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Re: Reinventing the wheel.

From David Brown <david.brown@hesbynett.no>
Newsgroups comp.lang.c
Subject Re: Reinventing the wheel.
Date 2020-02-20 20:06 +0100
Organization A noiseless patient Spider
Message-ID <r2mlau$tca$1@dont-email.me> (permalink)
References <7ca1c103-6910-48ad-8eee-b2f47e7b1db0@googlegroups.com>

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On 20/02/2020 19:33, James Kuyper wrote:
> It's reinventing the wheel time, again.
> 
> I have some code I need to write, with the externally imposed
> requirement that it be written in C and be self-contained. That is, it
> must not reference any external libraries other than the C standard
> library. If I could use C++, I'd use std::map<> to solve the problem
> described below, and even in C there's many widely available third-party
> libraries I could use to similar effect. Because those alternatives are
> normally available, the last time I had to implement any such algorithm
> was about 30 years ago. My BS was in Physics, not CS, so I never did a
> formal study of such algorithms. Therefore, I'm looking for advice as to
> the best algorithm to use, and ideally public-domain source code I could
> copy and modify as needed. Efficiency is the key thing I'm looking for,
> but keep in mind that std::map<> would have been acceptably efficient,
> if I could have used it. Since I'm going to have to write, understand,
> and test the code, simplicity of implementation is also important.
> 
> My code will be computing a large number of values, and will unavoidably
> have to deal with each value exactly twice. It would be nice to avoid
> calculating each value twice, but the computation time isn't large
> enough for that to be a significant issue. The main problem is that I
> need to store each value exactly once.
> 
> There's a key that I can easily calculate that uniquely identifies each
> value, so one approach would be
> 
> Compute key
> Search for key in a key-location list
> IF found THEN
> Retrieve previously calculated value from the corresponding location
> Remove key-location pair from list
> ELSE
> Calculate new value
> Allocate storage
> Store that value in that location.
> Add the key-location pair to the list
> ENDIF
> 
> In the worst case, my code will be generating on the order of
> 2^18 different values, but that list should never need to
> store more than about 2^9 key-location pairs (possible future
> developments might increase those values to 2^31 and 2^16,
> respectively).
> 
> What kind of data structure would you recommend for storing that list of
> key-location pairs?
> 

How big are your keys and values?

Since you suggest it would be straightforward with std::map<> and C++, 
can you give the code in that language so that it's clear exactly what 
you are wanting?  (Obviously the calculation stuff can be skipped or 
simplified.)

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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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