Language: C++

#include <iostream>

using namespace std;

int main()
{
    char b;
    while(cin>>b!=NULL)
    {
        if(b=='A')
            cout<<"T"<<endl;
        else if(b=='T')
            cout<<"A"<<endl;
        else if(b=='G')
            cout<<"C"<<endl;
        else cout<<"G"<<endl;
    }
    return 0;
}


Compiler output:
a.cc: In function 'int main()':
a.cc:8:17: error: no match for 'operator!=' (operand types are 'std::basic_istream<char>' and 'long int')
    8 |     while(cin>>b!=NULL)
      |                 ^
a.cc:8:17: note: candidate: 'operator!=(int, long int)' (built-in)
a.cc:8:17: note:   no known conversion for argument 1 from 'std::basic_istream<char>' to 'int'
In file included from /usr/include/c++/14/iosfwd:42,
                 from /usr/include/c++/14/ios:40,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:1:
/usr/include/c++/14/bits/postypes.h:197:5: note: candidate: 'template<class _StateT> bool std::operator!=(const fpos<_StateT>&, const fpos<_StateT>&)'
  197 |     operator!=(const fpos<_StateT>& __lhs, const fpos<_StateT>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/postypes.h:197:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/stdio.h:34,
                 from /usr/include/c++/14/cstdio:42,
                 from /usr/include/c++/14/ext/string_conversions.h:45,
                 from /usr/include/c++/14/bits/basic_string.h:4154,
                 from /usr/include/c++/14/string:54,
                 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:
a.cc:8:19: note:   'std::basic_istream<char>' is not derived from 'const std::fpos<_StateT>'
    8 |     while(cin>>b!=NULL)
      |                   ^~~~
In file included from /usr/include/c++/14/string:43:
/usr/include/c++/14/bits/allocator.h:243:5: note: candidate: 'template<class _T1, class _T2> bool std::operator!=(const allocator<_CharT>&, const allocator<_T2>&)'
  243 |     operator!=(const allocator<_T1>&, const allocator<_T2>&)
      |     ^~~~~~~~
/usr/include/c++/14/bits/allocator.h:243:5: note:   template argument deduction/substitution failed:
a.cc:8:19: note:   'std::basic_istream<char>' is not derived from 'const std::allocator<_CharT>'
    8 |     while(cin>>b!=NULL)
      |                   ^~~~
In file included from /usr/include/c++/14/string:48:
/usr/include/c++/14/bits/stl_iterator.h:455:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator!=(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)'
  455 |     operator!=(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:455:5: note:   template argument deduction/substitution failed:
a.cc:8:19: note:   'std::basic_istream<char>' is not derived from 'const std::reverse_iterator<_Iterator>'
    8 |     while(cin>>b!=NULL)
      |                   ^~~~
/usr/include/c++/14/bits/stl_iterator.h:500:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator!=(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  500 |     operator!=(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:500:5: note:   template argument deduction/substitution failed:
a.cc:8:19: note:   'std::basic_istream<char>' is not derived from 'const std::reverse_iterator<_Iterator>'
    8 |     while(cin>>b!=NULL)
      |                   ^~~~
/usr/include/c++/14/bits/stl_iterator.h:1686:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator!=(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1686 |     operator!=(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1686:5: note:   template argument deduction/substitution failed:
a.cc:8:19: note:   'std::basic_istream<char>' is not derived from 'const std::move_iterator<_IteratorL>'
    8 |     while(cin>>b!=NULL)
      |                   ^~~~
/usr/include/c++/14/bits/stl_iterator.h:1753:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator!=(const move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)'
 1753 |     operator!=(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1753:5: note:   template argument deduction/substitution failed:
a.cc:8:19: note:   'std::basic_istream<char>' is not derived from 'const std::move_iterator<_IteratorL>'
    8 |     while(cin>>b!=NULL)
      |                   ^~~~
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:1052:5: note: candidate: 'template<class _T1, class _T2> constexpr bool std::operator!=(const pair<_T1, _T2>&, const pair<_T1, _T2>&)'
 1052 |     operator!=(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1052:5: note:   template argument deduction/substitution failed:
a.cc:8:19: note:   'std::basic_istream<char>' is not derived from 'const std::pair<_T1, _T2>'
    8 |     while(cin>>b!=NULL)
