在c++中的字符串类数组中不打印任何内容，即使我可以访问单个元素

即使我可以在str1[I].a中打印字符串str2[I].a在按索引值打印字符串时，我正在获取字符串中的各个字符我还可以打印str1[I].一个我以前输入的字符串谢谢

     #include <iostream>
     #include <vector>
     #include <string>
     using namespace std;
        class st
     {
        public:
           string a;
           int col;
     }str1[200],str2[200];

     int main()
    {
        int i=0,j=0,num,l=0,len=0;
        string str;
        for(i=0;;i++)
       {
           cin>>num;
           str1[i].col=num;
           if(str1[i].col==0)
                break;
           cin>>str1[i].a;
           cout<<str1[i].a; // I get string entered before                    
      }

      for(i=0;;i++)
      {   
             if(str1[i].col==0)
                  break;
             len = str1[i].a.size()-1;
             //cout << len << endl;
             l=0,j=0;
             for(;;)
            {
                 str2[i].a[l]=str1[i].a[j];
                 l++;
                 j=((j+str1[i].col)%len);
                 if(j==0)
                   {
                      str2[i].a[l]=str1[i].a[len];
                      cout << str2[i].a[l];//I get the char assigned above
                      l++;
                      str2[i].a[l]='n';
                      break;
                   }
             }

          cout << str2[i].a; // I get nothing after I print
          cout << str2[i].a[0];// I get the required character of string
          cout << str2[i].a[5];//I get the required character of string
          cout << str2[i].a[8];//I get the required character of string
         }

       return 0;
    }