Are both of these proper use of recursion? Is one better than the other?

Question

I am trying to better understand recursion.

As an example, here are two definitions for calculating n! :

def factorial(num, p=1):
    if num == 0:
        return p
    else:
        p *= num
        return factorial(num-1,p)

and

def factorial(num):
   if num == 1:
       return num
   return num * factorial(num - 1)

Is one better than the other? I understand that the second one builds a call stack, while the first is calculating p at each step and just passing it to another iteration of the function. Are there separate names for these types of functions? Is the first one not recursion?

Just looking for some guidance on understanding the differences here. Thanks!

the former is tail call recursion, which can be optimized, but is not optimized in CPython. — juanpa.arrivillaga, Jun 21 '19 at 22:34
https://stackoverflow.com/questions/310974/what-is-tail-call-optimization — CristiFati, Jun 21 '19 at 22:56
@juanpa.arrivillaga Thanks! Knowing the name 'tail call recursion' definitely helps... I can do some more research with that. — David R., Jun 21 '19 at 23:33
@CristiFati This is perfect! Wish I would have found it before posting the question. Thank you! — David R., Jun 21 '19 at 23:34
@DavidR. please understnd, CPython *does not do tail-call optimization* — juanpa.arrivillaga, Jun 21 '19 at 23:47

score 0 · Answer 1 · answered Jun 22 '19 at 03:09

There is little difference between the two. I would say "simpler is better" and the 2nd one involves fewer variables so I would go for that one.

Or something even simpler:

  def factorial(N): return max(1,N) if N<3 else N*factorial(N-1)

In real life though, you shouldn't implement it recursively because iterative solutions will be faster.

Are both of these proper use of recursion? Is one better than the other?

1 Answers1