foo.cc: In function 'std::string cf(std::string)':
foo.cc:14:16: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   14 |   for(int i=0;i<=a.size();i++)
      |               ~^~~~~~~~~~
foo.cc:8:6: warning: unused variable 'len' [-Wunused-variable]
    8 |  int len=s.size();
      |      ^~~
foo.cc: In function 'int main()':
foo.cc:107:16: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
  107 |   for(int i=0;i<s.size();i++)
      |               ~^~~~~~~~~

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