Go: package main

import "fmt"

func abs(x float64) float64 {
	if x < 0 {
		return -x
	}
	return x
}

func main() {
	var n int
	var x1, y1, x2, y2, x3, y3, x4, y4 float64
	fmt.Scan(&n)
	for i := 0 ; i < n; i++ {
		fmt.Scanf("%f %f %f %f %f %f %f %f", &x1, &y1, &x2, &y2, &x3, &y3, &x4, &y4)
		if abs((x2-x1)*(y4-y3) - (y2-y1)*(x4-x3)) < 1e-10 {
			fmt.Println("YES")
		} else {
			fmt.Println("NO")
		}
	}
}

C++: #include <iostream>
using namespace std;
int main(){
	int n , i , j ;
	double x1, x2 , x3 , x4 , y1 , y2 , y3 , y4 ;
	cin >> n ;
	double ab , cd ;
	for( i=1 ; i<=n ; i++ ){
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4 ;
		ab = ( y2 - y1 ) / ( x2 - x1 );
		cd = ( y4 - y3 ) / ( x4 - x3 );
		if( ab==cd )
			cout <<"YES" << endl;
		else 
			cout << "NO" << endl;
	}
}

C++: #include<stdio.h>
#include<math.h>
int main(void)
{
	int n;
	int i,j;
	double x[5],y[5];
	double xx,yy;
	
	scanf("%d",&n);
	for(i=0;i<n;i++){
		for(j=0;j<4;j++){
			scanf("%lf %lf",&x[j],&y[j]);
		}
		if(x[0]-x[1]==0 || x[2]-x[3]==0){
			if(x[0]-x[1]==0 && x[2]-x[3]==0){
				printf("YES\n");
			}
			else {
				printf("NO\n");
			}
		}
		else {
			xx=(y[0]-y[1])/(x[0]-x[1]);
			yy=(y[2]-y[3])/(x[2]-x[3]);
//			printf("%lf %lf\n",xx,yy);
			xx=xx-yy;		
			if(xx<0.00001 || xx>-0.00001){
				printf("YES\n");
			}
			else {
				printf("NO\n");
			}
		}
	}	
	return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <cstdio>
using namespace std;

int main() {
	int n;
	double x1,x2,x3,x4,y1,y2,y3,y4,m,l;
	cin>>n;
	for(int i=0;i<n;i++){
		cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
		m=(y2-y1)/(x2-x1);
		l=(y4-y3)/(x4-x3);
		if(m==l)cout<<"YES"<<endl;
		else cout<<"NO"<<endl;
	}
	return 0;
}

C++: #include <complex>
#include <cmath>
#include <iostream>
#include <stdio.h>

using namespace std;
typedef complex<double> xy_t;

const double eps= 0.000000000001;
double x[4], y[4];
int N;
int main(){
  cin >> N;
  for(int t= 0; t<N; ++t){
    for (int i = 0; i < 4; ++i) cin >> x[i] >> y[i];
    xy_t a[2]={xy_t(x[0],y[0])-xy_t(x[1],y[1]), xy_t(x[2],y[2])-xy_t(x[3],y[3])};
    xy_t x=a[0],y=a[1];
    bool p = abs(x.real()*y.imag()-x.imag()*y.real()) < eps;
    cout << ( p ? "YES" : "NO" ) << endl ;
  }

}

C++: #include <bits/stdc++.h>
using namespace std;
#define FOR(i, s, n) for(int i = s; i < (int)n; i++)
#define per(i, n) for(int i = n; i >= 0; i--)
#define ROF(i, s, n) for(int i = s; i >= (int)n; i--)
#define FORIT(i, A) for (auto i : A)
#define PRINT(x) cout << (x) << "\n"
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define MP make_pair
#define EACH(i, n) for (__typeof((n).begin()) i = (n).begin(); i != (n).end(); ++i)
#define SZ(a) int((a).size())
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())
#define CLR(a) memset((a), 0 ,sizeof(a))
#define dump(x) cout << #x << " = " << (x) << "\n";
#define debug(x) cout << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << "\n";
#define sq(n) (n) * (n)
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef priority_queue<int> maxpq;
typedef priority_queue< int, vector<int>, greater<int> > minpq;
static const double EPS = 1e-10;
static const double PI = acos( -1.0 );
static const int mod = 10007;
static const int INF = 1 << 25;
static const LL LL_INF = 1152921504606846976;
static const int dx[] = { -1, 0, 1, 0,  1, -1,  1, -1 };
static const int dy[] = { 0, -1, 0, 1 , 1,  1, -1, -1 };

int n;
double x, x2, x3, x4, y, y2, y3, y4;

int main() {
	scanf( "%d", &n );
	for ( int i = 0; i < n; i++ ) {
		scanf( "%lf %lf %lf %lf %lf %lf %lf %lf", &x, &y, &x2, &y2, &x3, &y3, &x4, &y4 );
		if ( fabs( ( x2 - x ) * ( y4 - y3 ) - ( x4 - x3 ) * ( y2 - y ) ) < EPS ) {
			printf( "YES\n" );
		}
		else {
			printf( "NO\n" );
		}
	}
	return 0;
}

C++: #include <iostream>
using namespace std;
int main()
{
	int n, i, j;
	float x[4], y[4];

	cin >> n;

	for (i = 0; i < n; i++)
	{
		for (j = 0; j < 4; j++)
			cin >> x[j] >> y[j];

		if (((y[1]-y[0])/(x[1]-x[0])) == ((y[3]-y[2])/(x[3]-x[2])))
			cout << "YES" << endl;
		else
			cout << "NO" << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <complex>
using namespace std;
int main() {
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) {
    double x1, y1, x2, y2, x3, y3, x4, y4;
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    complex<double> c1(x1, y1);
    complex<double> c2(x2, y2);
    complex<double> c3(x3, y3);
    complex<double> c4(x4, y4);
    double a1 = arg(c2 - c1);
    double a2 = arg(c4 - c3);
    cout << ((a1 - a2 == 0 || a1 + a2 == 0) ? "YES" : "NO") << endl;
  }
}