Language: C++

#include <bits/stdc++.h>
#include <unordered_map>
typedef long long ll;
#define rep(i,m,n) for(ll i = (m); i < (n); i++)
#define rrep(i,m,n) for(ll i = (m); i >= (n); i--)
#define print(x) cout << (x) << endl;
#define print2(x,y) cout << (x) << " " << (y) << endl;
#define printa(x,n) for(ll i = 0; i < n; i++){ cout << (x[i]) << " \n"[i == n-1];}
#define printp(x,n) for(ll i = 0; i < n; i++){ cout << "(" << x[i].first << ", " << x[i].second << ") "; } cout << endl;
#define INF (1e18)
using namespace std;
const ll MOD = 1e9 + 7;
typedef pair<ll, ll> lpair;
ll f(ll n){
    return (n*n + 4) / 8;
}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    string s;
    cin >> s;
    rep(i,0,s.size()){
        if(i % 2 == 0) cout >> s[i];
    }
    cout << endl;

}

Compiler output:
a.cc: In function 'int main()':
a.cc:24:29: error: no match for 'operator>>' (operand types are 'std::ostream' {aka 'std::basic_ostream<char>'} and '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'})
   24 |         if(i % 2 == 0) cout >> s[i];
a.cc:24:29: note: candidate: 'operator>>(int, int)' (built-in)
a.cc:24:29: note:   no known conversion for argument 1 from 'std::ostream' {aka 'std::basic_ostream<char>'} to 'int'
In file included from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:41,
                 from a.cc:1:
/usr/include/c++/14/cstddef:131:5: note: candidate: 'template<class _IntegerType> constexpr std::__byte_op_t<_IntegerType> std::operator>>(byte, _IntegerType)'
  131 |     operator>>(byte __b, _IntegerType __shift) noexcept
      |     ^~~~~~~~
/usr/include/c++/14/cstddef:131:5: note:   template argument deduction/substitution failed:
a.cc:24:24: note:   cannot convert 'std::cout' (type 'std::ostream' {aka 'std::basic_ostream<char>'}) to type 'std::byte'
   24 |         if(i % 2 == 0) cout >> s[i];
      |                        ^~~~
In file included from /usr/include/c++/14/string:55,
                 from /usr/include/c++/14/bitset:52,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:52:
/usr/include/c++/14/bits/basic_string.tcc:835:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::basic_istream<_CharT, _Traits>& std::operator>>(basic_istream<_CharT, _Traits>&, __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
  835 |     operator>>(basic_istream<_CharT, _Traits>& __in,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.tcc:835:5: note:   template argument deduction/substitution failed:
a.cc:24:35: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::basic_istream<_CharT, _Traits>'
   24 |         if(i % 2 == 0) cout >> s[i];
      |                                   ^
/usr/include/c++/14/bitset:1597:5: note: candidate: 'template<class _CharT, class _Traits, long unsigned int _Nb> std::basic_istream<_CharT, _Traits>& std::operator>>(basic_istream<_CharT, _Traits>&, bitset<_Nb>&)'
 1597 |     operator>>(std::basic_istream<_CharT, _Traits>& __is, bitset<_Nb>& __x)
      |     ^~~~~~~~
/usr/include/c++/14/bitset:1597:5: note:   template argument deduction/substitution failed:
a.cc:24:35: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::basic_istream<_CharT, _Traits>'
   24 |         if(i % 2 == 0) cout >> s[i];
      |                                   ^
In file included from /usr/include/c++/14/istream:1109,
                 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/bits/istream.tcc:978:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(basic_istream<_CharT, _Traits>&, _CharT&)'
  978 |     operator>>(basic_istream<_CharT, _Traits>& __in, _CharT& __c)
      |     ^~~~~~~~
/usr/include/c++/14/bits/istream.tcc:978:5: note:   template argument deduction/substitution failed:
a.cc:24:35: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::basic_istream<_CharT, _Traits>'
   24 |         if(i % 2 == 0) cout >> s[i];
      |                                   ^
/usr/include/c++/14/istream:849:5: note: candidate: 'template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(basic_istream<char, _Traits>&, unsigned char&)'
  849 |     operator>>(basic_istream<char, _Traits>& __in, unsigned char& __c)
      |     ^~~~~~~~
/usr/include/c++/14/istream:849:5: note:   template argument deduction/substitution failed:
a.cc:24:35: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::basic_istream<char, _Traits>'
   24 |         if(i % 2 == 0) cout >> s[i];
      |                                   ^
/usr/include/c++/14/istream:854:5: note: candidate: 'template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(basic_istream<char, _Traits>&, signed char&)'
  854 |     operator>>(basic_istream<char, _Traits>& __in, signed char& __c)
      |     ^~~~~~~~
/usr/include/c++/14/istream:854:5: note:   template argument deduction/substitution failed:
a.cc:24:35: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::basic_istream<char, _Traits>'
   24 |         if(i % 2 == 0) cout >> s[i];
      |                                   ^
