Go: package main

import (
	"bufio"
	"os"
	"strconv"
	"fmt"
	"strings"
)

var scanner = bufio.NewScanner(os.Stdin)

func nextInt() int {
	scanner.Scan()
	i, e := strconv.Atoi(scanner.Text())
	if e != nil {
		panic(e)
	}
	return i
}

func main() {
	n := nextInt()
	for i := 0; i < n; i++ {
		scanner.Scan()
		points := strings.Split(scanner.Text(), " ")
		fpoints := make([]float64, 0)
		for _, v := range points {
			tmp, _ := strconv.ParseFloat(v, 64)
			fpoints = append(fpoints, tmp)
		}
		a := (fpoints[1]-fpoints[3])/(fpoints[0]-fpoints[2])
		b := (fpoints[5]-fpoints[7])/(fpoints[4]-fpoints[6])
		if a == b {
			fmt.Println("YES")
		} else {
			fmt.Println("NO")
		}
	}
}

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;
}

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

struct Point
{
    float x;
    float 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 << std::fixed << std::setprecision(100) << GetSlope(l1) << std::endl;
    // std::cerr << std::fixed << std::setprecision(100) << GetSlope(l2) << std::endl;
    return GetSlope(l1) == GetSlope(l2);
    // const double gosa = 1e-10;
    // double Slope1 = GetSlope(l1);
    // double Slope2 = GetSlope(l2);
    // return std::fabs(Slope1 - Slope2) < gosa;
}
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> p = Input(std::cin);
        // std::cout << (IsParallel(p) ? "YES" : "NO") << std::endl; 
        if((p[1].y-p[0].y)/(p[1].x-p[0].x) == (p[3].y-p[2].y)/(p[3].x-p[2].x)){
            std::cout << "YES";
        }else{
            std::cout << "NO";
        }
        std::cout << std::endl;
    }
    return 0;
}

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

int main()
{
    double a,s,d,f,g,h,j,k;
    double q,w,e,r;
        int n;
    cin>>n;
    for(int i=0; i<n; i++){
        cin>>a>>s>>d>>f>>g>>h>>j>>k;
        q=d-a;
    w=f-s;
    e=j-g;
    r=k-h;
        if(q*r==w*e&&q!=0&&w!=0&&e!=0&&r!=0)cout<<"YES"<<endl;
        else if(q==0&&e==0&&w==r)cout<<"YES"<<endl;
        else if(w==0&&r==0&&q==e)cout<<"YES"<<endl;
        else  cout<<"NO"<<endl;
    }
    return 0;
}

C++: #include<string>
#include<vector>
#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cstdlib>
#include<sstream>
#include<functional>
#include<map>
#include<stack>
#include<set>
#include<cmath>
 
#define REP(i,s,e) for(int i=int(s);i<=int(e);i++)
#define rep(i,n) for(int i=0;i<int(n);i++)
 
#define EPS 1e-6
 
using namespace std;
 
int main(){
 
    int n;
    double x1,y1,x2,y2,x3,y3,x4,y4;
 
    cin >> n;
 
    rep(i,n){
 
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
 
    double a=y2-y1; double b=x2-x1;
    double d=y4-y3; double e=x4-x3;
 
    if(a*e-d*b==0) cout << "YES" << endl;
    else cout << "NO" << endl;
 
    }
 
    return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <cmath>
#include <vector>
#include <deque>
#include <list>
#include <string>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <algorithm>

using namespace std;

int n;
double x1,x2,x3,x4,ya1,y2,y3,y4;

int main() {
	cin >> n;

	for (int i = 0; i < n; i++) {
		cin >> x1 >> ya1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		if (x1 > x2) {
			double tmp = x1;
			x1 = x2;
			x2 = tmp;
			tmp = ya1;
			ya1 = y2;
			y2 = tmp;
		}
		if (x3 > x4) {
			double tmp = x3;
			x3 = x4;
			x4 = tmp;
			tmp = y3;
			y3 = y4;
			y4 = tmp;
		}

		if (x1 == x2) {
			if (x3 == x4) cout << "YES" << endl;
			else cout << "NO" << endl;
		} else {
			if (x3 == x4) cout << "NO" << endl;
			else {
				if ((y2 - ya1) / (x2 - x1) - (y4 - y3) / (x4 - x3) <= 0.000000001)
					cout << "YES" << endl;
				else cout << "NO" << endl;
			}
		}
	}

	return 0;
}

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