Re: Reinventing the wheel.

From	Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups	comp.lang.c
Subject	Re: Reinventing the wheel.
Date	2020-02-22 14:06 -0800
Organization	A noiseless patient Spider
Message-ID	<86sgj2b7jw.fsf@linuxsc.com> (permalink)
References	(1 earlier) <r2ml8q$614$1@news.albasani.net> <VRA3G.968213$4c1.653214@fx31.am4> <87v9o1as7k.fsf@bsb.me.uk> <pXC3G.440164$Uz.43777@fx32.am4> <87ftf4c1g0.fsf@bsb.me.uk>

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Ben Bacarisse <ben.usenet@bsb.me.uk> writes:

> Bart <bc@freeuk.com> writes:
>
>> On 20/02/2020 21:01, Ben Bacarisse wrote:
>>
>>> Bart <bc@freeuk.com> writes:
>>>
>>>> A hashtable was my first thought.  However, items need to removed
>>>> when they are looked up.  This would be problematic in the kind
>>>> of hashtable I normally use.
>>>
>>> That's curious.  What do you usually use that makes removal
>>> problematic?
>>
>> Suppose a fragment of the hashtable already looks like this (index
>> and value):
>>
>>   ....
>>   100 ABC
>>   101 DEF
>>   102 --- (free)
>>   ....
>>
>> Say that ABC and DEF both have the same hash value of 100 (modulo
>> table size).  Then in looking up DEF, 100 doesn't match, but 101
>> does - fine.
>>
>> But suppose that before looking up DEF, a successful lookup for ABC
>> is done, which is then removed to result in:
>>
>>   ....
>>   100 --- (free)
>>   101 DEF
>>   102 --- (free)
>>
>> Now, you look up DEF, but location 100 is empty, a sign usually
>> that what you're looking for doesn't exist.  Lookup fails.
>
> Removal from a hash table using open addressing is a well-known
> algorithm (actually there are two) so I probably would not call
> implementing it "problematic", but it is certainly the most fiddly
> part.  I was imagining some design that made it well-neigh
> impossible.
>
>> Maybe your hash algorithm is different, but with mine:
>>
>> * Starting at the first hash index:  you compare successive entries
>> until either a match or an empty slot is found (wrapping as
>> needed).  Empty means not found.
>
> If you do what's called lazy deletion (I did not know it had a name
> until just now!) you delete by invalidating the entry while keeping
> the entry marked as occupied.  The search operation works exactly as
> above (the invalidated entries won't match any valid key), but you
> make a note of the first invalid-but-occupied slot you see on the
> way.  If the key is found, and there was an invalid-but-occupied
> slot on the way, you move the entry to that slot just before the
> return.

That helps with lookup times, but it doesn't help with occupancy
rate.  If the hash table is never cleaned up, a long term steady
state of adding and removing will eventually fill up the hash
table with "occupied but invalid" tombstones (ie, besides the
actual still-live entries).  So there needs to be more than just
marking a removed table entry with a tombstone (as I am used to
calling them).

