这是简单加密的一部份，请问出什么问题了？

为什么总提示：vector subscript out of range?
代码如下：
#include<iostream>
#include<sstream>
#include<vector>
#include<fstream>

using namespace std;

typedef vector <vector <string>> mat;
mat input();
void jiami(mat& a);
void print(const mat& a);

int main()
{
   mat a=input();
   jiami(a);
   print(a);
}
mat input()
    {
        mat a;
        for(string s;getline(cin,s);)
        {
            vector<string> b;
            istringstream sin(s);
            for(string ia;sin>>ia;)
                b.push_back(ia);
             a.push_back(b);
        }
        return a;
    }
void jiami(mat& a)
{
    
    for(int i=0;i<a.size();++i)
        for(int j=0;j<7;++j)
        {
            switch(j)
            {
            case 0 :a[i][j]+=4;
                break;
            case 1 :a[i][j]+=9;
                break;
            case 2 :a[i][j]+=6;
                break;
            case 3 :a[i][j]+=2;
                break;
            case 4 :a[i][j]+=8;
                break;
            case 5 :a[i][j]+=7;
                break;
            case 6 :a[i][j]+=3;
                break;
            default:cout<<"error"<<endl;

            }
        }


}
void print(const mat& a)
{
    for(int i=0;i<a.size();++i)
    {
        for(int j=0;j<a[i].size();++j)
            cout<<a[i][j]<<" ";
        cout<<endl;
    }
}