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Re: Losing my mind: results change with/without printf() statements

From Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups comp.lang.c
Subject Re: Losing my mind: results change with/without printf() statements
Date 2021-07-01 10:38 +0100
Organization A noiseless patient Spider
Message-ID <8735syjrwf.fsf@bsb.me.uk> (permalink)
References (2 earlier) <Yg2DI.3867$mR.1082@fx33.iad> <hY2DI.505463$N5n7.405017@fx05.ams4> <Gi4DI.1867$t64.414@fx13.iad> <87im1vj6q3.fsf@bsb.me.uk> <O%aDI.906$tE4.1@fx29.iad>

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DFS <nospam@dfs.com> writes:

> On 6/30/2021 7:03 PM, Ben Bacarisse wrote:
>> DFS <nospam@dfs.com> writes:
>> 
>>> On 6/30/2021 2:44 PM, Bart wrote:
>> <cut>
>>>> But I suspect that in all cases, what is dominant is the conversion
>>>> from int to text, from text back to int, and applying the mod
>>>> operator. All things that happen outside of the code generated from
>>>> your source.
>>>
>>> I don't understand 'happen outside'.
>> I think he means that your calls to atoi are doing most of the work, so
>> you can't get much more speed from tinkering with the code you actually
>> wrote.
>> That's not quite right in that you can get a good speed-up by not
>> generating all those sub-strings.  You already have the full number in
>> 's' so you can get sub-string numbers by doing this:
>>    char save = s[j+k];
>>    s[j+k] = 0;
>>    ssnbr = atoi(s+j);
>>    s[j+k] = save;
>
> Geez, that block means less code overall, and the prog runs
> significantly faster.  Where do you mad geniuses come from?

Actually there is no genius involved (IMO).  It's mostly the result of
having written and (possibly more importantly) read code for more than
40 years.  You pick up stuff along the way.

>> Having said that, Bart's point still holds.  This can be seen as a
>> purely arithmetical problem.  There is no need for strings conversions
>> (and hence atoi) at all.  Whether that would make the code faster is not
>> obvious, but I suspect it might.  I've not have time to try it.
>
> Careful of gotchas.  These are all the substrings of 104, including
> the leading-zero dropped '04':

Whether that's a gotcha or not depends on how you tackle the problem.

I've written an arithmetic version (i.e. no strings) and it is
significantly faster again.  I thought it probably would be.  You might
want to have a go...

>>> Thanks for looking at it.
>> Your code looks a little old fashioned.  But even sticking to old C
>> standards you could tidy it up a but.  For example, it's neater to
>> declare int div = 0; in the block that needs it, rather than "resetting"
>> at the bottom of the loop.
>
> First I had it just above the j loop, but moved it down to be near its
> last use.

I'm not sure that's really my point.  div is used only in that loop so
this pattern

  for (....) {
     int count = 0;
     ....
         code that might do count += 1;
     ....
  }

is clearer and better overall than this pattern:

  int count = 0;
  ....
  for (....) {
     ....
         code that might do count += 1;
     ....
     count = 0;
  }

-- 
Ben.

