Language: C++

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

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

Compiler output:
a.cc: In function 'int main()':
a.cc:9:25: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
    9 |                 if ( ch == '\n' )
      |                      ~~~^~~~~~~
a.cc:11:33: error: no matching function for call to 'islower(char [1])'
   11 |                 else if (islower(ch))
      |                          ~~~~~~~^~~~
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,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:1:
/usr/include/ctype.h:112:1: note: candidate: 'int islower(int)' (near match)
  112 | __exctype (islower);
      | ^~~~~~~~~
/usr/include/ctype.h:112:1: note:   conversion of argument 1 would be ill-formed:
a.cc:11:34: error: invalid conversion from 'char*' to 'int' [-fpermissive]
   11 |                 else if (islower(ch))
      |                                  ^~
      |                                  |
      |                                  char*
In file included from /usr/include/c++/14/bits/basic_ios.h:37,
                 from /usr/include/c++/14/ios:46:
/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
a.cc:12:33: error: no matching function for call to 'toupper(char [1])'
   12 |                     ch = toupper(ch);
      |                          ~~~~~~~^~~~
/usr/include/ctype.h:125:12: note: candidate: 'int toupper(int)' (near match)
  125 | extern int toupper (int __c) __THROW;
      |            ^~~~~~~
/usr/include/ctype.h:125:12: note:   conversion of argument 1 would be ill-formed:
a.cc:12:34: error: invalid conversion from 'char*' to 'int' [-fpermissive]
   12 |                     ch = toupper(ch);
      |                                  ^~
      |                                  |
      |                                  char*
/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
a.cc:13:33: error: no matching function for call to 'isupper(char [1])'
   13 |                 else if (isupper(ch))
      |                          ~~~~~~~^~~~
/usr/include/ctype.h:117:1: note: candidate: 'int isupper(int)' (near match)
  117 | __exctype (isupper);
      | ^~~~~~~~~
/usr/include/ctype.h:117:1: note:   conversion of argument 1 would be ill-formed:
a.cc:13:34: error: invalid conversion from 'char*' to 'int' [-fpermissive]
   13 |                 else if (isupper(ch))
      |                                  ^~
      |                                  |
      |                                  char*
/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
a.cc:14:33: error: no matching function for call to 'tolower(char [1])'
   14 |                     ch = tolower(ch);
      |                          ~~~~~~~^~~~
/usr/include/ctype.h:122:12: note: candidate: 'int tolower(int)' (near match)
  122 | extern int tolower (int __c) __THROW;
      |            ^~~~~~~
/usr/include/ctype.h:122:12: note:   conversion of argument 1 would be ill-formed:
a.cc:14:34: error: invalid conversion from 'char*' to 'int' [-fpermissive]
   14 |                     ch = tolower(ch);
      |                                  ^~
      |                                  |
      |                                  char*
/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