水題娛樂一下。

話說去年寒假學了點JAVA，寫了些推箱子之類的東東，但還從來沒有用JAVA A過題，很早就聽說JAVA直接用大數類做大數和高精度手段很是淫蕩，今天就來水幾把尋點樂子。

POJ1001

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

代碼：

import java.util.*;
import java.math.*;
import java.io.*;

public class poj1001 {
	public static void main(String args[])
	{
		Scanner cin = new Scanner(System.in);
		while(cin.hasNext())
		{
			String a = cin.next();
			int t = cin.nextInt();
			BigDecimal ans = new BigDecimal(a);
			ans = ans.pow(t);
			String result = ans.stripTrailingZeros().toPlainString();
			if(result.charAt(0)=='0') result=result.substring(1);
			System.out.println(result);
		}
	}
}

math包裏有兩個處理大數的類：BigInteger 和 BigDecimal,顧名思義一個是用來處理整數的一個是用來處理有理數數的；

add(),subtract(),pow(),abs()之類的常用運算方法都有，直接拿來用就行了。

BigDecimal關於格式控制的方法多了幾個，這對處理各種不同格式的輸出是很有用的。

stripTraillingZeros():把不影響數值大小的0全去掉；

1.50 ->1.5；

1.00->1;

這功能很有用吧。

大家都知道JAVA的類一般都要帶toString這個方法的，BigDecimal則有toString,toPlainString和toEngineeringString三種表示成字符串的方法，

下面是這三種方法各自的特點：

toString: using scientific notation if an exponent is needed;

toEngineeringString:using engineering notation if an exponent is needed.

toPlainString:without an exponent field.