Language: C++

#include <bits/stdc++.h>
int main() {
  
  string b;
  cin >> b;
  
  if (b == "A") {
    cout << "T" << endl;
  }
  if else (b == "T") {
    cout << "A" << endl;
  }
  if else (b == "G") {
    cout << "c" << endl;
  }
  else {
    cout << "G" << endl;
  }
}
    

Compiler output:
a.cc: In function 'int main()':
a.cc:4:3: error: 'string' was not declared in this scope
    4 |   string b;
      |   ^~~~~~
a.cc:4:3: note: suggested alternatives:
In file included from /usr/include/c++/14/string:41,
                 from /usr/include/c++/14/bitset:52,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:52,
                 from a.cc:1:
/usr/include/c++/14/bits/stringfwd.h:77:33: note:   'std::string'
   77 |   typedef basic_string<char>    string;
      |                                 ^~~~~~
/usr/include/c++/14/string:76:11: note:   'std::pmr::string'
   76 |     using string    = basic_string<char>;
      |           ^~~~~~
a.cc:5:3: error: 'cin' was not declared in this scope; did you mean 'std::cin'?
    5 |   cin >> b;
      |   ^~~
      |   std::cin
In file included from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:146:
/usr/include/c++/14/iostream:62:18: note: 'std::cin' declared here
   62 |   extern istream cin;           ///< Linked to standard input
      |                  ^~~
a.cc:5:10: error: 'b' was not declared in this scope
    5 |   cin >> b;
      |          ^
a.cc:8:5: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
    8 |     cout << "T" << endl;
      |     ^~~~
      |     std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:8:20: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
    8 |     cout << "T" << endl;
      |                    ^~~~
      |                    std::endl
In file included from /usr/include/c++/14/istream:41,
                 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:
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
a.cc:10:6: error: expected '(' before 'else'
   10 |   if else (b == "T") {
      |      ^~~~
      |      (
a.cc:13:6: error: expected '(' before 'else'
   13 |   if else (b == "G") {
      |      ^~~~
      |      (
a.cc:16:3: error: 'else' without a previous 'if'
   16 |   else {
      |   ^~~~
a.cc:17:5: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   17 |     cout << "G" << endl;
      |     ^~~~
      |     std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:17:20: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   17 |     cout << "G" << endl;
      |                    ^~~~
      |                    std::endl
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~