Language: C++

#include<cstdio>
#include<iostream>
#include<cctype>
using namespace std;

int main(){
 string s;
 while(1){
  cin >> s ;
  if(isalpha(s)){
   if(islower(s))cout<<(char)toupper(s);
   else if(isupper(s))cout<<(char)tolower(s);
  }
  else{
   cout<<s;
  }
  if(s=='\n')break;
 }
 return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:10:13: error: no matching function for call to 'isalpha(std::string&)'
   10 |   if(isalpha(s)){
      |      ~~~~~~~^~~
In file included from /usr/include/c++/14/bits/basic_ios.h:37,
                 from /usr/include/c++/14/ios:46,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:2:
/usr/include/c++/14/bits/locale_facets.h:2643:5: note: candidate: 'template<class _CharT> bool std::isalpha(_CharT, const locale&)'
 2643 |     isalpha(_CharT __c, const locale& __loc)
      |     ^~~~~~~
/usr/include/c++/14/bits/locale_facets.h:2643:5: note:   candidate expects 2 arguments, 1 provided
In file included from /usr/include/c++/14/cctype:42,
                 from /usr/include/c++/14/bits/localefwd.h:42,
                 from /usr/include/c++/14/ios:43:
/usr/include/ctype.h:109:1: note: candidate: 'int isalpha(int)'
  109 | __exctype (isalpha);
      | ^~~~~~~~~
/usr/include/ctype.h:109:1: note:   no known conversion for argument 1 from 'std::string' {aka 'std::__cxx11::basic_string<char>'} to 'int'
  109 | __exctype (isalpha);
      | ^~~~~~~~~
a.cc:11:14: error: no matching function for call to 'islower(std::string&)'
   11 |    if(islower(s))cout<<(char)toupper(s);
      |       ~~~~~~~^~~
/usr/include/c++/14/bits/locale_facets.h:2637:5: note: candidate: 'template<class _CharT> bool std::islower(_CharT, const locale&)'
 2637 |     islower(_CharT __c, const locale& __loc)
      |     ^~~~~~~
/usr/include/c++/14/bits/locale_facets.h:2637:5: note:   candidate expects 2 arguments, 1 provided
/usr/include/ctype.h:112:1: note: candidate: 'int islower(int)'
  112 | __exctype (islower);
      | ^~~~~~~~~
/usr/include/ctype.h:112:1: note:   no known conversion for argument 1 from 'std::string' {aka 'std::__cxx11::basic_string<char>'} to 'int'
  112 | __exctype (islower);
      | ^~~~~~~~~
a.cc:11:37: error: no matching function for call to 'toupper(std::string&)'
   11 |    if(islower(s))cout<<(char)toupper(s);
      |                              ~~~~~~~^~~
/usr/include/c++/14/bits/locale_facets.h:2687:5: note: candidate: 'template<class _CharT> _CharT std::toupper(_CharT, const locale&)'
 2687 |     toupper(_CharT __c, const locale& __loc)
      |     ^~~~~~~
/usr/include/c++/14/bits/locale_facets.h:2687:5: note:   candidate expects 2 arguments, 1 provided
/usr/include/ctype.h:125:12: note: candidate: 'int toupper(int)'
  125 | extern int toupper (int __c) __THROW;
      |            ^~~~~~~
/usr/include/ctype.h:125:25: note:   no known conversion for argument 1 from 'std::string' {aka 'std::__cxx11::basic_string<char>'} to 'int'
  125 | extern int toupper (int __c) __THROW;
      |                     ~~~~^~~
a.cc:12:19: error: no matching function for call to 'isupper(std::string&)'
   12 |    else if(isupper(s))cout<<(char)tolower(s);
      |            ~~~~~~~^~~
/usr/include/c++/14/bits/locale_facets.h:2631:5: note: candidate: 'template<class _CharT> bool std::isupper(_CharT, const locale&)'
 2631 |     isupper(_CharT __c, const locale& __loc)
      |     ^~~~~~~
/usr/include/c++/14/bits/locale_facets.h:2631:5: note:   candidate expects 2 arguments, 1 provided
/usr/include/ctype.h:117:1: note: candidate: 'int isupper(int)'
  117 | __exctype (isupper);
      | ^~~~~~~~~
/usr/include/ctype.h:117:1: note:   no known conversion for argument 1 from 'std::string' {aka 'std::__cxx11::basic_string<char>'} to 'int'
  117 | __exctype (isupper);
      | ^~~~~~~~~
a.cc:12:42: error: no matching function for call to 'tolower(std::string&)'
   12 |    else if(isupper(s))cout<<(char)tolower(s);
      |                                   ~~~~~~~^~~
/usr/include/c++/14/bits/locale_facets.h:2693:5: note: candidate: 'template<class _CharT> _CharT std::tolower(_CharT, const locale&)'
 2693 |     tolower(_CharT __c, const locale& __loc)
      |     ^~~~~~~
/usr/include/c++/14/bits/locale_facets.h:2693:5: note:   candidate expects 2 arguments, 1 provided
/usr/include/ctype.h:122:12: note: candidate: 'int tolower(int)'
  122 | extern int tolower (int __c) __THROW;
      |            ^~~~~~~
/usr/include/ctype.h:122:25: note:   no known conversion for argument 1 from 'std::string' {aka 'std::__cxx11::basic_string<char>'} to 'int'
  122 | extern int tolower (int __c) __THROW;
      |                     ~~~~^~~
a.cc:17:7: error: no match for 'operator==' (operand types are 'std::string' {aka 'std::__cxx11::basic_string<char>'} and 'char')
   17 |   if(s=='\n')break;
      |      ~^~~~~~
      |      |  |
