foo.cc: In function 'void change(std::string&)':
foo.cc:17:24: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   17 |         for (int i=0; i<s.size(); i++)
      |                       ~^~~~~~~~~
foo.cc:20:24: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   20 |         for (int i=0; i<s.size(); i++)
      |                       ~^~~~~~~~~
foo.cc:21:32: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
   21 |                 if (!(s[i]>='A'&&s[i]<='Z'||s[i]>='a'&&s[i]<='z'))
foo.cc: In function 'int main()':
foo.cc:35:24: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   35 |         for (int i=0; i<s.size(); i++)
      |                       ~^~~~~~~~~

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