Language: C++

#include<iostream>
#include<cctype>
#include<stdio.h>
using namespace std;

int main()
{
 char c[100];
 while(true)
 { 
   cin>>c;
   if(isupper(c) != 0)
   { 
     cout<<tolower(c)<<endl;
    }
   else if( islower(c) != 0)
   {
   cout<<toupper(c)<<endl;
    }
   else
   {
   cout<<c<<endl;
   }
  }
return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:12:14: error: no matching function for call to 'isupper(char [100])'
   12 |    if(isupper(c) != 0)
      |       ~~~~~~~^~~
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: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:12:15: error: invalid conversion from 'char*' to 'int' [-fpermissive]
   12 |    if(isupper(c) != 0)
      |               ^
      |               |
      |               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: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:14:19: error: no matching function for call to 'tolower(char [100])'
   14 |      cout<<tolower(c)<<endl;
      |            ~~~~~~~^~~
/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:14:20: error: invalid conversion from 'char*' to 'int' [-fpermissive]
   14 |      cout<<tolower(c)<<endl;
      |                    ^
      |                    |
      |                    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
a.cc:16:20: error: no matching function for call to 'islower(char [100])'
   16 |    else if( islower(c) != 0)
      |             ~~~~~~~^~~
/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:16:21: error: invalid conversion from 'char*' to 'int' [-fpermissive]
   16 |    else if( islower(c) != 0)
      |                     ^
      |                     |
      |                     char*
/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:18:17: error: no matching function for call to 'toupper(char [100])'
   18 |    cout<<toupper(c)<<endl;
      |          ~~~~~~~^~~
/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:18:18: error: invalid conversion from 'char*' to 'int' [-fpermissive]
   18 |    cout<<toupper(c)<<endl;
      |                  ^
      |                  |
      |                  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