Language: C++

#include <bits/stdc++.h>
using namespace std;
int main() {
  vector<string> a(3);
  for (int i=0; i<3; i++) {
    cin >> a[i];
    if (a[i].at(0)=='a') {
      cin >> 'A';
    }
    if (a[i].at(0)=='b') {
      cin >> 'B';
    }
    if (a[i].at(0)=='c') {
      cin >> 'C';
    }
    if (a[i].at(0)=='d') {
      cin >> 'D';
    }
    if (a[i].at(0)=='e') {
      cin >> 'E';
    }
    if (a[i].at(0)=='f') {
      cin >> 'F';
    }
    if (a[i].at(0)=='g') {
      cin >> 'G';
    }
    if (a[i].at(0)=='h') {
      cin >> 'H';
    }
    if (a[i].at(0)=='i') {
      cin >> 'I';
    }
    if (a[i].at(0)=='j') {
      cin >> 'J';
    }
    if (a[i].at(0)=='k') {
      cin >> 'K';
    }
    if (a[i].at(0)=='m') {
      cin >> 'M';
    }
    if (a[i].at(0)=='n') {
      cin >> 'N';
    }
    if (a[i].at(0)=='o') {
      cin >> 'O';
    }
    if (a[i].at(0)=='p') {
      cin >> 'P';
    }
    if (a[i].at(0)=='q') {
      cin >> 'Q';
    }
    if (a[i].at(0)=='r') {
      cin >> 'R';
    }
    if (a[i].at(0)=='s') {
      cin >> 'S';
    }
    if (a[i].at(0)=='t') {
      cin >> 'T';
    }
    if (a[i].at(0)=='u') {
      cin >> 'U';
    }
    if (a[i].at(0)=='v') {
      cin >> 'V';
    }
    if (a[i].at(0)=='w') {
      cin >> 'W';
    }
    if (a[i].at(0)=='x') {
      cin >> 'X';
    }
    if (a[i].at(0)=='y') {
      cin >> 'Y';
    }
    if (a[i].at(0)=='z') {
      cin >> 'Z';
    }
  }
  cout << endl;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:8:11: error: no match for 'operator>>' (operand types are 'std::istream' {aka 'std::basic_istream<char>'} and 'char')
    8 |       cin >> 'A';
      |       ~~~ ^~ ~~~
      |       |      |
      |       |      char
      |       std::istream {aka std::basic_istream<char>}
In file included from /usr/include/c++/14/sstream:40,
                 from /usr/include/c++/14/complex:45,
                 from /usr/include/c++/14/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:127,
                 from a.cc:1:
/usr/include/c++/14/istream:170:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(bool&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  170 |       operator>>(bool& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:170:7: note:   conversion of argument 1 would be ill-formed:
a.cc:8:14: error: cannot bind non-const lvalue reference of type 'bool&' to a value of type 'char'
    8 |       cin >> 'A';
      |              ^~~
/usr/include/c++/14/istream:174:7: note: candidate: 'std::basic_istream<_CharT, _Traits>& std::basic_istream<_CharT, _Traits>::operator>>(short int&) [with _CharT = char; _Traits = std::char_traits<char>]' (near match)
  174 |       operator>>(short& __n);
      |       ^~~~~~~~
/usr/include/c++/14/istream:174:7: note:   conversion of argument 1 would be ill-formed:
a.cc:8:14: error: cannot bind non-const lvalue reference of type 'short int&' to a value of type 'char'
    8 |       cin >> 'A';
      |              ^~~
/usr/include/c++/14/istream:177:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(short unsigned int&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  177 |       operator>>(unsigned short& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:177:7: note:   conversion of argument 1 would be ill-formed:
a.cc:8:14: error: cannot bind non-const lvalue reference of type 'short unsigned int&' to a value of type 'char'
    8 |       cin >> 'A';
      |              ^~~
/usr/include/c++/14/istream:181:7: note: candidate: 'std::basic_istream<_CharT, _Traits>& std::basic_istream<_CharT, _Traits>::operator>>(int&) [with _CharT = char; _Traits = std::char_traits<char>]' (near match)
  181 |       operator>>(int& __n);
      |       ^~~~~~~~
/usr/include/c++/14/istream:181:7: note:   conversion of argument 1 would be ill-formed:
a.cc:8:14: error: cannot bind non-const lvalue reference of type 'int&' to a value of type 'char'
    8 |       cin >> 'A';
      |              ^~~
/usr/include/c++/14/istream:184:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(unsigned int&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  184 |       operator>>(unsigned int& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:184:7: note:   conversion of argument 1 would be ill-formed:
a.cc:8:14: error: cannot bind non-const lvalue reference of type 'unsigned int&' to a value of type 'char'
    8 |       cin >> 'A';
      |              ^~~
/usr/include/c++/14/istream:188:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long int&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  188 |       operator>>(long& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:188:7: note:   conversion of argument 1 would be ill-formed:
a.cc:8:14: error: cannot bind non-const lvalue reference of type 'long int&' to a value of type 'char'
    8 |       cin >> 'A';
      |              ^~~
