java大数乘法的简单实现 浮点数乘法运算

import java.io.bufferedreader;
import java.io.ioexception;
import java.io.inputstreamreader;
import java.util.regex.matcher;
import java.util.regex.pattern;

/**
 * 大数乘法的简单实现, 目前还不是很完善
 * fix:
 * 1. 修改前后删除0的一些错误情况
 * 2. 支持负数运算
 * 3. 判断输入字符串是否符合小数定义, 用正则表达式判断
 * @author icejoywoo
 * @since 2012.2.16
 * @version 0.1.1
 */
public class bignumber {
    public static void main(string[] args) throws ioexception {
        system.out.println("input two large integers:");
        bufferedreader buffer = new bufferedreader(new inputstreamreader(system.in));
        string[] strarray = buffer.readline().split("\\*");
        system.out.println(bignumbermultiply(strarray[0], strarray[1]));
    }

    /**
     * 计算两个任意大小和精度的数的乘积
     * @param first 第一个参数
     * @param second 第二个参数
     * @return 两个数的乘积
     */
    private static string bignumbermultiply(string first, string second) {
        // 正负号判断标志
        boolean flag = false;

        if (first.charat(0) == '-') {
            flag = !flag;
            first = first.substring(1);
        }

        if (second.charat(0) == '-') {
            flag = !flag;
            second = second.substring(1);
        }

        // 小数点的位置
        int apoints = first.length() - first.indexof('.') - 1;
        int bpoints = second.length() - second.indexof('.') - 1;
        int pointpos = apoints + bpoints; // 结果的小数点位置

        // 删除小数点
        stringbuffer abuffer = new stringbuffer(first.replaceall("\\.", ""));
        stringbuffer bbuffer = new stringbuffer(second.replaceall("\\.", ""));

        int[] a = string2intarray(abuffer.tostring());
        int[] b = string2intarray(bbuffer.tostring());

        int[] result = new int[a.length + b.length - 1]; // 保存结果的数组

        // 计算
        for (int i = 0; i < a.length; i++) {
            for (int j = 0; j < b.length; j++) {
                result[i + j] += a[i] * b[j];
            }
        }

        // result中的某一位大于9的话需要进位
        for (int i = result.length - 1; i >= 0; --i) {
            if (result[i] > 9) {
                result[i - 1] += result[i] / 10;
                result[i] = result[i] % 10;
            }
        }

        stringbuffer buffer = new stringbuffer(); // 将result数组转换为字符串
        for (int i = 0; i < result.length; ++i) {
            // 添加小数点
            if(result.length - i == pointpos) {
                buffer.append(".");
            }
            buffer.append(string.valueof(result[i]));
        }

        if (buffer.indexof(".") != -1)
        {
            // 删除最开始的0
            int i = 0;
            while (i < buffer.length()) {
                if (buffer.length() > 2 && buffer.charat(i+1) == '.') { // 小数点前只有一个数 0.
                    break;
                } else if (buffer.charat(i) == '0') { // 删除最前边的0
                    buffer.deletecharat(i);
                    i = 0;
                    continue;
                } else { // 当第一位不是0的时候
                    break;
                }
            }

            // 删除末尾的0
            i = buffer.length() - 1;
            while (i >= 0) {
                if (buffer.length() > 2 && buffer.charat(i-1) == '.') { // 小数点后直接是数字
                    break;
                } else if (buffer.charat(i) == '0') { // 删除末尾的0
                    buffer.deletecharat(i);
                    i = buffer.length() - 1;
                    continue;
                } else { // 当最后一位不是0的时候
                    break;
                }
            }
        }

        // 根据符号位, 返回值的正负标志
        if (flag) {
            return "-" + buffer.tostring();
        } else {
            return buffer.tostring();
        }
    }

    /**
     * 将字符串装换为数组
     * @param number
     * @return
     */
    private static int[] string2intarray(string number) {
        // 判断输入是否符合浮点数的要求
        pattern pattern = pattern.compile("^(-?\\d+|\\d*)\\.?\\d*$");
        matcher matcher = pattern.matcher(number);
        if (!matcher.find()) {
            throw new illegalargumentexception("输入的数不正确!");
        }

        int[] result = new int[number.length()];
        for (int i = 0; i < number.length(); i++) {
            result[i] = (int) (number.charat(i) - '0');
        }
        return result;
    }
}

运行结果如下:

1. 错误输入的判断

input two large integers:
1a*a22
exception in thread "main" java.lang.illegalargumentexception: 输入的数不正确!
at bignumber.string2intarray(bignumber.java:132)
at bignumber.bignumbermultiply(bignumber.java:54)
at bignumber.main(bignumber.java:22)

 2. 带负数的运算, 前后带有0的情况

input two large integers:
-23424.2300*02345.23400000
-54935300.61982

 python中计算的结果如下

python 2.6.5
>>> -23424.2300*02345.23400000
-54935300.619819999

 可以看出python的结果是有失真的