字符串匹配

//字符串的查找
//BF算法
#include<iostream>
#include<string>
using namespace std;

int index(const string& Tag, const string& Ptn, int pos)
{
int i = pos;
int j = 0;
int tlen = Tag.size();
int plen = Ptn.size();
while (i<tlen&&j<plen)
{
if (Ptn[j] == Tag[i])
{
i++;
j++;
}
else
{
i = i - j + 1;//如果不匹配，i回溯
j = 0;//j归0；
}


}
if (j >= plen)
return i - plen;
else
return -1;
}
//测试程序：
int main()
{
char ch;
do 
{
string Tag;
string ptn;
int pos;
cout << "输入主串" << endl;
cin >> Tag;
cout <<"输入子串" << endl;
cin >> ptn;
cout << "输入主串开始的位置（首字符位置为0）" << endl;
cin >> pos;

int result = index(Tag, ptn, pos);
if (result != -1)
cout << "子串在主串的第" << result+1 << "个字符" << endl;
else
cout << " 无匹配字符串" << endl;


cout << " 是否继续测试" << ":Y or y，任意键结束" << endl;
cin >> ch;
} while (ch=='y'||ch=='Y');
return 0;
}