/in/foo.cc: In function 'int kto10(int, std::__cxx11::string)':
/in/foo.cc:7:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0; i<s.size(); i++) {
               ~^~~~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:31:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=1; i<=s.size(); i++) {
               ~^~~~~~~~~~
/in/foo.cc:32:7: error: no match for 'operator%' (operand types are 'std::__cxx11::string {aka std::__cxx11::basic_string<char>}' and 'int')
   if(m%10*(i-1)/10=='1') {
      ~^~~
In file included from /usr/include/c++/6/valarray:587:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:95,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/valarray_after.h:406:5: note: candidate: template<class _Dom1, class _Dom2> std::_Expr<std::_BinClos<std::__modulus, std::_Expr, std::_Expr, _Dom1, _Dom2>, typename std::__fun<std::__modulus, typename _Dom1::value_type>::result_type> std::operator%(const std::_Expr<_Dom1, typename _Dom1::value_type>&, const std::_Expr<_Dom2, typename _Dom2::value_type>&)
     _DEFINE_EXPR_BINARY_OPERATOR(%, __modulus)
     ^
/usr/include/c++/6/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
/in/foo.cc:32:8: note:   'std::__cxx11::string {aka std::__cxx11::basic_string<char>}' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
   if(m%10*(i-1)/10=='1') {
        ^~
In file included from /usr/include/c++/6/valarray:587:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:95,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/valarray_after.h:406:5: note: candidate: template<class _Dom> std::_Expr<std::_BinClos<std::__modulus, std::_Expr, std::_Constant, _Dom, typename _Dom::value_type>, typename std::__fun<std::__modulus, typename _Dom1::value_type>::result_type> std::operator%(const std::_Expr<_Dom1, typename _Dom1::value_type>&, const typename _Dom::value_type&)
     _DEFINE_EXPR_BINARY_OPERATOR(%, __modulus)
     ^
/usr/include/c++/6/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
/in/foo.cc:32:8: note:   'std::__cxx11::string {aka std::__cxx11::basic_string<char>}' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
   if(m%10*(i-1)/10=='1') {
        ^~
In file included from /usr/include/c++/6/valarray:587:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:95,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/valarray_after.h:406:5: note: candidate: template<class _Dom> std::_Expr<std::_BinClos<std::__modulus, std::_Constant, std::_Expr, typename _Dom::value_type, _Dom>, typename std::__fun<std::__modulus, typename _Dom1::value_type>::result_type> std::operator%(const typename _Dom::value_type&, const std::_Expr<_Dom1, typename _Dom1::value_type>&)
     _DEFINE_EXPR_BINARY_OPERATOR(%, __modulus)
     ^
/usr/include/c++/6/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
/in/foo.cc:32:8: note:   mismatched types 'const std::_Expr<_Dom1, typename _Dom1::value_type>' and 'int'
   if(m%10*(i-1)/10=='1') {
        ^~
In file included from /usr/include/c++/6/valarray:587:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:95,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/valarray_after.h:406:5: note: candidate: template<class _Dom> std::_Expr<std::_BinClos<std::__modulus, std::_Expr, std::_ValArray, _Dom, typename _Dom::value_type>, typename std::__fun<std::__modulus, typename _Dom1::value_type>::result_type> std::operator%(const std::_Expr<_Dom1, typename _Dom1::value_type>&, const std::valarray<typename _Dom::value_type>&)
     _DEFINE_EXPR_BINARY_OPERATOR(%, __modulus)
     ^
/usr/include/c++/6/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
/in/foo.cc:32:8: note:   'std::__cxx11::string {aka std::__cxx11::basic_string<char>}' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
   if(m%10*(i-1)/10=='1') {
        ^~
In file included from /usr/include/c++/6/valarray:587:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:95,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/valarray_after.h:406:5: note: candidate: template<class _Dom> std::_Expr<std::_BinClos<std::__modulus, std::_ValArray, std::_Expr, typename _Dom::value_type, _Dom>, typename std::__fun<std::__modulus, typename _Dom1::value_type>::result_type> std::operator%(const std::valarray<typename _Dom::value_type>&, const std::_Expr<_Dom1, typename _Dom1::value_type>&)
     _DEFINE_EXPR_BINARY_OPERATOR(%, __modulus)
     ^
/usr/include/c++/6/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
/in/foo.cc:32:8: note:   mismatched types 'const std::_Expr<_Dom1, typename _Dom1::value_type>' and 'int'
   if(m%10*(i-1)/10=='1') {
        ^~
In file included from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:95:0,
                 from /in/foo.cc:1:
/usr/include/c++/6/valarray:1171:1: note: candidate: template<class _Tp> std::_Expr<std::_BinClos<std::__modulus, std::_ValArray, std::_ValArray, _Tp, _Tp>, typename std::__fun<std::__modulus, _Tp>::result_type> std::operator%(const std::valarray<_Tp>&, const std::valarray<_Tp>&)
 _DEFINE_BINARY_OPERATOR(%, __modulus)
 ^
/usr/include/c++/6/valarray:1171:1: note:   template argument deduction/substitution failed:
/in/foo.cc:32:8: note:   'std::__cxx11::string {aka std::__cxx11::basic_string<char>}' is not derived from 'const std::valarray<_Tp>'
   if(m%10*(i-1)/10=='1') {
        ^~
In file included from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:95:0,
                 from /in/foo.cc:1:
/usr/include/c++/6/valarray:1171:1: note: candidate: template<class _Tp> std::_Expr<std::_BinClos<std::__modulus, std::_ValArray, std::_Constant, _Tp, _Tp>, typename std::__fun<std::__modulus, _Tp>::result_type> std::operator%(const std::valarray<_Tp>&, const _Tp&)
 _DEFINE_BINARY_OPERATOR(%, __modulus)
 ^
/usr/include/c++/6/valarray:1171:1: note:   template argument deduction/substitution failed:
/in/foo.cc:32:8: note:   'std::__cxx11::string {aka std::__cxx11::basic_string<char>}' is not derived from 'const std::valarray<_Tp>'
   if(m%10*(i-1)/10=='1') {
        ^~
In file included from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:95:0,
                 from /in/foo.cc:1:
/usr/include/c++/6/valarray:1171:1: note: candidate: template<class _Tp> std::_Expr<std::_BinClos<std::__modulus, std::_Constant, std::_ValArray, _Tp, _Tp>, typename std::__fun<std::__modulus, _Tp>::result_type> std::operator%(const _Tp&, const std::valarray<_Tp>&)
 _DEFINE_BINARY_OPERATOR(%, __modulus)
 ^
/usr/include/c++/6/valarray:1171:1: note:   template argument deduction/substitution failed:
/in/foo.cc:32:8: note:   mismatched types 'const std::valarray<_Tp>' and 'int'
   if(m%10*(i-1)/10=='1') {
        ^~
/in/foo.cc:28:6: warning: unused variable 'b' [-Wunused-variable]
  int b;
      ^