/in/foo.cc: In function 'int main()':
/in/foo.cc:25:9: warning: unused variable 'a1' [-Wunused-variable]
  double a1;
         ^~
/in/foo.cc:24:6: warning: 'mid' may be used uninitialized in this function [-Wmaybe-uninitialized]
  int mid;
      ^~~