Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #168666

Re: Compute Unique Numbers in a Set

From Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups comp.lang.c
Subject Re: Compute Unique Numbers in a Set
Date 2022-12-27 20:08 +0000
Organization A noiseless patient Spider
Message-ID <87bknolie2.fsf@bsb.me.uk> (permalink)
References (3 earlier) <DTEqL.146517$gGD7.19214@fx11.iad> <87sfh0lr50.fsf@bsb.me.uk> <iTFqL.10904$OD18.573@fx08.iad> <87h6xglom6.fsf@bsb.me.uk> <Y7HqL.20387$wfQc.14603@fx43.iad>

Show all headers | View raw

Richard Damon <Richard@Damon-Family.org> writes:

> On 12/27/22 12:53 PM, Ben Bacarisse wrote:
>> Richard Damon <Richard@Damon-Family.org> writes:
>> 
>>> On 12/27/22 11:59 AM, Ben Bacarisse wrote:
>>>> Richard Damon <Richard@Damon-Family.org> writes:
>>>>
>>>>> On 12/27/22 10:57 AM, Ben Bacarisse wrote:
>>>>>> Richard Damon <Richard@Damon-Family.org> writes:
>>>>>>
>>>>>>> On 12/26/22 6:45 PM, Albert wrote:
>>>>>>>> Is this the best way to generate unique random numbers in a set of 6
>>>>>>>> numbers?
>>>>>>>>
>>>>>>>
>>>>>>> An alternate way to generate numbers without repeat, and avoid
>>>>>>> recalling the random number generator, is first generate the number
>>>>>>> between 0 and N-1,
>>>>>>>
>>>>>>> Then generate the second number between 0 and N-2, and if the result
>>>>>>> is greater than or equal the first number, increment it.
>>>>>>>
>>>>>>> Then generate the third number between 0 and N-3, and if the result is
>>>>>>> greater than or equal to the first number, increment it. THen if it is
>>>>>>> greater than or equal to the second number, increment it.
>>>>>>>
>>>>>>> Just keep repeating the pattern with smaller and smaller ranges, and
>>>>>>> compare to the previous numbers in order, and increment if the value
>>>>>>> at that point is greater than or equoal.
>>>>>> I don't see how this does the job at all.  For example, when picking 3
>>>>>> from, say, [0,5] we might pick 2 and then 1.  Now if the third choice is
>>>>>> 1 it is not equal to 2 (so no increment) but it is equal to 1 so
>>>>>> increment to 2.
>>>>>
>>>>> As I think about it, if you increment, you need to restart the scan
>>>>> with the beginning number, but not repeat a test.  Another way is have
>>>>> a sorted list of picks and work smallest to largest.
>>>>>
>>>>> So:
>>>>>
>>>>> #1
>>>>>   From [0, 5] get 2
>>>>>
>>>>> #2
>>>>>   From [0, 4] get 1
>>>>> 1 not >= 2 so ok.
>>>>>
>>>>> #3
>>>>>
>>>>>   From [0, 3] get 1
>>>>> 1 not >= 2 so next
>>>>> 1 IS >= 1 so make 2 and restart
>>>>> 1 >= 2 so make 3
>>>> Presumably "2 >= 2 so make 3"
>>>>
>>>>> don't repeat test 2.
>>>> Now you just get highly biased results (if I've got then hang of it).
>>>>
>>>
>>> No, it is unbiased, we skip the test of the second number, as that
>>> rule was already used for this number.
>>
>> What's the algorithm, then?  My attempt to code it from your description
>> must have gone wrong.
>
> You choose a random number based on the number of UNCHOSEN numbers
> left.

Can you give the algorithm using some pseudo code?

> since they weren't all the lowest numbers, you increase the value by
> one for every chosen value that is less than or equal to the number
> picked (including after any other increases).

Surely it's a few lines of pseudo code.  I'm getting lost in the words.

-- 
Ben.

