/in/foo.c: In function 'f':
/in/foo.c:8:2: warning: implicit declaration of function 'sprinf' [-Wimplicit-function-declaration]
  sprinf(a, "%d", n);
  ^~~~~~
/in/foo.c: In function 'main':
/in/foo.c:26:2: error: 'cin' undeclared (first use in this function)
  cin >> n;
  ^~~
/in/foo.c:26:2: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:27:2: error: 'cout' undeclared (first use in this function)
  cout << j(n);
  ^~~~