foo.cc: In function 'bool find(int, int, std::string)':
foo.cc:25:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   25 |         for (int i=0;i<b.size()-a.size()+1;i++) {
      |                      ~^~~~~~~~~~~~~~~~~~~~
foo.cc:27:31: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   27 |                 for (int j=0;j<a.size();j++) {
      |                              ~^~~~~~~~~
foo.cc: In function 'std::string replace(std::string&, int, int, int)':
foo.cc:48:25: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   48 |         for (int i=c+1;i<s.size();i++) s2+=s[i];
      |                        ~^~~~~~~~~
foo.cc: In function 'bool work(std::string)':
foo.cc:56:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   56 |         for (int i=0;i<s.size();i++) {
      |                      ~^~~~~~~~~
foo.cc:80:31: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   80 |         for (int k=first_pos;k<s.size();k++) {
      |                              ~^~~~~~~~~
foo.cc:82:41: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   82 |                         for (int i=k+1;i<s.size();i++) {
      |                                        ~^~~~~~~~~
foo.cc:85:57: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   85 |                                         for (int j=i+1;j<s.size();j++) {
      |                                                        ~^~~~~~~~~

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