Language: C++

#include<iostream>
#include<string>
#include<algorithm>
#include<vector>
using namespace std;

int main(){
    int n,k;
    string s;
    cin >> n >> k >> s;
    if(s.at(k=='A') swap(s.at(k),"a");
    else if(s.at(k=='B') swap(s.at(k),"b");
    else swap(s.at(k),"c");
    cout << s << endl;
    }

Compiler output:
a.cc: In function 'int main()':
a.cc:11:20: error: expected ';' before 'swap'
   11 |     if(s.at(k=='A') swap(s.at(k),"a");
      |                    ^~~~~
      |                    ;
a.cc:11:12: warning: ignoring return value of 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::reference std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::at(size_type) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; reference = char&; size_type = long unsigned int]', declared with attribute 'nodiscard' [-Wunused-result]
   11 |     if(s.at(k=='A') swap(s.at(k),"a");
      |        ~~~~^~~~~~~~
In file included 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,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:1:
/usr/include/c++/14/bits/basic_string.h:1309:7: note: declared here
 1309 |       at(size_type __n)
      |       ^~
a.cc:12:5: error: expected primary-expression before 'else'
   12 |     else if(s.at(k=='B') swap(s.at(k),"b");
      |     ^~~~
a.cc:11:39: error: expected ')' before 'else'
   11 |     if(s.at(k=='A') swap(s.at(k),"a");
      |       ~                               ^
      |                                       )
   12 |     else if(s.at(k=='B') swap(s.at(k),"b");
      |     ~~~~                               
a.cc:13:14: error: no matching function for call to 'swap(__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type&, const char [2])'
   13 |     else swap(s.at(k),"c");
      |          ~~~~^~~~~~~~~~~~~
In file included from /usr/include/c++/14/exception:166,
                 from /usr/include/c++/14/ios:41:
/usr/include/c++/14/bits/exception_ptr.h:229:5: note: candidate: 'void std::__exception_ptr::swap(exception_ptr&, exception_ptr&)'
  229 |     swap(exception_ptr& __lhs, exception_ptr& __rhs)
      |     ^~~~
/usr/include/c++/14/bits/exception_ptr.h:229:25: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} to 'std::__exception_ptr::exception_ptr&'
  229 |     swap(exception_ptr& __lhs, exception_ptr& __rhs)
      |          ~~~~~~~~~~~~~~~^~~~~
In file included from /usr/include/c++/14/bits/exception_ptr.h:41:
/usr/include/c++/14/bits/move.h:226:5: note: candidate: 'template<class _Tp> std::_Require<std::__not_<std::__is_tuple_like<_Tp> >, std::is_move_constructible<_Tp>, std::is_move_assignable<_Tp> > std::swap(_Tp&, _Tp&)'
  226 |     swap(_Tp& __a, _Tp& __b)
      |     ^~~~
/usr/include/c++/14/bits/move.h:226:5: note:   template argument deduction/substitution failed:
a.cc:13:14: note:   deduced conflicting types for parameter '_Tp' ('char' and 'const char [2]')
   13 |     else swap(s.at(k),"c");
      |          ~~~~^~~~~~~~~~~~~
/usr/include/c++/14/bits/move.h:250:5: note: candidate: 'template<class _Tp, long unsigned int _Nm> std::__enable_if_t<((bool)std::__is_swappable<_Tp>::value)> std::swap(_Tp (&)[_Nm], _Tp (&)[_Nm])'
  250 |     swap(_Tp (&__a)[_Nm], _Tp (&__b)[_Nm])
      |     ^~~~
/usr/include/c++/14/bits/move.h:250:5: note:   template argument deduction/substitution failed:
a.cc:13:14: note:   mismatched types '_Tp [_Nm]' and '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'}
   13 |     else swap(s.at(k),"c");
      |          ~~~~^~~~~~~~~~~~~
