指数函数的近似

#include "Graph.h"
#include "Simple_window.h"
using namespace Graph_lib;
double fac(int n);
double term(double x, int n);
double expe(double x, int n);
int expN_number_of_terms = 10;
double expN(double x)
{
    return expe(x, expN_number_of_terms);
}
int main()
{
    constexpr int xmax = 600;
    constexpr int ymax = 400;
    constexpr int x_orig = xmax / 2;
    constexpr int y_orig = ymax / 2;
    Point orig{ x_orig,y_orig };
    constexpr double r_min = -10;
    constexpr double r_max = 11;
    constexpr int n_points = 400;
    constexpr int x_scale = 30;
    constexpr int y_scale = 30;
    constexpr int xlength = xmax - 40;
    constexpr int ylength = ymax - 40;
    Simple_window win{ Point{100,100},xmax,ymax,"Wanbabe" };
    Axis x{ Axis::x,Point{20,y_orig},xlength,xlength / x_scale,"one notch==1" };
    Axis y{ Axis::y,Point{x_orig,ylength+20},ylength,ylength / y_scale,"one notch==1" };
    x.set_color(Color::red);
    x.label.set_color(Color::blue);
    y.set_color(Color::red);
    Function real_exp{ exp,r_min,r_max,orig,200,double(x_scale),double(y_scale) };
    real_exp.set_color(Color::blue);
    win.attach(real_exp);
    win.attach(x);
    win.attach(y);
    for (int n = 0; n < 20; ++n) {
        ostringstream ss;
        ss<< "exp approximation;n=" << n;
        win.set_label(ss.str());
        expN_number_of_terms = n;
        Function e(expN, r_min, r_max, orig, 200, x_scale, y_scale);
        win.attach(e);
        win.wait_for_button();
        win.detach(e);
    }
}
double fac(int n) {
    int r = 1;
    while (n > 1) {
        r *= n;
        --n;
    }
    return r;
}
double term(double x, int n) {
    return pow(x, n) / fac(n);
}
double expe(double x, int n) {
    double sum = 0;
    for (int i = 0; i < n; ++i)
        sum += term(x, i);
    return sum;
}