/usr/include/c++/14/istream:192:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long unsigned int&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  192 |       operator>>(unsigned long& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:192:7: note:   conversion of argument 1 would be ill-formed:
a.cc:8:14: error: cannot bind non-const lvalue reference of type 'long unsigned int&' to a value of type 'char'
    8 |       cin >> 'A';
      |              ^~~
/usr/include/c++/14/istream:199:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long long int&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  199 |       operator>>(long long& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:199:7: note:   conversion of argument 1 would be ill-formed:
a.cc:8:14: error: cannot bind non-const lvalue reference of type 'long long int&' to a value of type 'char'
    8 |       cin >> 'A';
      |              ^~~
/usr/include/c++/14/istream:203:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long long unsigned int&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  203 |       operator>>(unsigned long long& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:203:7: note:   conversion of argument 1 would be ill-formed:
a.cc:8:14: error: cannot bind non-const lvalue reference of type 'long long unsigned int&' to a value of type 'char'
    8 |       cin >> 'A';
      |              ^~~
/usr/include/c++/14/istream:219:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(float&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  219 |       operator>>(float& __f)
      |       ^~~~~~~~
/usr/include/c++/14/istream:219:7: note:   conversion of argument 1 would be ill-formed:
a.cc:8:14: error: cannot bind non-const lvalue reference of type 'float&' to a value of type 'char'
    8 |       cin >> 'A';
      |              ^~~
/usr/include/c++/14/istream:223:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(double&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  223 |       operator>>(double& __f)
      |       ^~~~~~~~
/usr/include/c++/14/istream:223:7: note:   conversion of argument 1 would be ill-formed:
a.cc:8:14: error: cannot bind non-const lvalue reference of type 'double&' to a value of type 'char'
    8 |       cin >> 'A';
      |              ^~~
/usr/include/c++/14/istream:227:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long double&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  227 |       operator>>(long double& __f)
      |       ^~~~~~~~
/usr/include/c++/14/istream:227:7: note:   conversion of argument 1 would be ill-formed:
a.cc:8:14: error: cannot bind non-const lvalue reference of type 'long double&' to a value of type 'char'
    8 |       cin >> 'A';
      |              ^~~
/usr/include/c++/14/istream:328:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(void*&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  328 |       operator>>(void*& __p)
      |       ^~~~~~~~
/usr/include/c++/14/istream:328:7: note:   conversion of argument 1 would be ill-formed:
a.cc:8:14: error: invalid conversion from 'char' to 'void*' [-fpermissive]
    8 |       cin >> 'A';
      |              ^~~
      |              |
      |              char
a.cc:8:14: error: cannot bind rvalue '(void*)65' to 'void*&'
/usr/include/c++/14/istream:122:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(__istream_type& (*)(__istream_type&)) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  122 |       operator>>(__istream_type& (*__pf)(__istream_type&))
      |       ^~~~~~~~
/usr/include/c++/14/istream:122:7: note:   conversion of argument 1 would be ill-formed:
a.cc:8:14: error: invalid conversion from 'char' to 'std::basic_istream<char>::__istream_type& (*)(std::basic_istream<char>::__istream_type&)' {aka 'std::basic_istream<char>& (*)(std::basic_istream<char>&)'} [-fpermissive]
    8 |       cin >> 'A';
      |              ^~~
      |              |
      |              char
/usr/include/c++/14/istream:126:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(__ios_type& (*)(__ios_type&)) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>; __ios_type = std::basic_ios<char>]' (near match)
  126 |       operator>>(__ios_type& (*__pf)(__ios_type&))
      |       ^~~~~~~~
/usr/include/c++/14/istream:126:7: note:   conversion of argument 1 would be ill-formed:
a.cc:8:14: error: invalid conversion from 'char' to 'std::basic_istream<char>::__ios_type& (*)(std::basic_istream<char>::__ios_type&)' {aka 'std::basic_ios<char>& (*)(std::basic_ios<char>&)'} [-fpermissive]
    8 |       cin >> 'A';
      |              ^~~
      |              |
      |              char
/usr/include/c++/14/istream:133:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(std::ios_base& (*)(std::ios_base&)) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  133 |       operator>>(ios_base& (*__pf)(ios_base&))
      |       ^~~~~~~~
/usr/include/c++/14/istream:133:7: note:   conversi