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Re: Optimization of cascading recursions in "C" compilers?

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On 2025-10-24, Janis Papanagnou <janis_papanagnou+ng@hotmail.com> wrote:
> On 24.10.2025 19:14, Kaz Kylheku wrote:
>> On 2025-10-24, Janis Papanagnou <janis_papanagnou+ng@hotmail.com> wrote:
>>> On 24.10.2025 16:35, Michael S wrote:
>>>> On Fri, 24 Oct 2025 15:36:58 +0200
>>>> Janis Papanagnou <janis_papanagnou+ng@hotmail.com> wrote:
>>>>
>>>>> On 24.10.2025 09:53, Rosario19 wrote:
>>>>>>
>>>>>>   
>>>>>>> into the above pair of functions, or anything similar.
>>>>>>>
>>>>>>> That's not ordinary optimization of the type which improves the
>>>>>>> code generated for the user's algorithm; that's recognizing and
>>>>>>> redesigning the algorithm.
>>>>>>>
>>>>>>> The line between those is blurred, but not /that/ blurred.  
>>>>>>
>>>>>> int fib1(int n){return fib_tail(0, 1, n);}
>>>>>>
>>>>>> is different from
>>>>>>
>>>>>> int fib(int n){if(n<2) return 1;return fib(n - 1) + fib(n - 2);
>>>>>>
>>>>>>
>>>>>> both these function calculate from the 0 1  the number result, but
>>>>>> fib() start to calculate from n goes dowwn until n=0, than goes up
>>>>>> until n=input, instead fib1() go only down for n. It seen the calls
>>>>>> (fib has 2 call each call... instead fib_tail() just 1), I think
>>>>>> fib() is O(n^2) instead fib1 is O(n).  
>>>>>
>>>>> It's worse; it's O(2^N).
>>>>>
>>>>> Janis
>>>>>
>>>>
>>>> Actually, the number of calls in naive recursive variant = 2*FIB(N)-1.
>>>> So, O(1.618**n) - a little less bad than your suggestion.
>>>
>>> It's new to me that O-complexities would be written with decimals.
>>>
>>> You also don't say that 'insertion sort' is O((N^2)/2); it's O(N^2).
>> 
>> I think, whereas constant factors and constant terms are understood not
>> to matter in asymptotic complexity, the choice of exponential base in
>> exponential complexity does. Similarly to the choice of exponent
>> in the category of polynomial time: e.g. cubic is slower than quadratic.
>
> I don't know about you, but my (40 years old) knowledge is reflected
> by the table you find in https://de.wikipedia.org/wiki/Landau-Symbole.
> We have differentiated O(1), O(log log N), O(log N), O(N), O(N log N),
> O(N^2), O(N^3) - other O(c^N) for c>3 were practically "irrelevant";
> no need to show me an example of O(N^4), or O(N^3 log N), I know there
> are examples - O(2^N), and O(n!). (There are two more listed on Wiki.)
> (Just note that all listed complexity classes are here(!) functions on
> N alone, where "1" is "N^0" and the "N^M" just an abbreviated notation
> to not list the individual mostly anyway irrelevant exponents.)
>
> I think that if we compare, as done here, different complexity classes,
> O(N) vs. O(2^N) the decimals are anyway irrelevant! If we're comparing
> functions within the same class O(2^N) it makes sense to differentiate
> an O(2^N) complexity with concrete numbers of 1.618**n vs. 2**n.
> But we've never said that these two were different complexity classes.
>
> The class O(a^N) (for a>1) is anyway just called exponential complexity.

But that's the same as "polynomial time" including quadratic, cubic,
quartic, ..

> And since O defines an _order_ (with a specific characteristic) there's
> no point in differentiating a class O(1.618^N) from a class O(1.617^N),

But these orders are different.

The complexity upper bound O(1.618^n) includes faster function growth than
the complexity upper bound O(1.617^n).

If we know something runs in K * 1.618^n time, then it isn't
O(1.617^n); it blows past that tighter upper bound.

For any positive constant factor K, given a sufficiently large n:

   1.617^n < K 1.68^n

No matter how small you make K, after a certain n value, the right side
will permanently overtake the left.

Both complexities are in the exponential class, but they are not
of the same order.

