#include <iostream>#include <algorithm>#include <vector>#include <fstream>#include <cmath>using namespace std; struct Point {      double x, y;};//计算叉乘，平面上的点，所以向量总是沿z轴方向double cross(const Point &A, const Point &B, const Point &C) {    return (B.x - A.x) * (C.y - A.y) - (B.y - A.y) * (C.x - A.x);}//默认以(0,0)为向量起点，循环计算各三角形的面积double polygon_area(vector<Point>& v){    double area=0;    auto it=v.begin();    for(;it<v.end();++it){        area+=cross({0,0},*it,*(it+1));    }    return abs(area/2.0);}int main(){fstream fs("C:\\Documents and Settings\\Administrator\\桌面\\poly.txt");Point tmp;vector<Point> v;while(!fs.eof()){fs>>tmp.x>>tmp.y;v.push_back(tmp);}double area=polygon_area(v);cout<<"多边形面积为："<<area<<endl;return 0;}