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Re: An experimental max_n for int's...

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Bart <bc@freeuk.com> writes:

> On 30/08/2020 11:35, Tim Rentsch wrote:
>
>> "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> writes:
>>
>>> On 8/29/2020 5:33 AM, Tim Rentsch wrote:
>>>
>>>> "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> writes:
>>>>
>>>>> What do you think of this [int array maximum function]
>>>>> [code]
>>>>> Is there a better way to write this?
>>>>
>>>> I would be inclined to write something more like this:
>>>>
>>>>       #include <limits.h>
>>>>       #include <stddef.h>
>>>>
>>>>       static int
>>>>       maximum( int *v, size_t n, int r ){
>>>>           return  n == 0  ? r  : maximum( v+1, n-1, r < *v ? *v : r );
>>>>       }
>>>>
>>>>       int
>>>>       max_n_int( int *p, size_t n ){
>>>>           return  maximum( p, n, INT_MIN );
>>>>       }
>>>>
>>>> The INT_MIN is there to handle the degenerate case of arrays
>>>> with no elements.
>>>
>>> I like the idea of using INT_MIN for the degenerate case.  However,
>>> your maximum is recursive.  Can it blow the stack apart if feed with
>>> a large enough array?
>>
>> Oh, excellent question.  I'm glad you asked.
>>
>> Here is the generated code under gcc -O2 (white space editing
>> only).  Note the lack of any function call instructions:
>
> <snip>
>
>> I write small recursive functions like this all the time.  My
>> experience is that compilers are almost always smart enough to
>> turn tail-call recursion into loops, like what was done in the
>> two compilations shown above.  Of course it is possible, if a
>> naive compiler is used or if optimization settings are too low,
>> that the tail-call optimization won't happen, and truly recursive
>> object code will be the result.  Usually though that doesn't
>> happen.  If we are being careful -- and we should be -- the lack
>> of actual recursion in the object code should be verified;  after
>> all, we don't want any nasty surprises.  But this isn't hard to
>> do.  It's a fairly simple matter to write a program in the
>> scripting language of your choice (I used awk) that can scan code
>> in this format and report cases of recursive calls in the object
>> code.  Having the tool in hand, making use of tail calls and tail
>> recursion can be done with confidence.
>>
>> I recommend developing an ability to write simple functions like
>> this using a recursive, tail-call-optimizable, style.  Once you
>> get the knack I think you will find it quite empowering.
>
> This sounds wrong in several ways:
>
> [objections to using a functional style like the example]

You should learn to read more carefully.  My suggestion is to
develop an ability to write simple functions in a functional
style like that of the example.  The question of _when_ to write
code that way is an entirely separate conversation.

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Re: An experimental max_n for int's... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-08-29 18:01 -0700
  Re: An experimental max_n for int's... Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-08-30 03:35 -0700
    Re: An experimental max_n for int's... Bart <bc@freeuk.com> - 2020-08-30 12:12 +0100
      Re: An experimental max_n for int's... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-08-30 20:33 -0700
        Re: An experimental max_n for int's... James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-08-31 07:50 -0400
          Re: An experimental max_n for int's... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-08-31 17:46 -0700
            Re: An experimental max_n for int's... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-09-01 02:32 +0100
              Re: An experimental max_n for int's... Richard Damon <Richard@Damon-Family.org> - 2020-09-01 07:27 -0400
                Re: An experimental max_n for int's... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-09-01 13:36 +0100
                Re: An experimental max_n for int's... James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-09-01 10:21 -0400
                Re: An experimental max_n for int's... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-09-01 15:57 +0100
                Re: An experimental max_n for int's... Richard Damon <Richard@Damon-Family.org> - 2020-09-01 21:45 -0400
                Re: An experimental max_n for int's... David Brown <david.brown@hesbynett.no> - 2020-09-01 14:39 +0200
                Re: An experimental max_n for int's... James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-09-01 10:28 -0400
      Re: An experimental max_n for int's... Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-09-07 08:28 -0700
    Re: An experimental max_n for int's... antispam@math.uni.wroc.pl - 2020-08-30 12:37 +0000
      Re: An experimental max_n for int's... Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-09-07 08:18 -0700

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