Language: C++

#include <iostream>
#include <string>
#include <cctype>
using namespace std;
int main(){
string str;
getline(cin,str);
char ch;
for (int i = 0;i <= str,size() ;i++){
	cin >> ch;
	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:9:18: error: no match for 'operator<=' (operand types are 'int' and 'std::string' {aka 'std::__cxx11::basic_string<char>'})
    9 | for (int i = 0;i <= str,size() ;i++){
      |                ~ ^~ ~~~
      |                |    |
      |                int  std::string {aka std::__cxx11::basic_string<char>}
In file included from /usr/include/c++/14/string:48,
                 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,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_iterator.h:469:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator<=(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)'
  469 |     operator<=(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:469:5: note:   template argument deduction/substitution failed:
a.cc:9:21: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and 'int'
    9 | for (int i = 0;i <= str,size() ;i++){
      |                     ^~~
/usr/include/c++/14/bits/stl_iterator.h:513:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator<=(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  513 |     operator<=(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:513:5: note:   template argument deduction/substitution failed:
a.cc:9:21: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and 'int'
    9 | for (int i = 0;i <= str,size() ;i++){
      |                     ^~~
/usr/include/c++/14/bits/stl_iterator.h:1704:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator<=(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1704 |     operator<=(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1704:5: note:   template argument deduction/substitution failed:
a.cc:9:21: note:   mismatched types 'const std::move_iterator<_IteratorL>' and 'int'
    9 | for (int i = 0;i <= str,size() ;i++){
      |                     ^~~
/usr/include/c++/14/bits/stl_iterator.h:1767:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator<=(const move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)'
 1767 |     operator<=(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1767:5: note:   template argument deduction/substitution failed:
a.cc:9:21: note:   mismatched types 'const std::move_iterator<_IteratorL>' and 'int'
    9 | for (int i = 0;i <= str,size() ;i++){
      |                     ^~~
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:1064:5: note: candidate: 'template<class _T1, class _T2> constexpr bool std::operator<=(const pair<_T1, _T2>&, const pair<_T1, _T2>&)'
 1064 |     operator<=(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1064:5: note:   template argument deduction/substitution failed:
a.cc:9:21: note:   mismatched types 'const std::pair<_T1, _T2>' and 'int'
    9 | for (int i = 0;i <= str,size() ;i++){
      |                     ^~~
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:717:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator<=(basic_string_view<_CharT, _Traits>, basic_string_view<_CharT, _Traits>)'
  717 |     operator<=(basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:717:5: note:   template argument deduction/substitution failed:
a.cc:9:21: note:   mismatched types 'std::basic_string_view<_CharT, _Traits>' and 'int'
    9 | for (int i = 0;i <= str,size() ;i++){
      |                     ^~~
/usr/include/c++/14/string_view:724:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator<=(basic_string_view<_CharT, _Traits>, __type_identity_t<basic_string_view<_CharT, _Traits> >)'
  724 |     operator<=(basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:724:5: note:   template argument deduction/substitution failed:
a.cc:9:21: note:   mismatched types 'std::basic_string_view<_CharT, _Traits>' and 'int'
    9 | for (int i = 0;i <= str,size() ;i++){
      |                     ^~~
/usr/include/c++/14/string_view:732:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator<=(__type_identity_t<basic_string_view<_CharT, _Traits> >, basic_string_view<_CharT, _Traits>)'
  732 |     operator<=(__type_identity_t<basic_string_view<_CharT, _Traits>> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:732:5: note:   template argument deduction/substitution failed:
a.cc:9:21: note:   'std::__cxx11::basic_string<char>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
    9 | for (int i = 0;i <= str,size() ;i++){
      |                     ^~~
/usr/include/c++/14/bits/basic_string.h:3956:5: 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>&)'
 3956 |     operator<=(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3956:5: note:   template argument deduction/substitution failed:
a.cc:9:21: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>' and 'int'
    9 | for (int i = 0;i <= str,size() ;i++){
      |                     ^~~
/usr/include/c++/14/bits/basic_string.h:3970:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator<=(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const _CharT*)'
 3970 |     operator<=(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3970:5: note:   template argument deduction/substitution failed:
a.cc:9:21: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>' and 'int'
    9 | for (int i = 0;i <= str,size() ;i++){
      |                     ^~~
/usr/include/c++/14/bits/basic_string.h:3983:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator<=(const _CharT*, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3983 |     operator<=(const _CharT* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3983:5: note:   template argument deduction/substitution failed:
a.cc:9:21: note:   mismatched types 'const _CharT*' and 'int'
    9 | for (int i = 0;i <= str,size() ;i++){
      |                     ^~~
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:2625:5: note: candidate: 'template<class ... _TElements, class ... _UElements> constexpr bool std::operator<=(const tuple<_UTypes ...>&, const tuple<_Elements ...>&)'
 2625 |     operator<=(const tuple<_TElements...>& __t,
      |     ^~~~~~~~
/usr/include/c++/14/tuple:2625:5: note:   template argument deduction/substitution failed:
a.cc:9:21: note:   mismatched types 'const std::tuple<_UTypes ...>' and 'int'
    9 | for (int i = 0;i <= str,size() ;i++){
      |                     ^~~
a.cc:9:29: error: no matching function for call to 'size()'
    9 | for (int i = 0;i <= str,size() ;i++){
      |                         ~~~~^~
In file included from /usr/include/c++/14/string:53:
/usr/include/c++/14/bits/range_access.h:262:5: note: candidate: 'template<class _Container> constexpr decltype (__cont.size()) std::size(const _Container&)'
  262 |     size(const _Container& __cont) noexcept(noexcept(__cont.size()))
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:262:5: note:   candidate expects 1 argument, 0 provided
/usr/include/c++/14/bits/range_access.h:272:5: note: candidate: 'template<class _Tp, long unsigned int _Nm> constexpr std::size_t std::size(const _Tp (&)[_Nm])'
  272 |     size(const _Tp (&)[_Nm]) noexcept
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:272:5: note:   candidate expects 1 argument, 0 provided