/in/foo.cc: In function 'int itos(std::__cxx11::string)':
/in/foo.cc:8:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int j=0;j<s.length()-1-i;j++){
               ~^~~~~~~~~~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:18:8: error: too few arguments to function 'int scanf(const char*, ...)'
  scanf()
        ^
In file included from /usr/include/c++/6/cstdio:42:0,
                 from /usr/include/c++/6/ext/string_conversions.h:43,
                 from /usr/include/c++/6/bits/basic_string.h:5417,
                 from /usr/include/c++/6/string:52,
                 from /usr/include/c++/6/bits/locale_classes.h:40,
                 from /usr/include/c++/6/bits/ios_base.h:41,
                 from /usr/include/c++/6/ios:42,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/stdio.h:433:12: note: declared here
 extern int scanf (const char *__restrict __format, ...) __wur;
            ^~~~~