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<bits/stdc++.h>
#define rep(i,n) for(int i=0;i<n;++i)
#define rrep(i,n) for(int i=n-1;i>=0;--i)
#define yesno(flg) if(flg){cout<<"YES"<<endl;}else{cout<<"NO"<<endl;}
#define MAX_N 1002
#define i197 1000000007

using namespace std;
typedef long long ll;
typedef pair<ll,ll> P1;
typedef pair<int,int> Pi;
typedef pair<double,Pi> Pdi;
typedef pair<ll,int>Pli;
typedef pair<P1,ll> P2;
typedef pair<string,string> Ps;
const ll INF=1000000000000000001;
struct edge{int to,cost;};
int dy[]={0, 0, 1, -1};
int dx[]={1, -1, 0, 0};
struct Road{double cost;int a,b;};
struct pos{
	// 1 変数を入れる;
	int x,y,sx,sy,cost;
};

int main() {

	int n;
	cin>>n;
	rep(i,n){
		long double x1,x2,x3,x4,y1,y2,y3,y4;
		cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
		long double a,b,c,d;
		a=x2-x1;
		b=y2-y1;
		c=x4-x3;
		d=y4-y3;
		if(b/a==d/c)cout<<"YES"<<endl;
		else cout<<"NO"<<endl;
	}

	return 0;
}

C++: #include <iostream>
#include <math.h>
using namespace std;

int main()
{
	double x1, y1, x2, y2, x3, y3, x4, y4;
	double slope_ab, slope_cd;
	int loop;
	int i = 0;

	cin >> loop;

	while(cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4){

		++i;

		slope_ab = (y2-y1) / (x2-x1);
		slope_cd = (y4-y3) / (x4-x3);

		if(fabs(slope_ab - slope_cd) < 1.0E-10){
			cout << "YES" << endl;
		}else{
			cout << "NO" << endl;
		}

		if(i==loop) break;
	}
}

C++: #include <cstdio>
#include <complex>
#include <vector>
 
using namespace std;
 
typedef complex<double> P;
struct L : public vector<P> {
    L(const P &a, const P &b) {
        push_back(a);
        push_back(b);
    }
};
 
const double EPS = 1e-8;
 
double cross(P a, P b)
{
    return imag(conj(a) * b);
}
 
bool intersectLL(const L &l, const L &m)
{
    return fabs(cross(l[1]-l[0], m[1]-m[0])) > EPS;
}
 
int main()
{
    int n;
 
    scanf("%d", &n);
    for (int i = 0; i < n; ++i) {
        P p[4];
        double x, y;
 
        scanf("%lf %lf", &x, &y);
        p[0] = P(x, y);
        scanf("%lf %lf", &x, &y);
        p[1] = P(x, y);
        scanf("%lf %lf", &x, &y);
        p[2] = P(x, y);
        scanf("%lf %lf", &x, &y);
        p[3] = P(x, y);
 
        puts(intersectLL(L(p[0], p[1]), L(p[2], p[3])) ? "NO" : "YES");
    }
 
    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
typedef complex<double> xy_t; 
xy_t a[100], b[100], c[100], d[100];
double x, y;
int n;
int main()
{
	cin >> n;
	for(int i=0; i<n; i++)
	{
		cin >> x >> y;
		a[i]=xy_t(x, y);
		cin >> x >> y;
		b[i]=xy_t(x, y);
		cin >> x >> y;
		c[i]=xy_t(x, y);
		cin >> x >> y;
		d[i]=xy_t(x, y);
	}
	for(int i=0; i<n; i++)
	{
		if((conj(b[i]-a[i])*(d[i]-c[i])).imag()==0)
		{
			cout << "YES" << endl;
		}
		else
		{
			cout << "NO" << endl;
		}
	}
	return 0;
}

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

int main(){

	typedef struct{
		double x, y;
	}_td;

	int n;
	_td p1, p2, p3, p4, v_ab, v_cd;
	double del, eps;
	eps = 0.000001;
	
	cin >> n;
	
	for(int i=0; i<n; i++){
		cin >> p1.x >> p1.y >> p2.x >> p2.y >> p3.x >> p3.y >> p4.x >> p4.y;
	
		v_ab.x = p1.x - p2.x;
		v_ab.y = p1.y - p2.y;
		v_cd.x = p3.x - p3.y;
		v_cd.y = p4.x - p4.y;
		
		del = v_ab.x * v_cd.y - v_ab.y * v_cd.x;
		if(del < eps) cout << "YES" << endl;
		else cout << "NO" << endl;
	}

	return 0;

}

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

int main(void)
{
    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;
        if (((x2==x1)&&(y2==y1)) || (x4==x3)&&(y4==y3))
            cout << "NO" << endl;
        else if ((x2-x1)==0 || (x4-x3)==0)
            cout << (((x2-x1)==0&&(x4-x3)==0)? "YES" : "NO") << endl;
        else
            cout << (((int)((y2-y1)*(x4-x3)) == (int)((y4-y3)*(x2-x1)))? "YES" : "NO") << endl;
    }
    return 0;
}

C++: #define	_USE_MATH_DEFINES
#include <iostream>
#include <algorithm>
#include <functional>
#include <vector>
#include <cstdio>
#include <string>
#include <cmath>
#include <cfloat>
#include <map>
#include <queue>
#include <stack>
#include <list>
using namespace std;

int main(){

	int n;
	cin>>n;
	double x1,y1,x2,y2,x3,y3,x4,y4;
	
	while(n>0){
		double a,b;
	cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
		
		a=(y2-y1)/(x2-x1);
		b=(y4-y3)/(x4-x3);
		
		if(a==b)
			cout<<"YES"<<endl;
		else
			cout<<"NO"<<endl;
		
		n--;
	}
}

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

int main()
{
	int n;
	cin >> n;
	double a[100][4][2];
	for (int i = 0;i < n;i++)
		for (int j = 0;j < 4;j++)
			cin >> a[i][j][0] >> a[i][j][1];
	for (int k = 0;k < n;k++) {
		if (a[k][0][0] - a[k][1][0] == 0 && a[k][2][0] - a[k][3][0])
			cout << "YES" << endl;
		else {
			if ((a[k][2][0] - a[k][3][0])*(a[k][0][1] - a[k][1][1]) == (a[k][0][0] - a[k][1][0])*(a[k][2][1] - a[k][3][1]))
				cout << "YES" << endl;
			else
				cout << "NO" << endl;
		}
	}
    return 0;
}

C++: #include <iostream>
using namespace std;
 
 int main(void){
	 double x1,y1,x2,y2,x3,y3,x4,y4;
	 int n;
   cin >> n;
   for( int i = 0 ; i < n ; i++ ){
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
	  if( (y2-y1)/(x2-x1) == (y4-y3)/(x4-x3) ) cout << "YES" << endl;
    else cout << "NO" << endl;
	 }
   return 0;
 }

C++: include <iostream>
#include <complex>
using namespace std;
typedef complex<double> xy_t;
double cross_product(xy_t a, xy_t b) { return (conj(a)*b).imag(); }
const double eps = 1e-11;
double x[4],y[4];
int N;
int main(int argc, const char * argv[]) {
    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])
        };
        bool p = abs(cross_product(a[0], a[1])) < eps;
        cout << (p ? "YES" : "NO") << endl;
        }
    return 0;
}