/in/foo.cc: In function 'int main()':
/in/foo.cc:7:17: warning: format '%s' expects argument of type 'char*', but argument 2 has type 'char (*)[20]' [-Wformat=]
   scanf("%s", &a);
                 ^
/in/foo.cc:8:31: error: 'strlen' was not declared in this scope
   for (int i = 0; i < strlen(a); ++i)
                               ^