(*  Collatz conjecture  *)
(*  If, odd, increase the number, otherwise, decrease it  *)

x = 27;
y = {x};
While(x > 1,
 x = If(Mod(x, 2) == 1, x*3+1, x/2);
 y = Append(y, x)
);
y
Print(Length(y), " iterations");


(*  Collatz conjecture  *)
(*  Anonymous function and built-in Wolfram function  *)

x = 27;
y = NestWhileList(If(Mod(#, 2) == 1, #*3+1, #/2)&, x, # > 1&);
y
Print(Length(y), " iterations");