/usr/include/c++/14/istream:896:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(basic_istream<_CharT, _Traits>&, _CharT*)'
  896 |     operator>>(basic_istream<_CharT, _Traits>& __in, _CharT* __s)
      |     ^~~~~~~~
/usr/include/c++/14/istream:896:5: note:   template argument deduction/substitution failed:
a.cc:24:35: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::basic_istream<_CharT, _Traits>'
   24 |         if(i % 2 == 0) cout >> s[i];
      |                                   ^
/usr/include/c++/14/istream:939:5: note: candidate: 'template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(basic_istream<char, _Traits>&, unsigned char*)'
  939 |     operator>>(basic_istream<char, _Traits>& __in, unsigned char* __s)
      |     ^~~~~~~~
/usr/include/c++/14/istream:939:5: note:   template argument deduction/substitution failed:
a.cc:24:35: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::basic_istream<char, _Traits>'
   24 |         if(i % 2 == 0) cout >> s[i];
      |                                   ^
/usr/include/c++/14/istream:945:5: note: candidate: 'template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(basic_istream<char, _Traits>&, signed char*)'
  945 |     operator>>(basic_istream<char, _Traits>& __in, signed char* __s)
      |     ^~~~~~~~
/usr/include/c++/14/istream:945:5: note:   template argument deduction/substitution failed:
a.cc:24:35: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::basic_istream<char, _Traits>'
   24 |         if(i % 2 == 0) cout >> s[i];
      |                                   ^
/usr/include/c++/14/istream:1099:5: note: candidate: 'template<class _Istream, class _Tp> _Istream&& std::operator>>(_Istream&&, _Tp&&)'
 1099 |     operator>>(_Istream&& __is, _Tp&& __x)
      |     ^~~~~~~~
/usr/include/c++/14/istream:1099:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/istream: In substitution of 'template<class _Istream, class _Tp> _Istream&& std::operator>>(_Istream&&, _Tp&&) [with _Istream = std::basic_ostream<char>&; _Tp = char&]':
a.cc:24:35:   required from here
   24 |         if(i % 2 == 0) cout >> s[i];
      |                                   ^
/usr/include/c++/14/istream:1099:5: error: no type named 'type' in 'struct std::enable_if<false, void>'
 1099 |     operator>>(_Istream&& __is, _Tp&& __x)
      |     ^~~~~~~~
/usr/include/c++/14/complex:509:5: note: candidate: 'template<class _Tp, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(basic_istream<_CharT, _Traits>&, complex<_Tp>&)'
  509 |     operator>>(basic_istream<_CharT, _Traits>& __is, complex<_Tp>& __x)
      |     ^~~~~~~~
/usr/include/c++/14/complex:509:5: note:   template argument deduction/substitution failed:
a.cc:24:35: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::basic_istream<_CharT, _Traits>'
   24 |         if(i % 2 == 0) cout >> s[i];
      |                                   ^
In file included from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:143:
/usr/include/c++/14/iomanip:76:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(basic_istream<_CharT, _Traits>&, _Resetiosflags)'
   76 |     operator>>(basic_istream<_CharT, _Traits>& __is, _Resetiosflags __f)
      |     ^~~~~~~~
/usr/include/c++/14/iomanip:76:5: note:   template argument deduction/substitution failed:
a.cc:24:35: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::basic_istream<_CharT, _Traits>'
   24 |         if(i % 2 == 0) cout >> s[i];
      |                                   ^
/usr/include/c++/14/iomanip:106:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(basic_istream<_CharT, _Traits>&, _Setiosflags)'
  106 |     operator>>(basic_istream<_CharT, _Traits>& __is, _Setiosflags __f)
      |     ^~~~~~~~
/usr/include/c++/14/iomanip:106:5: note:   template argument deduction/substitution failed:
a.cc:24:35: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::basic_istream<_CharT, _Traits>'
   24 |         if(i % 2 == 0) cout >> s[i];
      |                                   ^
/usr/include/c++/14/iomanip:137:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(basic_istream<_CharT, _Traits>&, _Setbase)'
  137 |     operator>>(basic_istream<_CharT, _Traits>& __is, _Setbase __f)
      |     ^~~~~~~~
/usr/include/c++/14/iomanip:137:5: note:   template argument deduction/substitution failed:
a.cc:24:35: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::basic_istream<_CharT, _Traits>'
   24 |         if(i % 2 == 0) cout >> s[i];
      |                                   ^
/usr/include/c++/14/iomanip:177:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(basic_istream<_CharT, _Traits>&, _Setfill<_CharT>)'
  177 |     operator>>(basic_istream<_CharT, _Traits>& __is, _Setfill<_CharT> __f)
      |     ^~~~~~~~
/usr/include/c++/14/iomanip:177:5: note:   template argument deduction/substitution failed:
a.cc:24:35: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::basic_istream<_CharT, _Traits>'
   24 |         if(i % 2 == 0) cout >> s[i];
      |