foo.cc: In function 'std::string sub(std::string, std::string)':
foo.cc:31:5: warning: this 'while' clause does not guard... [-Wmisleading-indentation]
   31 |     while(!na[--lmax]&&lmax>0)  ;lmax++;
      |     ^~~~~
foo.cc:31:34: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the 'while'
   31 |     while(!na[--lmax]&&lmax>0)  ;lmax++;
      |                                  ^~~~
foo.cc: In function 'std::string div(std::string, int)':
foo.cc:53:18: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   53 |     for(int i=0;i<a.size();i++)
      |                 ~^~~~~~~~~
foo.cc:59:18: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   59 |     for(int i=0;i<r.size();i++)
      |                 ~^~~~~~~~~
foo.cc: In function 'int mod(std::string, int)':
foo.cc:66:18: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   66 |     for(int i=0;i<a.size();i++) d=(d*10+(a[i]-'0'))%b;//求出余数
      |                 ~^~~~~~~~~

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