/in/foo.cc: In function 'bool isPrime(int)':
/in/foo.cc:9:2: warning: this 'for' clause does not guard... [-Wmisleading-indentation]
  for (int i=2; i<=sqrt(x); i++)
  ^~~
/in/foo.cc:12:3: note: ...this statement, but the latter is misleadingly indented as if it is guarded by the 'for'
   return ((x == 1 || x==0) ? false : true);
   ^~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:23:9: warning: array subscript has type 'char' [-Wchar-subscripts]
   a[s[i]]++;
         ^
/in/foo.cc:18:7: warning: variable 'letter_max' set but not used [-Wunused-but-set-variable]
  char letter_max, letter_min;
       ^~~~~~~~~~
/in/foo.cc:18:19: warning: variable 'letter_min' set but not used [-Wunused-but-set-variable]
  char letter_max, letter_min;
                   ^~~~~~~~~~