foo.cc: In function 'std::string up(std::string)':
foo.cc:7:27: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    7 |         for (int i = 0; i < x.size(); i++)
      |                         ~~^~~~~~~~~~
foo.cc: In function 'std::string Mul(std::string, std::string)':
foo.cc:23:35: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   23 |                 for (int i = 0; i < out.size(); i++)
      |                                 ~~^~~~~~~~~~~~
foo.cc:26:35: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   26 |                 for (int j = 0; j < s2.size(); j++)
      |                                 ~~^~~~~~~~~~~
foo.cc: In function 'std::string tran_z_s(std::string)':
foo.cc:36:27: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   36 |         for (int i = 0; i < x.size(); i++)
      |                         ~~^~~~~~~~~~
foo.cc: In function 'std::string tran_s_z(std::string)':
foo.cc:42:27: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   42 |         for (int i = 0; i < x.size(); i++)
      |                         ~~^~~~~~~~~~

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