声明：本算法可以实现整数乘以整数，小数乘以小数功能。

但是小数只能是小数点前不为0 的小数。

比如0.1之类的不适用。

#include<iostream>
#include<string>
using namespace std;
void multiply(const char*a,const char*b)
{
int length1=strlen(a);
int length2=strlen(b);
int*p=new int[length1+length2];
for(int i=0;i<length1+length2;i++)
{
p[i]=0;
}
for(int i=0;i<length1;i++)
{
for(int j=0;j<length2;j++)
{
p[i+j+1]+=(a[i]-'0')*(b[j]-'0');
}
}
for(int i=length1+length2-1;i>=0;i--)
{
if(p[i]>=10)
{
p[i-1]+=p[i]/10;
p[i]=p[i]%10;
}
}
char*pp=new char[length1+length2+1];
int count=0;
while(p[count]==0)
{
count++;
}
int i1;
for(i1=0;count<length1+length2;i1++,count++)
{
pp[i1]=(p[count]+'0');
}
pp[i1]='\0';
cout<<pp<<endl;
delete[]p;
delete[]pp;
}
void dianmultiply(const char*a,const char*b)
{
int place1=0;
int place2=0;
char*newp1=new char[strlen(a)];
char*newp2=new char[strlen(b)];
int k1=0;
int k2=0;
for(int i=0;i<strlen(a);i++)
{
if(a[i]!='.')
{
newp1[k1]=a[i];
k1++;
}
else
{
place1=i;
}
}
newp1[k1]='\0';
/*cout << newp1 << endl;*/
for(int i=0;i<strlen(b);i++)
{
if(b[i]!='.')
{
newp2[k2]=b[i];
k2++;
}
else
{
place2=i;
}
}
newp2[k2]='\0';
/*cout << newp2 << endl;*/
int length1=strlen(newp1);
int length2=strlen(newp2);
//cout << length1 << " " << length2 << endl;
//cout << place1 << " " << place2 << endl;
int*p=new int[length1+length2];
for(int i=0;i<length1+length2;i++)
{
p[i]=0;
}
for(int i=0;i<length1;i++)
{
for(int j=0;j<length2;j++)
{
p[i+j+1]+=(newp1[i]-'0')*(newp2[j]-'0');
}
}
for(int i=length1+length2-1;i>=0;i--)
{
if(p[i]>=10)
{
p[i-1]+=p[i]/10;
p[i]=p[i]%10;
}
}
//cout << p << endl;
char*pp=new char[length1+length2+2];
int count=0;
while(p[count]==0)
{
count++;
}
int weizhi=0;
if(place1!=0)
{
place1=strlen(a)-place1-1;
weizhi+=place1;
}
if(place2!=0)
{
place2=strlen(b)-place2-1;
weizhi+=place2;
}
int i1;
/*cout << count << endl;*/
int length=length1+length2-count;
int flag=0;
int tem=count;
for(i1=0;count<length1+length2;i1++,count++) {
if(tem==0)
{
if(count==length-weizhi&&flag==0)
{
pp[i1]='.';
count--;
flag++;
}
else
{
pp[i1]=(p[count]+'0');
}
}
else
{
if(count==length-weizhi+1&&flag==0)
{
pp[i1]='.';
count--;
flag++;
}
else
{
pp[i1]=(p[count]+'0');
}
}
}
pp[i1]='\0';
cout<<pp<<endl;
delete[]p;
delete[]pp;
}
bool judge(const char*p,const char*q)
{
for(int i=0;i<strlen(p);i++)
{
if(p[i]=='.')
{
return true;
}
}
for(int i=0;i<strlen(q);i++)
{
if(q[i]=='.')
{
return true;
}
}
return false;
}
int main()
{
string dashu1;
string dashu2;
cin>>dashu1;
cin>>dashu2;
const char*p1=dashu1.c_str();
const char*p2=dashu2.c_str();
cout<<p1<<"*"<<p2<<"=";
if(judge(p1,p2))
{
dianmultiply(p1,p2);
}
else
{
multiply(p1,p2);
}
system("pause");
return0;
}。