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>
 
using namespace std;
 
typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef complex<double> P;
typedef pair<int,int> pii;
#define REP(i,n) for(ll i=0;i<n;++i)
#define REPR(i,n) for(ll i=1;i<n;++i)
#define FOR(i,a,b) for(ll i=a;i<b;++i)
 
#define DEBUG(x) cout<<#x<<": "<<x<<endl
#define DEBUG_VEC(v) cout<<#v<<":";REP(i,v.size())cout<<" "<<v[i];cout<<endl
#define ALL(a) (a).begin(),(a).end()
 
#define MOD (ll)(1e9+7)
#define ADD(a,b) a=((a)+(b))%MOD
#define FIX(a) ((a)%MOD+MOD)%MOD
 
int main(){
int n;
cin>>n;
double x1,y1,x2,y2,x3,y3,x4,y4;
while(~scanf("%lf%lf%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3,&x4,&y4)){
P a(x2-x1,y2-y1);
P b(x4-x3,y4-y3);
double cr = imag(conj(a)*b);
if(abs(cr)<1e-9){
  cout<<"YES"<<endl;
}else{
  cout<<"NO"<<endl;
}
}
  return 0;
}

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

int main()
{
  int n;
  cin >> n;
  for (int i = 0; i < n; i++){
    double x1, x2, x3, x4, y1, y2, y3, y4, 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;
  }
}

C++: #include<iostream>
#include<queue>
#include<string>
#include<stack>
#include<cstdio>
#include<algorithm>

using namespace std;

int main(void)
{
  double x,y;
  double x1,y1;
  int n;
  int i;
  long double a=0,b=0;
  int k=0;
  int j=0;

  cin>>n;

  for(i=0;i<n;i++)
    {
      cin>>x>>y>>x1>>y1;
      if(x1-x!=0)
	{
          a=(y1-y)/(x1-x);
          j++;
	}
      else
        k++;

      cin>>x>>y>>x1>>y1;
      if(x1-x!=0)
	{
          b=(y1-y)/(x1-x);
          j++;
	}
      else
        k++;

      if((a==b && j==2) || k==2)
        puts("YES");
      else
        puts("NO");
    }
}

C++: #include <cstdio>
#include <cstring>
#include <string>
#include <cmath>
#include <cassert>
#include <iostream>
#include <algorithm>
#include <stack>
#include <queue>
#include <vector>
#include <set>
#include <map>
#include <bitset>
#include <functional>
#include <sstream>
#include <utility>
#include <cstdlib>
#include <cctype>
#include <climits>
#include <fstream>

using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<double> vd;
typedef vector<pair<int, int> > vpii;
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define rep(i,n) for(int i=0;i<(n);++i)
#define foreach(itr,c) for(__typeof(c.begin()) itr=c.begin(); itr!=c.end(); itr++)
#define dbg(x) cout << #x"="<< (x) << endl
#define mp(a,b) make_pair((a),(b))
#define pb(a) push_back(a) 
#define in(x) cin >> x;
#define all(x) (x).begin(), (x).end()
#define INF 2147483600
#define fi first
#define se second
using namespace std;

int main(){
	int N;
	vd a(8);
	cin >> N;
	rep(i,N){
		rep(i,8){
			cin>>a[i];
		}
		if((a[1]-a[3])*(a[4]-a[6])==(a[5]-a[7])*(a[0]-a[2])){
			cout<<"YES"<<endl;
		}else{
			cout<<"NO"<<endl;
		}
	}

}

C++: #include <iostream>
 
int main()
{
    int n;
    std::cin >> n;
 
    for (int i = 0; i < n; ++i) {
        double x1, y1, x2, y2, x3, y3, x4, y4;
        std::cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
        double abx = x2 - x1, aby = y2 - y1, cdx = x4 - x3, cdy = y4 - y3;
        if (abx * cdy - aby * cdx == 0.0) std::cout << "YES" << std::endl;
        else std::cout << "NO" << std::endl;
    }
}

C++: #include <stdio.h>

int main(){
    int n,i,j;
    double X[5],Y[5];
    long x[5],y[5];
    scanf("%d",&n);
    for(i=0;i<n;i++){
        for(j=1;j<=4;j++){
            scanf("%lf %lf",&X[j],&Y[j]);
            x[j] = (long)(X[j]*100000);
            y[j] = (long)(Y[j]*100000);
        }
        if((x[2]-x[1])*(y[4]-y[3])==(x[4]-x[3])*(y[2]-y[1])){
            if((y[2]-y[1])*(x[3]-x[1])==(x[2]-x[1])*(y[3]-y[1])){
                printf("NO\n");
            }else{
                printf("YES\n");
            }
        }else{
            printf("NO\n");
        }
    }
}

