输入一串字符，统计ff,fl,fi出现的次数，为什么输入奇数个f就会出错啊？

 #include <iostream>
#include <vector>
#include <string>
using namespace std;


int main()
{
    int a=0,b=0,c=0;
    string aa;
    char bb=' ';
    vector<char> ss;

    cin>>aa;

    for(string::size_type i = 0; i != aa.size(); ++i)
    {
        ss.push_back(aa.at(i));
    }
    for(vector<char>::iterator i=ss.begin();i!=ss.end();++i)
    {
        if(bb=='f')
        {
            switch(*i)
            {
            case 'f':
                ++a;
                break;
            case 'l':
                ++b;
                break;
            case 'i':
                ++c;
                break;
            }
            ++i;
        }
        bb=*i;
    }
    cout<<"the number of ff is:"<<a<<endl;
    cout<<"the number of fl is:"<<b<<endl;
    cout<<"the number of fi is:"<<c<<endl;
    system("pause");
    return 0;
}