Language: C++

#include<iostream>
#include<algorithm>

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

Compiler output:
a.cc: In function 'int main()':
a.cc:6:3: error: 'cin' was not declared in this scope; did you mean 'std::cin'?
    6 |   cin >> b;
      |   ^~~
      |   std::cin
In file included from a.cc:1:
/usr/include/c++/14/iostream:62:18: note: 'std::cin' declared here
   62 |   extern istream cin;           ///< Linked to standard input
      |                  ^~~
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/iostream:41:
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
a.cc:10:5: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   10 |     cout >> 'A' >> 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:10:20: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   10 |     cout >> 'A' >> endl;
      |                    ^~~~
      |                    std::endl
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
a.cc:12:5: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   12 |     cout >> 'C' >> 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:12:20: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   12 |     cout >> 'C' >> endl;
      |                    ^~~~
      |                    std::endl
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
a.cc:14:5: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   14 |     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:14:20: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   14 |     cout >> 'G' >> endl;
      |                    ^~~~
      |                    std::endl
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~