Ordinary Differential Equation

#include <stdio.h>
double f(double x, double y) {
  return - x * y;
}
int main(void) {
  double a = 0.;
  double b = 3.;
  int n = 1000;
  double h = (b - a) / n;
  double x = 0., ye = 1., yh = 1.;
  for (int i = 0; i < n; i++) {
    x += h;
    ye += h * f(x, ye);
    double k1 = f(x, yh);
    double k2 = f(x + h, yh + h * k1);
    yh += h * (k1 + k2) / 2.;
    printf("%lf %lf %lf\n", x, ye, yh);
  }
  return 0;
}