Poj1131-Octal Fractions

Octal Fractions
Time Limit: 1000MS
 
Memory Limit: 10000K
Total Submissions: 6669
 
Accepted: 3641
Description
Fractions in octal (base 8) notation can be expressed exactly in decimal notation. For example, 0.75 in octal is 0.953125 (7/8 + 5/64) in decimal. All octal numbers of n digits to the right of the octal point can be expressed in no more than 3n decimal digits to the right of the decimal point. 
Write a program to convert octal numerals between 0 and 1, inclusive, into equivalent decimal numerals.
Input
The input to your program will consist of octal numbers, one per line, to be converted. Each input number has the form 0.d1d2d3 ... dk, where the di are octal digits (0..7). There is no limit on k.
Output
Your output will consist of a sequence of lines of the form 
0.d1d2d3 ... dk [8] = 0.D1D2D3 ... Dm [10]
where the left side is the input (in octal), and the right hand side the decimal (base 10) equivalent. There must be no trailing zeros, i.e. Dm is not equal to 0.
Sample Input

0.75
0.0001
0.01234567

Sample Output

0.75 [8] = 0.953125 [10]
0.0001 [8] = 0.000244140625 [10]
0.01234567 [8] = 0.020408093929290771484375 [10]

 
Javaの大きい数を使って、感じはとても良いです

import java.util.*;
import java.io.*;
import java.math.*;
import java.text.*;
public class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(new BufferedInputStream(System.in));   /// BufferedInputStream     
        BigDecimal ans, t, tmp;
        while(sc.hasNext()){
            String st = sc.nextLine();     ///     
            t = BigDecimal.valueOf(1);
            ans = BigDecimal.valueOf(0);
            
            int i, sta = st.indexOf('.');//       '.'     
            for(i = sta+1; i < st.length(); ++i){              ///          length（）  
                tmp = BigDecimal.valueOf(st.charAt(i) - '0');  //charAt（）         
                t = t.divide(new BigDecimal("8"));           //           ，Int   Int String   String
                tmp = tmp.multiply(t);// 
                ans = ans.add(tmp);// 
            }
        System.out.println(st + " [8] = " + ans +  " [10]"); ///   System.out.printf();
        }
    }
}