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	2021-07-02 14:44 +0100
Organization	A noiseless patient Spider
Message-ID	<87fswwhlu4.fsf@bsb.me.uk> (permalink)
References	(4 earlier) <Gi4DI.1867$t64.414@fx13.iad> <87im1vj6q3.fsf@bsb.me.uk> <O%aDI.906$tE4.1@fx29.iad> <8735syjrwf.fsf@bsb.me.uk> <54aa8ed0-7069-44c8-a582-d05aa8541ac8n@googlegroups.com>

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Michael S <already5chosen@yahoo.com> writes:

> On Thursday, July 1, 2021 at 12:38:21 PM UTC+3, Ben Bacarisse wrote:
>> DFS <nos...@dfs.com> writes: 
>> 
>> > On 6/30/2021 7:03 PM, Ben Bacarisse wrote: 
>> >> DFS <nos...@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.
>
> Does it include replacement of division by reciprocal multiplication?

No, it's a plain integer / and % version.  Have you got one that does
something clever with the divisions?

-- 
Ben.

Thread

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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