Thread

Reinventing the wheel. James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-02-20 10:33 -0800
  Re: Reinventing the wheel. Bonita Montero <Bonita.Montero@gmail.com> - 2020-02-20 20:04 +0100
    Re: Reinventing the wheel. Bart <bc@freeuk.com> - 2020-02-20 19:11 +0000
      Re: Reinventing the wheel. David Brown <david.brown@hesbynett.no> - 2020-02-20 21:50 +0100
      Re: Reinventing the wheel. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-02-20 21:01 +0000
        Re: Reinventing the wheel. Bart <bc@freeuk.com> - 2020-02-20 21:34 +0000
          Re: Reinventing the wheel. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-02-20 22:56 +0000
            Re: Reinventing the wheel. Bart <bc@freeuk.com> - 2020-02-21 00:34 +0000
            Re: Reinventing the wheel. David Brown <david.brown@hesbynett.no> - 2020-02-21 09:40 +0100
            Re: Reinventing the wheel. Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-02-22 14:06 -0800
  Re: Reinventing the wheel. David Brown <david.brown@hesbynett.no> - 2020-02-20 20:06 +0100
    Re: Reinventing the wheel. Bonita Montero <Bonita.Montero@gmail.com> - 2020-02-20 20:08 +0100
    Re: Reinventing the wheel. James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-02-20 12:46 -0800
      Re: Reinventing the wheel. already5chosen@yahoo.com - 2020-02-20 13:55 -0800
        Re: Reinventing the wheel. James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-02-21 00:12 -0500
          Re: Reinventing the wheel. scott@slp53.sl.home (Scott Lurndal) - 2020-02-21 16:16 +0000
            Re: Reinventing the wheel. James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-02-21 09:20 -0800
              Re: Reinventing the wheel. jadill33@gmail.com - 2020-02-21 10:07 -0800
              Re: Reinventing the wheel. scott@slp53.sl.home (Scott Lurndal) - 2020-02-21 23:23 +0000
                Re: Reinventing the wheel. Richard Damon <Richard@Damon-Family.org> - 2020-02-21 18:41 -0500
                Re: Reinventing the wheel. scott@slp53.sl.home (Scott Lurndal) - 2020-02-22 16:20 +0000
                Re: Reinventing the wheel. already5chosen@yahoo.com - 2020-02-22 10:33 -0800
                Re: Reinventing the wheel. James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-02-22 15:00 -0500
                Re: Reinventing the wheel. Richard Damon <Richard@Damon-Family.org> - 2020-02-22 15:16 -0500
          Re: Reinventing the wheel. James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-02-21 08:42 -0800
            Re: Reinventing the wheel. David Brown <david.brown@hesbynett.no> - 2020-02-21 18:23 +0100
      Re: Reinventing the wheel. Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-02-22 17:06 -0800
        Re: Reinventing the wheel. James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-02-23 06:18 -0800
  Re: Reinventing the wheel. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-02-20 21:14 +0000
    Re: Reinventing the wheel. James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-02-21 00:24 -0500
  Re: Reinventing the wheel. "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-02-20 13:15 -0800
    Re: Reinventing the wheel. James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-02-20 13:44 -0800
      Re: Reinventing the wheel. "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-02-20 14:21 -0800
        Re: Reinventing the wheel. "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-02-20 14:22 -0800
  Re: Reinventing the wheel. Robert Wessel <robertwessel2@yahoo.com> - 2020-02-20 15:19 -0600
    Re: Reinventing the wheel. James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-02-21 00:28 -0500
  Re: Reinventing the wheel. "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-02-20 23:27 -0800
    Re: Reinventing the wheel. "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-02-20 23:52 -0800
  Re: Reinventing the wheel. jadill33@gmail.com - 2020-02-21 08:20 -0800
  Re: Reinventing the wheel. Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2020-02-22 03:15 -0800
    Re: Reinventing the wheel. already5chosen@yahoo.com - 2020-02-22 10:38 -0800
      Re: Reinventing the wheel. Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2020-02-24 02:50 -0800
        Re: Reinventing the wheel. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-02-24 16:36 +0000
          Re: Reinventing the wheel. Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2020-02-25 04:08 -0800
            Re: Reinventing the wheel. David Brown <david.brown@hesbynett.no> - 2020-02-25 13:42 +0100
              Re: Reinventing the wheel. already5chosen@yahoo.com - 2020-02-25 05:00 -0800
                Re: Reinventing the wheel. David Brown <david.brown@hesbynett.no> - 2020-02-26 10:53 +0100
          Re: Reinventing the wheel. chad <cdalten@gmail.com> - 2020-02-26 06:51 -0800
    Re: Reinventing the wheel. Anton Shepelev <anton.txt@gmail.com> - 2020-02-24 22:40 +0300
      Re: Reinventing the wheel. Robert Wessel <robertwessel2@yahoo.com> - 2020-02-24 16:37 -0600
        Re: Reinventing the wheel. Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2020-02-25 01:37 -0800
          Re: Reinventing the wheel. Anton Shepelev <anton.txt@g{oogle}mail.com> - 2020-02-25 13:07 +0300
            Re: Reinventing the wheel. already5chosen@yahoo.com - 2020-02-25 04:50 -0800
              Re: Reinventing the wheel. Anton Shepelev <anton.txt@g{oogle}mail.com> - 2020-02-25 19:30 +0300
                Re: Reinventing the wheel. already5chosen@yahoo.com - 2020-02-25 08:43 -0800
                Re: Reinventing the wheel. Anton Shepelev <anton.txt@gmail.com> - 2020-02-29 01:25 +0300
              Re: Reinventing the wheel. Robert Wessel <robertwessel2@yahoo.com> - 2020-02-25 15:40 -0600
                Re: Reinventing the wheel. Anton Shepelev <anton.txt@g{oogle}mail.com> - 2020-02-26 13:40 +0300
            Re: Reinventing the wheel. James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-02-25 11:29 -0800
              Re: Reinventing the wheel. Anton Shepelev <anton.txt@g{oogle}mail.com> - 2020-02-26 13:47 +0300
                Re: Reinventing the wheel. James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-02-26 09:32 -0500
                Re: Reinventing the wheel. Anton Shepelev <anton.txt@g{oogle}mail.com> - 2020-02-26 21:26 +0300
  Re: Reinventing the wheel. "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-02-22 23:32 -0800
    Re: Reinventing the wheel. "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-03-07 22:22 -0800
      Re: Reinventing the wheel. "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-03-07 22:38 -0800

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