      |                   ^~~~
In file included from /usr/include/c++/14/bits/basic_string.h:47:
/usr/include/c++/14/string_view:651:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator!=(basic_string_view<_CharT, _Traits>, basic_string_view<_CharT, _Traits>)'
  651 |     operator!=(basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:651:5: note:   template argument deduction/substitution failed:
a.cc:8:19: note:   'std::basic_istream<char>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
    8 |     while(cin>>b!=NULL)
      |                   ^~~~
/usr/include/c++/14/string_view:658:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator!=(basic_string_view<_CharT, _Traits>, __type_identity_t<basic_string_view<_CharT, _Traits> >)'
  658 |     operator!=(basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:658:5: note:   template argument deduction/substitution failed:
a.cc:8:19: note:   'std::basic_istream<char>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
    8 |     while(cin>>b!=NULL)
      |                   ^~~~
/usr/include/c++/14/string_view:666:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator!=(__type_identity_t<basic_string_view<_CharT, _Traits> >, basic_string_view<_CharT, _Traits>)'
  666 |     operator!=(__type_identity_t<basic_string_view<_CharT, _Traits>> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:666:5: note:   template argument deduction/substitution failed:
a.cc:8:19: note:   mismatched types 'std::basic_string_view<_CharT, _Traits>' and 'long int'
    8 |     while(cin>>b!=NULL)
      |                   ^~~~
/usr/include/c++/14/bits/basic_string.h:3833:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator!=(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3833 |     operator!=(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3833:5: note:   template argument deduction/substitution failed:
a.cc:8:19: note:   'std::basic_istream<char>' is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
    8 |     while(cin>>b!=NULL)
      |                   ^~~~
/usr/include/c++/14/bits/basic_string.h:3847:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator!=(const _CharT*, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3847 |     operator!=(const _CharT* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3847:5: note:   template argument deduction/substitution failed:
a.cc:8:19: note:   mismatched types 'const _CharT*' and 'std::basic_istream<char>'
    8 |     while(cin>>b!=NULL)
      |                   ^~~~
/usr/include/c++/14/bits/basic_string.h:3860:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator!=(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const _CharT*)'
 3860 |     operator!=(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3860:5: note:   template argument deduction/substitution failed:
a.cc:8:19: note:   'std::basic_istream<char>' is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
    8 |     while(cin>>b!=NULL)
      |                   ^~~~
In file included from /usr/include/c++/14/bits/memory_resource.h:47,
                 from /usr/include/c++/14/string:68:
/usr/include/c++/14/tuple:2613:5: note: candidate: 'template<class ... _TElements, class ... _UElements> constexpr bool std::operator!=(const tuple<_UTypes ...>&, const tuple<_Elements ...>&)'
 2613 |     operator!=(const tuple<_TElements...>& __t,
      |     ^~~~~~~~
/usr/include/c++/14/tuple:2613:5: note:   template argument deduction/substitution failed:
a.cc:8:19: note:   'std::basic_istream<char>' is not derived from 'const std::tuple<_UTypes ...>'
    8 |     while(cin>>b!=NULL)
      |                   ^~~~
In file included from /usr/include/c++/14/bits/locale_facets.h:48,
                 from /usr/include/c++/14/bits/basic_ios.h:37,
                 from /usr/include/c++/14/ios:46:
/usr/include/c++/14/bits/streambuf_iterator.h:242:5: note: candidate: 'template<class _CharT, class _Traits> bool std::operator!=(const istreambuf_iterator<_CharT, _Traits>&, const istreambuf_iterator<_CharT, _Traits>&)'
  242 |     operator!=(const istreambuf_iterator<_CharT, _Traits>& __a,
      |     ^~~~~~~~
/usr/include/c++/14/bits/streambuf_iterator.h:242:5: note:   template argument deduction/substitution failed:
a.cc:8:19: note:   'std::basic_istream<char>' is not derived from 'const std::istreambuf_iterator<_CharT, _Traits>'
    8 |     while(cin>>b!=NULL)
      |                   ^~~~
In file included from /usr/include/c++/14/bits/ios_base.h:46:
/usr/include/c++/14/system_error:525:3: note: candidate: 'bool std::operator!=(const error_code&, const error_code&)'