C++ Blatt 11 A27.cc

#include <iostream>
#include <cmath>
using namespace std;

bool signchange (double a, double b) {
  return (a > 0 && b < 0) || (a < 0 && b > 0);
}

string bisection(double a, double b, double epsilon_x, double epsilon_y, double (*f)(double), int n) {
  double average = (a + b) / 2.0;

  if (!signchange(f(a), f(b)))
    return "Error: no change of sign";

  cout << "step: " << n << " interval: [" << a << ", " << b << "] f(" << average << "): " << f(average) << endl;

  if (abs(a - b) < epsilon_x || abs(f(average)) < epsilon_y)
    return "root: " + to_string(average);

  if (signchange(f(a), f(average)))
    return bisection(a, average, epsilon_x, epsilon_y, f, n + 1);

  if (signchange(f(average), f(b)))
    return bisection(average, b, epsilon_x, epsilon_y, f, n + 1);
}

double f(double x) {
  // return cos(x) - x;
  return exp(x) - pow(x, 3);
}

int main() {
  double a, b;

  cout << "x0: ";
  cin >> a;
  cout << "x1: ";
  cin >> b;

  cout << bisection(a, b, pow(10, -4), pow(10, -3), &f, 1) << endl;
}