大数乘法的实现

/******************************************************************************
*Author: steven
*Datatime: 2013-05-23
*此程序用来演示大数乘法的算法
*******************************************************************************/

#include <iostream>
using std::cin;
using std::cout;
using std::endl;

#include <string>
using std::string;

void calculate(void);

int main()
{
 calculate();
 return 0;
}

void calculate(void)
{
 string str1,str2;

 cout<<"请输入两个大的正整数:\n";
 cin>>str1>>str2;

 cout<<str1<<" * "<<str2<<" = ";

 int i,j,length,length1,length2;
 length1 = str1.size();
 length2 = str2.size();
 length = length1 + length2;
 int *pch = new int[length];

 for (i = 0; i < length; i++)
 {
  pch[i] = 0;
 }

 for (i = 0; i < length1; i++)
 {
  for (j = 0; j < length2; j++)
  {
   pch[i+j+1] += (str1[i] - '0') * (str2[j] - '0');
  }
 }

 //下面代码实现进位操作
 for (i = length; i > 0; i--)
 {
  if (pch[i] >= 10)
  {
   pch[i-1] += pch[i] / 10;
   pch[i] = pch[i] % 10;
  }
 }

 //跳过前导的零值
 while(pch[++i] == 0);

 for (; i < length; i++)  {   cout<<pch[i];  }  cout<<endl; }