View Compilation Error

0_0_28847428_18433.cpp: In function 'int main()':
0_0_28847428_18433.cpp:15:3: error: 'a1' was not declared in this scope
   a1="";
   ^
0_0_28847428_18433.cpp:16:3: error: 'a2' was not declared in this scope
   a2="";
   ^
0_0_28847428_18433.cpp:17:15: error: 'i' was not declared in this scope
   a1.insert(0,i,' ');
               ^
0_0_28847428_18433.cpp:18:6: error: expected primary-expression before '.' token
   a2=.insert(0,i,c);
      ^
0_0_28847428_18433.cpp:25:17: error: expected primary-expression before ';' token
   for(int i=0;i<;i++)
                 ^