linear_regression.cpp

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

vector<double> x, y;
int n;

const double eps = 0.001;
void printPoly(int n, double a[]) {
    cout << fixed << setprecision(3);
    while (n >= 0 && fabs(a[n]) < eps) {
        n--;
    }
    if (n < 0) {
        cout << 0;
        return;
    }
    for (int i = n; i >= 0; i--) {
        if (fabs(a[i]) < eps) {
            continue;
        }
        if (a[i] > 0 && i < n) {
            cout << "+";
        }
        if (1-eps > fabs(a[i]) || fabs(a[i]) > 1+eps || i == 0) {
            cout << a[i];
        } else if (-1-eps < a[i] && a[i] < -1+eps) {
            cout << "-";
        }
        if (i > 1) {
            cout << "x^" << i;
        } else if (i == 1) {
            cout << "x";
        }
    }
}

int main() {
    cin >> n;
    x.push_back(0);
    y.push_back(0);
    for (int i = 1, _x, _y; i <= n; i++) {
        cin >> _x >> _y;
        x.push_back(_x);
        y.push_back(_y);
    }
    double sx  = 0;
    double sy  = 0;
    double sxy = 0;
    double sx2 = 0;
    for (int i = 1; i <= n; i++) {
        sx  += x[i];
        sy  += y[i];
        sxy += x[i]*y[i];
        sx2 += x[i]*x[i];
    }
    double k = (n*sxy-sx*sy) / (n*sx2-sx*sx);
    double b = sy/n - sx/n*k;
    double a[] = {b, k};
    cout << "y=";
    printPoly(1, a);
    return 0;
}