foo.cc: In function ‘int main()’:
foo.cc:9:19: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
    9 |     for (int i=0;i<=s3.size()-1;i++)
      |                  ~^~~~~~~~~~~~~
foo.cc:12:9: warning: this ‘if’ clause does not guard... [-Wmisleading-indentation]
   12 |         if (s3[i]>'Z')
      |         ^~
foo.cc:14:13: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the ‘if’
   14 |             if (s3[i]<'A')
      |             ^~