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Groups > comp.lang.c > #161565

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

From David Brown <david.brown@hesbynett.no>
Newsgroups comp.lang.c
Subject Re: Losing my mind: results change with/without printf() statements
Date 2021-07-01 10:56 +0200
Organization A noiseless patient Spider
Message-ID <sbk00a$m0i$1@dont-email.me> (permalink)
References (2 earlier) <Yg2DI.3867$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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On 01/07/2021 05:54, DFS wrote:
> 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?
> 

I haven't studied the code (or the problem), but if that change also
removes the call to the function with malloc(), it will help significantly.

Whenever you see "malloc" in your code, think "slow".  When you have a
malloc in the middle of your inner loop, think "very slow".  Avoid it if
at all possible - and in your original code, it is useless.

To see the cost of malloc, go back to the original code (with the "int
cnt = 0;" fix).  Figure out the maximum length (9, for 32-bit int) and
have a single "static char buff[9];".  Use that instead of malloc in
psubstr, and measure the difference in time.

> 
>> 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':
> 
> [1 10 104 0 4 4]
> 
> Note that it's a one-child number because out of all those substrings,
> only 0 is evenly divisible by 3 (the length of 104).
> 
> 
> 
> 
>>> 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.
> 
>

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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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