On Wed, 11 Feb 2009 04:31:10 -0500, Terry Reedy wrote:
>> Steven D'Aprano <ste...@remove.this.cybersource.com.au> writes:
>>> def fact(n):
>>> if n < 0: raise ValueError
>>> if n = 0: return 1
>>> return fact(n-1)*n
>>>
>>> At the risk of premature optimization, I wonder if there is an idiom
>>> for avoiding the unnecessary test for n <= 0 in the subsequent
>>> recursive calls?
>
> Reverse the test order
>
> def fact(n):
> if n > 0: return fact(n-1)*n
> if n == 0: return 1
> raise ValueError
>
> You must test recursive versus terminal case every call in any case.
> Nearly always, the first test passes and second is not done. You only
> test n==0 once, either to terminate or raise exception. This works for
> any integral value and catches non-integral values. (There is some delay
> for that, but only bad calls are penalized.)
Nice try, but in fact no.
>>> fact(None) # works okay
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 4, in fact
ValueError
>>>
>>> fact({})
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 2, in fact
TypeError: unsupported operand type(s) for -: 'dict' and 'int'
You're relying on arbitrary ordering of types in Python 2.x, and of
course in Python 3 that fails altogether.
Thanks for everyone who responded, I now have some food for thought.
--
Steven
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