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:1089:5: note: candidate: 'template<class _T1, class _T2> typename std::enable_if<std::__and_<std::__is_swappable<_T1>, std::__is_swappable<_T2> >::value>::type std::swap(pair<_T1, _T2>&, pair<_T1, _T2>&)'
 1089 |     swap(pair<_T1, _T2>& __x, pair<_T1, _T2>& __y)
      |     ^~~~
/usr/include/c++/14/bits/stl_pair.h:1089:5: note:   template argument deduction/substitution failed:
a.cc:13:14: note:   mismatched types 'std::pair<_T1, _T2>' and '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'}
   13 |     else swap(s.at(k),"c");
      |          ~~~~^~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1106:5: note: candidate: 'template<class _T1, class _T2> typename std::enable_if<(! std::__and_<std::__is_swappable<_T1>, std::__is_swappable<_T2> >::value)>::type std::swap(pair<_T1, _T2>&, pair<_T1, _T2>&)' (deleted)
 1106 |     swap(pair<_T1, _T2>&, pair<_T1, _T2>&) = delete;
      |     ^~~~
/usr/include/c++/14/bits/stl_pair.h:1106:5: note:   template argument deduction/substitution failed:
a.cc:13:14: note:   mismatched types 'std::pair<_T1, _T2>' and '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'}
   13 |     else swap(s.at(k),"c");
      |          ~~~~^~~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:4039:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> void std::swap(__cxx11::basic_string<_CharT, _Traits, _Allocator>&, __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 4039 |     swap(basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~
/usr/include/c++/14/bits/basic_string.h:4039:5: note:   template argument deduction/substitution failed:
a.cc:13:14: note:   mismatched types 'std::__cxx11::basic_string<_CharT, _Traits, _Allocator>' and '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'}
   13 |     else swap(s.at(k),"c");
      |          ~~~~^~~~~~~~~~~~~
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:2805:5: note: candidate: 'template<class ... _Elements> typename std::enable_if<std::__and_<std::__is_swappable<_Elements>...>::value>::type std::swap(tuple<_UTypes ...>&, tuple<_UTypes ...>&)'
 2805 |     swap(tuple<_Elements...>& __x, tuple<_Elements...>& __y)
      |     ^~~~
/usr/include/c++/14/tuple:2805:5: note:   template argument deduction/substitution failed:
a.cc:13:14: note:   mismatched types 'std::tuple<_UTypes ...>' and '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'}
   13 |     else swap(s.at(k),"c");
      |          ~~~~^~~~~~~~~~~~~
/usr/include/c++/14/tuple:2823:5: note: candidate: 'template<class ... _Elements> typename std::enable_if<(! std::__and_<std::__is_swappable<_Elements>...>::value)>::type std::swap(tuple<_UTypes ...>&, tuple<_UTypes ...>&)' (deleted)
 2823 |     swap(tuple<_Elements...>&, tuple<_Elements...>&) = delete;
      |     ^~~~
/usr/include/c++/14/tuple:2823:5: note:   template argument deduction/substitution failed:
a.cc:13:14: note:   mismatched types 'std::tuple<_UTypes ...>' and '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'}
   13 |     else swap(s.at(k),"c");
      |          ~~~~^~~~~~~~~~~~~
In file included from /usr/include/c++/14/vector:66,
                 from a.cc:4:
/usr/include/c++/14/bits/stl_vector.h:2122:5: note: candidate: 'template<class _Tp, class _Alloc> void std::swap(vector<_Tp, _Alloc>&, vector<_Tp, _Alloc>&)'
 2122 |     swap(vector<_Tp, _Alloc>& __x, vector<_Tp, _Alloc>& __y)
      |     ^~~~
/usr/include/c++/14/bits/stl_vector.h:2122:5: note:   template argument deduction/substitution failed:
a.cc:13:14: note:   mismatched types 'std::vector<_Tp, _Alloc>' and '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'}
   13 |     else swap(s.at(k),"c");
      |          ~~~~^~~~~~~~~~~~~