Complexity classes are coarse-grained containers of complexity orders:

https://en.wikipedia.org/wiki/Complexity_class

For instance everything polynomial is lumped into the complexity
class P, even though you make an algorithm have significantly better
complexity if you can reduce it from cubed to quadratic.

The exponential time complexity class contains a recognized subclass
called E which is O(2^l(n)) where l(n) is a linear polynomial of n.

Both O(2^n) and O(1.68^n) are found within E yet are not of the
same order.

1.68^n can be rewritten as approximately 2^(0.7485n), showing
that it belongs to class E.

But that 0.7485 coefficient on n cannot be ignored when it is inside
exponentiation, it leads to slower growth of a lower order.

-- 
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
Mastodon: @Kazinator@mstdn.ca

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Optimization of cascading recursions in "C" compilers? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-09-30 17:54 +0200
  Re: Optimization of cascading recursions in "C" compilers? David Brown <david.brown@hesbynett.no> - 2025-09-30 18:04 +0200
    Re: Optimization of cascading recursions in "C" compilers? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-01 00:15 +0200
  Re: Optimization of cascading recursions in "C" compilers? Kaz Kylheku <643-408-1753@kylheku.com> - 2025-09-30 16:12 +0000
    Re: Optimization of cascading recursions in "C" compilers? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-01 00:07 +0200
      Re: Optimization of cascading recursions in "C" compilers? Ben Bacarisse <ben@bsb.me.uk> - 2025-10-01 10:05 +0100
        Re: Optimization of cascading recursions in "C" compilers? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-01 19:01 +0200
          Re: Optimization of cascading recursions in "C" compilers? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-01 19:08 +0200
            Re: Optimization of cascading recursions in "C" compilers? Ben Bacarisse <ben@bsb.me.uk> - 2025-10-02 00:48 +0100
              Re: Optimization of cascading recursions in "C" compilers? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-02 05:25 +0200
                Re: Optimization of cascading recursions in "C" compilers? Ben Bacarisse <ben@bsb.me.uk> - 2025-10-03 15:09 +0100
                Re: Optimization of cascading recursions in "C" compilers? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-03 16:47 +0200
    Re: Optimization of cascading recursions in "C" compilers? bart <bc@freeuk.com> - 2025-10-01 00:02 +0100
      Re: Optimization of cascading recursions in "C" compilers? antispam@fricas.org (Waldek Hebisch) - 2025-10-01 01:22 +0000
    Re: Optimization of cascading recursions in "C" compilers? Rosario19 <Ros@invalid.invalid> - 2025-10-23 00:37 +0200
      Re: Optimization of cascading recursions in "C" compilers? Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-22 23:36 +0000
        Re: Optimization of cascading recursions in "C" compilers? Rosario19 <Ros@invalid.invalid> - 2025-10-24 09:53 +0200
          Re: Optimization of cascading recursions in "C" compilers? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-24 15:36 +0200
            Re: Optimization of cascading recursions in "C" compilers? Michael S <already5chosen@yahoo.com> - 2025-10-24 17:35 +0300
              Re: Optimization of cascading recursions in "C" compilers? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-24 17:22 +0200
                Re: Optimization of cascading recursions in "C" compilers? Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-24 17:14 +0000
                Re: Optimization of cascading recursions in "C" compilers? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-24 20:31 +0200
                Re: Optimization of cascading recursions in "C" compilers? Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-24 23:59 +0000
                Re: Optimization of cascading recursions in "C" compilers? James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-10-27 05:48 -0400
                Re: Optimization of cascading recursions in "C" compilers? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-27 12:52 +0100
                Re: Optimization of cascading recursions in "C" compilers? James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-10-27 14:41 -0400
                Re: Optimization of cascading recursions in "C" compilers? Michael S <already5chosen@yahoo.com> - 2025-10-27 22:33 +0200
                Re: Optimization of cascading recursions in "C" compilers? James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-10-27 16:52 -0400
                Re: Optimization of cascading recursions in "C" compilers? Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-27 21:03 +0000
                Re: Optimization of cascading recursions in "C" compilers? antispam@fricas.org (Waldek Hebisch) - 2025-10-28 01:58 +0000

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