View Compilation Error

0_0_13129104_31531.cpp: In function 'int main()':
0_0_13129104_31531.cpp:16:7: error: 'i' was not declared in this scope
  for (i = 2 ; i<= 50 ; i++) 
       ^
0_0_13129104_31531.cpp:23:27: error: 'n' was not declared in this scope
     while (~scanf ("%d", &n)) 
                           ^