View Compilation Error

0_0_38274572_11538.cpp: In function 'int main()':
0_0_38274572_11538.cpp:12:2: error: 'a' was not declared in this scope
  a[1] = 1; a[2] =2;
  ^
0_0_38274572_11538.cpp:16:23: error: 'n' was not declared in this scope
   for (int i = 3; i <=n; i++)
                       ^
0_0_38274572_11538.cpp:20:13: error: 'n' was not declared in this scope
   cout << a[n] << endl;
             ^