C++: #include <cassert>
#include <cmath>
#include <iostream>
#include <vector>

struct Point
{
    double x;
    double y;
};
struct Line
{
    Line(const Point& a, const Point& b)
    : p1(a), p2(b) {}
    Point p1;
    Point p2;
};
double GetSlope(const Line& line)
{
    return
    (line.p2.y - line.p1.y) /
    (line.p2.x - line.p1.x);
}
bool IsParallel(const Line& l1, const Line&l2)
{
    // std::cerr << GetSlope(l1) << std::endl;
    // std::cerr << GetSlope(l2) << std::endl;
    return std::fabs(GetSlope(l1) - GetSlope(l2)) < 1e-10;
}
bool IsParallel(const std::vector<Point>& points)
{
    assert(points.size() == 4);
    return IsParallel(Line(points[0], points[1]), Line(points[2], points[3]));
}
std::vector<Point> Input(std::istream& is)
{
    std::vector<Point> points(4);
    for(int i = 0; i < points.size(); ++i){
        is >> points[i].x >> points[i].y;
    }
    return points;
}
int main()
{
    int n;
    std::cin >> n;
    for(int i = 0; i < n; ++i) {
        std::vector<Point> points = Input(std::cin);
        std::cout << (IsParallel(points) ? "YES" : "NO") << std::endl; 
    }
    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;

using db = double;
const db eps = 1e-9;

struct poi {
	db x, y;
	void r() { cin >> x >> y; }
	poi operator -(poi p) {
		return {x - p.x, y - p.y};
	}
	db cross(poi p) {
		return x * p.y - y * p.x;
	}
};

int main() {
	ios :: sync_with_stdio(false);
	poi a, b, c, d;
	while(cin >> a.x >> a.y) {
		b.r(); c.r(); d.r();
		db t = (b - a).cross(d - c);
		if(t > -eps && t < eps)
			cout << "YES\n";
		else
			cout << "NO\n";
	}
	return 0;
}

C++: #include<iostream>
#define rep(i,x) for(int i=0;i<x;i++)
using namespace std;
int main(){
    int n;
    cin>>n;
    while(n--){
        double a[4][2];
        rep(i,4)rep(j,2)cin>>a[i][j];
        double line1=(a[0][0]-a[1][0])/(a[0][1]-a[1][1]);
        double line2=(a[2][0]-a[3][0])/(a[2][1]-a[3][1]);
        if(line1==line2)cout<<"YES"<<endl;
        else cout<<"NO"<<endl;
    }
}

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

float han(float a, float b, float c, float d){
	return (d - b) / (c - a);
}

int main(){
	int n;
	float ax, ay, bx, by, cx, cy, dx, dy, A, B;
	cin >> n;
	for(int i=0;i<n;i++){
		cin >> ax >> ay >> bx >> by >> cx >> cy >> dx >> dy;
		if(ax == bx && cx == dx){
			cout << "YES" << endl;
			continue;
		}
		if(ax == bx || cx == dx){
			cout << "NO" << endl;
			continue;
		}
		A = han(ax, ay, bx, by);
		B = han(cx, cy, dx, dy);
		if (A==B){
			cout << "YES" << endl;
		}
		else{
			cout << "NO" << endl;
		}
	}
	return 0;
}

C++: #include <cstdio>
#include <cstdlib>
#include <iostream>
#include <algorithm>
#include <cstring>
#include <string.h>
#include <queue>
#include <stack>
#include <utility>
#include <queue>
#define mp make_pair
#define pii pair<int,int> 
#define ff first
#define ss second

using namespace std;

double x1,x2,x3,x4,y1,y2,y3,y4;
double g1,g2;
int tc;

int main()
{
	scanf("%d",&tc);
	while(tc--)
	{
		//scanf("%lf %lf %lf %lf %lf %lf %lf %lf",&x1,&y1,&x2,&y2,&x3,&y3,&x4,&y4);
		cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
		g1=(y2-y1)/(x2-x1);
		g2=(y4-y3)/(x4-x3);
		if(g1==g2)
		{
			printf("YES\n");
		}
		else
		{
			printf("NO\n");
		}
		//printf("%.2lf,%.2lf\n",g1,g2);
	}
	return 0;
}