Why is this blowing the stack, thought it was tail-recursive...

Steven D'Aprano steve at REMOVE-THIS-cybersource.com.au
Sun Jul 13 08:56:20 CEST 2008


On Sat, 12 Jul 2008 19:25:18 -0400, Terry Reedy wrote:

> ssecorp wrote:
>> def fib(n):
>>     def fibt(a, b, n):
>>         if n <= 1:
>>             return b
>>         else:
>>             return fibt(b, a + b, n - 1)
>>     if n == 0:
>>         return 0
>>     else:
>>         return fibt(0, 1, n);
>> 
>> and can memoization speed up this even more? tesintg with memoization
>> doesnt really say anything because it is so fast it is instant anyway.
> 
> Except for the fact that a+b gets slower as a and b get bigger, this
> would already be linear time in n.  Memoization (here by means of a
> linear list) only helps if the list is preserved and one makes repeated
> requests for various fib values.
> 
> I am just curious what input you tried that blew the stack?  It had to
> be pretty large.

No, not really. Try it for yourself: on my system, I get RuntimeError: 
maximum recursion depth exceeded with fib(999).

fib(999) is a large number, with 208 digits, but not that large:

268638100244853593861467272021429239676166093189869523401
231759976179817002478816893383696544833565641918278561614
433563129766736422103503246348504103776803673341511728991
69723197082763985615764450078474174626L


-- 
Steven



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