Language: C++

#include<iostream>
#include<cmath>
using namespace std;
#define EPS 1e-10
double x1,y1,x2,y2,x3,y3,x4,y4;
double det(){
	double dx1,dy1,dx2,dy2;
	dx1 = x1-x2;
	dy1 = y1-y2;
	dx2 = x3-x4;
	dy2 = y3-y4;
	return dx1*dy2 - dy1*dx2;
}
int main(){
	int n;
	cin>>n;
	while(n--){
		cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
		cout<<(fabs(det())<EPS?"YES":"NO")<<endl;
	}
	return 0;
}

Compiler output:
a.cc:5:11: error: 'double y1' redeclared as different kind of entity
    5 | double x1,y1,x2,y2,x3,y3,x4,y4;
      |           ^~
In file included from /usr/include/features.h:523,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/os_defines.h:39,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/c++config.h:683,
                 from /usr/include/c++/14/bits/requires_hosted.h:31,
                 from /usr/include/c++/14/iostream:38,
                 from a.cc:1:
/usr/include/x86_64-linux-gnu/bits/mathcalls.h:257:1: note: previous declaration 'double y1(double)'
  257 | __MATHCALL (y1,, (_Mdouble_));
      | ^~~~~~~~~~
a.cc: In function 'double det()':
a.cc:9:17: error: invalid operands of types 'double(double) noexcept' and 'double' to binary 'operator-'
    9 |         dy1 = y1-y2;
      |               ~~^~~
      |               |  |
      |               |  double
      |               double(double) noexcept
a.cc: In function 'int main()':
a.cc:18:24: error: no match for 'operator>>' (operand types are 'std::basic_istream<char>::__istream_type' {aka 'std::basic_istream<char>'} and 'double(double) noexcept')
   18 |                 cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
      |                 ~~~~~~~^~~~
      |                    |     |
      |                    |     double(double) noexcept
      |                    std::basic_istream<char>::__istream_type {aka std::basic_istream<char>}
In file included from /usr/include/c++/14/iostream:42:
/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:18:26: error: cannot bind non-const lvalue reference of type 'bool&' to a value of type 'double (*)(double) noexcept'
   18 |                 cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
      |                          ^~
/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:18:26: error: invalid conversion from 'double (*)(double) noexcept' to 'short int' [-fpermissive]
   18 |                 cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
      |                          ^~
      |                          |
      |                          double (*)(double) noexcept
a.cc:18:26: error: cannot bind rvalue '(short int)y1' to 'short int&'
/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:18:26: error: invalid conversion from 'double (*)(double) noexcept' to 'short unsigned int' [-fpermissive]
   18 |                 cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
      |                          ^~
      |                          |
      |                          double (*)(double) noexcept
a.cc:18:26: error: cannot bind rvalue '(short unsigned int)y1' to 'short unsigned int&'
/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:18:26: error: invalid conversion from 'double (*)(double) noexcept' to 'int' [-fpermissive]
   18 |                 cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
      |                          ^~
      |                          |
      |                          double (*)(double) noexcept
a.cc:18:26: error: cannot bind rvalue '(int)y1' to 'int&'
/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:18:26: error: invalid conversion from 'double (*)(double) noexcept' to 'unsigned int' [-fpermissive]
   18 |                 cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
      |                          ^~
      |                          |
      |                          double (*)(double) noexcept
a.cc:18:26: error: cannot bind rvalue '(unsigned int)y1' to 'unsigned int&'
/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:18:26: error: invalid conversion from 'double (*)(double) noexcept' to 'long int' [-fpermissive]
   18 |                 cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
      |                          ^~
      |                          |
      |                          double (*)(double) noexcept
a.cc:18:26: error: cannot bind rvalue '(long int)y1' to 'long int&'
/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:18:26: error: invalid conversion from 'double (*)(double) noexcept' to 'long unsigned int' [-fpermissive]
   18 |                 cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
      |                          ^~
      |                          |
      |                          double (*)(double) noexcept
a.cc:18:26: error: cannot bind rvalue '(long unsigned int)y1' to 'long unsigned int&'
/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:18:26: error: invalid conversion from 'double (*)(double) noexcept' to 'long long int' [-fpermissive]
   18 |                 cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
      |                          ^~
      |                          |
      |                          double (*)(double) noexcept
a.cc:18:26: error: cannot bind rvalue '(long long int)y1' to 'long long int&'
/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:18:26: error: invalid conversion from 'double (*)(double) noexcept' to 'long long unsigned int' [-fpermissive]
   18 |                 cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
      |                          ^~
      |                          |
      |                          double (*)(double) noexcept
a.cc:18:26: error: cannot bind rvalue '(long long unsigned int)y1' to 'long long unsigned int&'
/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:18:26: error: invalid conversion from 'double (*)(double) noexcept' to 'void*' [-fpermissive]
   18 |                 cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
      |                          ^~
      |                          |
      |                          double (*)(double) noexcept
a.cc:18:26: error: cannot bind rvalue '(void*)y1' 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:18:26: error: invalid conversion from 'double (*)(double) noexcept' to 'std::basic_istream<char>::__istream_type& (*)(std::basic_istream<char>::__istream_type&)' {aka 'std::basic_istream<char>& (*)(std::basic_istream<char>&)'} [-fpermissive]
   18 |                 cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
      |                          ^~
      |                          |
      |                          double (*)(double) noexcept
/usr/includ