Language: C++

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

int main(){
 char s[1200];
 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(char [1200])'
   10 |   if(isalpha(s)){
      |      ~~~~~~~^~~
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:2:
/usr/include/ctype.h:109:1: note: candidate: 'int isalpha(int)' (near match)
  109 | __exctype (isalpha);
      | ^~~~~~~~~
/usr/include/ctype.h:109:1: note:   conversion of argument 1 would be ill-formed:
a.cc:10:14: error: invalid conversion from 'char*' to 'int' [-fpermissive]
   10 |   if(isalpha(s)){
      |              ^
      |              |
      |              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: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
a.cc:11:14: error: no matching function for call to 'islower(char [1200])'
   11 |    if(islower(s))cout<<(char)toupper(s);
      |       ~~~~~~~^~~
/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:11:15: error: invalid conversion from 'char*' to 'int' [-fpermissive]
   11 |    if(islower(s))cout<<(char)toupper(s);
      |               ^
      |               |
      |               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:11:37: error: no matching function for call to 'toupper(char [1200])'
   11 |    if(islower(s))cout<<(char)toupper(s);
      |                              ~~~~~~~^~~
/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:11:38: error: invalid conversion from 'char*' to 'int' [-fpermissive]
   11 |    if(islower(s))cout<<(char)toupper(s);
      |                                      ^
      |                                      |
      |                                      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:12:19: error: no matching function for call to 'isupper(char [1200])'
   12 |    else if(isupper(s))cout<<(char)tolower(s);
      |            ~~~~~~~^~~
/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:12:20: error: invalid conversion from 'char*' to 'int' [-fpermissive]
   12 |    else if(isupper(s))cout<<(char)tolower(s);
      |                    ^
      |                    |
      |                    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:12:42: error: no matching function for call to 'tolower(char [1200])'
   12 |    else if(isupper(s))cout<<(char)tolower(s);
      |                                   ~~~~~~~^~~
/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:12:43: error: invalid conversion from 'char*' to 'int' [-fpermissive]
   12 |    else if(isupper(s))cout<<(char)tolower(s);
      |                                           ^
      |                                           |
      |                                           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:17:7: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   17 |   if(s=='\n')break;
      |      ~^~~~~~