const FMT=" %7.3f";

fcn ivp_euler(f,y,step,end_t){
   print(" Step %2d: ".fmt(step));
   foreach t in ([0..end_t,step]){
      if (t % 10 == 0) print(FMT.fmt(y));
      y += f(t,y) * step;
   }
   println();
}

fcn analytic{
   print("    Time: ");
   foreach t in  ([0..100,10]){ print(" %7g".fmt(t)) }
   print("\nAnalytic: ");
   foreach t in ([0..100,10]){ print(FMT.fmt(20.0 + 80.0 * (-0.07 * t).exp())) }
   println();
}

fcn cooling(_,temp){ return(-0.07 * (temp - 20)) }

analytic();
ivp_euler(cooling, 100.0, 2,  100);
ivp_euler(cooling, 100.0, 5,  100);
ivp_euler(cooling, 100.0, 10, 100);