      |      |  char
      |      std::string {aka std::__cxx11::basic_string<char>}
In file included from /usr/include/c++/14/iosfwd:42,
                 from /usr/include/c++/14/ios:40:
/usr/include/c++/14/bits/postypes.h:192:5: note: candidate: 'template<class _StateT> bool std::operator==(const fpos<_StateT>&, const fpos<_StateT>&)'
  192 |     operator==(const fpos<_StateT>& __lhs, const fpos<_StateT>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/postypes.h:192:5: note:   template argument deduction/substitution failed:
a.cc:17:9: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::fpos<_StateT>'
   17 |   if(s=='\n')break;
      |         ^~~~
In file included from /usr/include/c++/14/string:43,
                 from /usr/include/c++/14/bits/locale_classes.h:40,
                 from /usr/include/c++/14/bits/ios_base.h:41,
                 from /usr/include/c++/14/ios:44:
/usr/include/c++/14/bits/allocator.h:235:5: note: candidate: 'template<class _T1, class _T2> bool std::operator==(const allocator<_CharT>&, const allocator<_T2>&)'
  235 |     operator==(const allocator<_T1>&, const allocator<_T2>&)
      |     ^~~~~~~~
/usr/include/c++/14/bits/allocator.h:235:5: note:   template argument deduction/substitution failed:
a.cc:17:9: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::allocator<_CharT>'
   17 |   if(s=='\n')break;
      |         ^~~~
In file included from /usr/include/c++/14/string:48:
/usr/include/c++/14/bits/stl_iterator.h:441:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator==(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)'
  441 |     operator==(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:441:5: note:   template argument deduction/substitution failed:
a.cc:17:9: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::reverse_iterator<_Iterator>'
   17 |   if(s=='\n')break;
      |         ^~~~
/usr/include/c++/14/bits/stl_iterator.h:486:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator==(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  486 |     operator==(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:486:5: note:   template argument deduction/substitution failed:
a.cc:17:9: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::reverse_iterator<_Iterator>'
   17 |   if(s=='\n')break;
      |         ^~~~
/usr/include/c++/14/bits/stl_iterator.h:1667:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator==(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1667 |     operator==(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1667:5: note:   template argument deduction/substitution failed:
a.cc:17:9: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::move_iterator<_IteratorL>'
   17 |   if(s=='\n')break;
      |         ^~~~
/usr/include/c++/14/bits/stl_iterator.h:1737:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator==(const move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)'
 1737 |     operator==(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1737:5: note:   template argument deduction/substitution failed:
a.cc:17:9: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::move_iterator<_IteratorL>'
   17 |   if(s=='\n')break;
      |         ^~~~
In file included from /usr/include/c++/14/bits/stl_algobase.h:64,
                 from /usr/include/c++/14/string:51:
/usr/include/c++/14/bits/stl_pair.h:1033:5: note: candidate: 'template<class _T1, class _T2> constexpr bool std::operator==(const pair<_T1, _T2>&, const pair<_T1, _T2>&)'
 1033 |     operator==(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1033:5: note:   template argument deduction/substitution failed:
a.cc:17:9: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::pair<_T1, _T2>'
   17 |   if(s=='\n')break;
      |         ^~~~
In file included from /usr/include/c++/14/bits/basic_string.h:47,
                 from /usr/include/c++/14/string:54:
/usr/include/c++/14/string_view:629:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator==(basic_string_view<_CharT, _Traits>, __type_identity_t<basic_string_view<_CharT, _Traits> >)'
  629 |     operator==(basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:629:5: note:   template argument deduction/substitution failed:
a.cc:17:9: note:   'std::__cxx11::basic_string<char>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
   17 |   if(s=='\n')break;
      |         ^~~~
/usr/include/c++/14/string_view:637:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator==(basic_string_view<_CharT, _Traits>, basic_string_view<_CharT, _Traits>)'
  637 |     operator==(basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:637:5: note:   template argument deduction/substitution failed:
a.cc:17:9: note:   'std::__cxx11::basic_string<char>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
   17