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Losing my mind: results change with/without printf() statements DFS <nospam@dfs.com> - 2021-06-30 12:26 -0400
  Re: Losing my mind: results change with/without printf() statements David Brown <david.brown@hesbynett.no> - 2021-06-30 19:03 +0200
    Re: Losing my mind: results change with/without printf() statements DFS <nospam@dfs.com> - 2021-06-30 13:58 -0400
      Re: Losing my mind: results change with/without printf() statements Bart <bc@freeuk.com> - 2021-06-30 19:44 +0100
        Re: Losing my mind: results change with/without printf() statements DFS <nospam@dfs.com> - 2021-06-30 16:16 -0400
          Re: Losing my mind: results change with/without printf() statements Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-01 00:03 +0100
            Re: Losing my mind: results change with/without printf() statements DFS <nospam@dfs.com> - 2021-06-30 23:54 -0400
              Re: Losing my mind: results change with/without printf() statements David Brown <david.brown@hesbynett.no> - 2021-07-01 10:56 +0200
              Re: Losing my mind: results change with/without printf() statements Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-01 10:38 +0100
                Re: Losing my mind: results change with/without printf() statements Michael S <already5chosen@yahoo.com> - 2021-07-02 02:49 -0700
                Re: Losing my mind: results change with/without printf() statements Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-02 14:44 +0100
                Re: Losing my mind: results change with/without printf() statements Michael S <already5chosen@yahoo.com> - 2021-07-02 07:48 -0700
                Re: Losing my mind: results change with/without printf() statements Michael S <already5chosen@yahoo.com> - 2021-07-02 09:25 -0700
                Re: Losing my mind: results change with/without printf() statements Bart <bc@freeuk.com> - 2021-07-02 18:33 +0100
                Re: Losing my mind: results change with/without printf() statements Michael S <already5chosen@yahoo.com> - 2021-07-04 01:27 -0700
                Re: Losing my mind: results change with/without printf() statements Bart <bc@freeuk.com> - 2021-07-04 15:19 +0100
                Re: Losing my mind: results change with/without printf() statements Michael S <already5chosen@yahoo.com> - 2021-07-04 09:04 -0700
                Re: Losing my mind: results change with/without printf() statements Michael S <already5chosen@yahoo.com> - 2021-07-04 09:51 -0700
          Re: Losing my mind: results change with/without printf() statements Michael S <already5chosen@yahoo.com> - 2021-07-02 03:20 -0700
            Re: Losing my mind: results change with/without printf() statements Bart <bc@freeuk.com> - 2021-07-02 11:42 +0100
              Re: Losing my mind: results change with/without printf() statements Michael S <already5chosen@yahoo.com> - 2021-07-02 04:47 -0700
          Re: Losing my mind: results change with/without printf() statements Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-02 15:21 +0100
            Re: Losing my mind: results change with/without printf() statements DFS <nospam@dfs.com> - 2021-07-02 11:37 -0400
            Re: Losing my mind: results change with/without printf() statements Bart <bc@freeuk.com> - 2021-07-02 17:06 +0100
            Re: Losing my mind: results change with/without printf() statements Kaz Kylheku <563-365-8930@kylheku.com> - 2021-07-02 17:41 +0000
              Re: Losing my mind: results change with/without printf() statements Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-02 19:09 +0100
                Re: Losing my mind: results change with/without printf() statements Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-02 19:48 +0100
                Re: Losing my mind: results change with/without printf() statements Michael S <already5chosen@yahoo.com> - 2021-07-04 03:21 -0700
                Re: Losing my mind: results change with/without printf() statements Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-04 14:01 +0100
                Re: Losing my mind: results change with/without printf() statements Michael S <already5chosen@yahoo.com> - 2021-07-05 02:44 -0700
                Re: Losing my mind: results change with/without printf() statements Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-05 14:43 +0100
                Re: Losing my mind: results change with/without printf() statements kegs@provalid.com (Kent Dickey) - 2021-07-05 19:42 -0500
                Re: Losing my mind: results change with/without printf() statements Michael S <already5chosen@yahoo.com> - 2021-07-06 03:36 -0700
                Re: Losing my mind: results change with/without printf() statements kegs@provalid.com (Kent Dickey) - 2021-07-06 12:27 -0500
                Re: Losing my mind: results change with/without printf() statements Michael S <already5chosen@yahoo.com> - 2021-07-06 16:02 -0700
                Re: Losing my mind: results change with/without printf() statements DFS <nospam@dfs.com> - 2021-07-06 20:30 -0400
                Re: Losing my mind: results change with/without printf() statements Michael S <already5chosen@yahoo.com> - 2021-07-08 01:02 -0700
                Re: Losing my mind: results change with/without printf() statements Michael S <already5chosen@yahoo.com> - 2021-07-09 05:52 -0700
                Re: Losing my mind: results change with/without printf() statements kegs@provalid.com (Kent Dickey) - 2021-07-09 08:24 -0500
                Re: Losing my mind: results change with/without printf() statements Michael S <already5chosen@yahoo.com> - 2021-07-09 07:20 -0700
                Re: Losing my mind: results change with/without printf() statements Michael S <already5chosen@yahoo.com> - 2021-07-10 15:27 -0700
                Re: Losing my mind: results change with/without printf() statements Michael S <already5chosen@yahoo.com> - 2021-07-12 00:56 -0700
                Re: Losing my mind: results change with/without printf() statements Michael S <already5chosen@yahoo.com> - 2021-07-12 12:01 -0700
                Re: Losing my mind: results change with/without printf() statements Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-12 21:30 +0100
                Re: Losing my mind: results change with/without printf() statements Michael S <already5chosen@yahoo.com> - 2021-07-13 00:33 -0700
                Re: Losing my mind: results change with/without printf() statements Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-13 22:58 +0100
                Re: Losing my mind: results change with/without printf() statements Michael S <already5chosen@yahoo.com> - 2021-07-20 13:38 -0700
                Re: Losing my mind: results change with/without printf() statements DFS <nospam@dfs.com> - 2021-07-09 11:23 -0400
                Re: Losing my mind: results change with/without printf() statements Michael S <already5chosen@yahoo.com> - 2021-07-09 08:43 -0700
                Re: Losing my mind: results change with/without printf() statements Mike Terry <news.dead.person.stones@darjeeling.plus.com> - 2021-07-11 16:40 +0100
                Re: Losing my mind: results change with/without printf() statements Michael S <already5chosen@yahoo.com> - 2021-07-12 02:51 -0700
                Re: Losing my mind: results change with/without printf() statements Mike Terry <news.dead.person.stones@darjeeling.plus.com> - 2021-07-12 16:57 +0100
                Re: Losing my mind: results change with/without printf() statements Mike Terry <news.dead.person.stones@darjeeling.plus.com> - 2021-07-12 17:25 +0100
                Re: Losing my mind: results change with/without printf() statements Mike Terry <news.dead.person.stones@darjeeling.plus.com> - 2021-07-20 18:03 +0100
      Re: Losing my mind: results change with/without printf() statements David Brown <david.brown@hesbynett.no> - 2021-07-01 10:46 +0200
        Re: Losing my mind: results change with/without printf() statements DFS <nospam@dfs.com> - 2021-07-02 11:52 -0400
          Re: Losing my mind: results change with/without printf() statements Bart <bc@freeuk.com> - 2021-07-02 17:16 +0100
            Re: Losing my mind: results change with/without printf() statements DFS <nospam@dfs.com> - 2021-07-02 13:27 -0400
      Re: Losing my mind: results change with/without printf() statements luser droog <luser.droog@gmail.com> - 2021-07-12 16:19 -0700
  Re: Losing my mind: results change with/without printf() statements Barry Schwarz <schwarzb@delq.com> - 2021-06-30 11:36 -0700
    Re: Losing my mind: results change with/without printf() statements DFS <nospam@dfs.com> - 2021-06-30 16:03 -0400
      Re: Losing my mind: results change with/without printf() statements Barry Schwarz <schwarzb@delq.com> - 2021-06-30 13:22 -0700
      Re: Losing my mind: results change with/without printf() statements David Brown <david.brown@hesbynett.no> - 2021-07-01 11:03 +0200
  Re: Losing my mind: results change with/without printf() statements Rosario19 <Ros@invalid.invalid> - 2021-07-01 10:49 +0200

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