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Compute Unique Numbers in a Set Albert <invalid@gmail.com> - 2022-12-26 23:45 +0000
  Re: Compute Unique Numbers in a Set "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-12-26 16:52 -0800
    Re: Compute Unique Numbers in a Set tTh <tth@none.invalid> - 2022-12-27 02:44 +0100
      Re: Compute Unique Numbers in a Set "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-12-27 13:35 -0800
  Re: Compute Unique Numbers in a Set Richard Damon <Richard@Damon-Family.org> - 2022-12-26 20:47 -0500
    Re: Compute Unique Numbers in a Set Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-12-27 15:57 +0000
      Re: Compute Unique Numbers in a Set Richard Damon <Richard@Damon-Family.org> - 2022-12-27 11:16 -0500
        Re: Compute Unique Numbers in a Set Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-12-27 16:59 +0000
          Re: Compute Unique Numbers in a Set Richard Damon <Richard@Damon-Family.org> - 2022-12-27 12:24 -0500
            Re: Compute Unique Numbers in a Set Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-12-27 17:53 +0000
              Re: Compute Unique Numbers in a Set Richard Damon <Richard@Damon-Family.org> - 2022-12-27 13:50 -0500
                Re: Compute Unique Numbers in a Set Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-12-27 20:08 +0000
                Re: Compute Unique Numbers in a Set Richard Damon <Richard@Damon-Family.org> - 2022-12-27 15:31 -0500
                Re: Compute Unique Numbers in a Set Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-12-28 02:57 +0000
                Re: Compute Unique Numbers in a Set Richard Damon <Richard@Damon-Family.org> - 2022-12-27 23:02 -0500
                Re: Compute Unique Numbers in a Set Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-12-29 02:06 +0000
                Re: Compute Unique Numbers in a Set Richard Damon <Richard@Damon-Family.org> - 2022-12-28 23:42 -0500
                Re: Compute Unique Numbers in a Set Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-12-29 12:26 +0000
  Re: Compute Unique Numbers in a Set Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-12-27 07:34 -0800
  Re: Compute Unique Numbers in a Set Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-12-27 16:18 +0000
    Re: Compute Unique Numbers in a Set Mike Terry <news.dead.person.stones@darjeeling.plus.com> - 2022-12-28 01:08 +0000
      Re: Compute Unique Numbers in a Set Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-12-28 03:30 +0000
    Re: Compute Unique Numbers in a Set Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-12-27 19:42 -0800
  Re: Compute Unique Numbers in a Set Manu Raju <MR@invalid.invalid> - 2022-12-27 18:11 +0000
  Re: Compute Unique Numbers in a Set antispam@math.uni.wroc.pl - 2022-12-28 01:32 +0000
    Re: Compute Unique Numbers in a Set Richard Damon <Richard@Damon-Family.org> - 2022-12-27 21:13 -0500
      Re: Compute Unique Numbers in a Set Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-12-27 19:48 -0800
        Re: Compute Unique Numbers in a Set Richard Damon <Richard@Damon-Family.org> - 2022-12-27 23:06 -0500
      Re: Compute Unique Numbers in a Set antispam@math.uni.wroc.pl - 2022-12-29 19:47 +0000
  Re: Compute Unique Numbers in a Set jak <nospam@please.ty> - 2022-12-28 11:28 +0100
  Re: Compute Unique Numbers in a Set Rosario19 <Ros@invalid.invalid> - 2023-01-01 21:06 +0100
    Re: Compute Unique Numbers in a Set Rosario19 <Ros@invalid.invalid> - 2023-01-02 06:43 +0100
  Re: Compute Unique Numbers in a Set gazelle@shell.xmission.com (Kenny McCormack) - 2023-01-01 23:19 +0000
    Re: Compute Unique Numbers in a Set jak <nospam@please.ty> - 2023-01-02 07:28 +0100
      Re: Compute Unique Numbers in a Set jak <nospam@please.ty> - 2023-01-02 08:48 +0100
      Re: Compute Unique Numbers in a Set Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-01-01 23:53 -0800
        Re: Compute Unique Numbers in a Set Öö Tiib <ootiib@hot.ee> - 2023-01-02 01:18 -0800
    Re: Compute Unique Numbers in a Set gazelle@shell.xmission.com (Kenny McCormack) - 2023-01-02 13:49 +0000
  Re: Compute Unique Numbers in a Set Bart <bc@freeuk.com> - 2023-01-02 12:27 +0000
  Re: Compute Unique Numbers in a Set Richard Damon <Richard@Damon-Family.org> - 2023-01-02 12:13 -0500
    Re: Compute Unique Numbers in a Set Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-01-02 17:31 +0000
      Re: Compute Unique Numbers in a Set Richard Damon <Richard@Damon-Family.org> - 2023-01-02 12:46 -0500
      Re: Compute Unique Numbers in a Set Siri Cruise <chine.bleu@yahoo.com> - 2023-01-02 18:54 -0800
      Re: Compute Unique Numbers in a Set Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-01-02 20:52 -0800
      Re: Compute Unique Numbers in a Set David Brown <david.brown@hesbynett.no> - 2023-01-03 09:01 +0100
      Re: Compute Unique Numbers in a Set Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-01-03 07:28 -0800
        Re: Compute Unique Numbers in a Set gazelle@shell.xmission.com (Kenny McCormack) - 2023-01-03 15:46 +0000
          Re: Compute Unique Numbers in a Set David Brown <david.brown@hesbynett.no> - 2023-01-03 18:19 +0100
  Re: Compute Unique Numbers in a Set Bonita Montero <Bonita.Montero@gmail.com> - 2023-01-08 03:48 +0100
    Re: Compute Unique Numbers in a Set Bonita Montero <Bonita.Montero@gmail.com> - 2023-01-08 04:18 +0100
    Re: Compute Unique Numbers in a Set gazelle@shell.xmission.com (Kenny McCormack) - 2023-01-08 03:48 +0000
      Re: Compute Unique Numbers in a Set Bonita Montero <Bonita.Montero@gmail.com> - 2023-01-08 05:12 +0100
        Re: Compute Unique Numbers in a Set Bonita Montero <Bonita.Montero@gmail.com> - 2023-01-08 06:01 +0100
          Re: Compute Unique Numbers in a Set Bart <bc@freeuk.com> - 2023-01-08 14:48 +0000
            Re: Compute Unique Numbers in a Set Bonita Montero <Bonita.Montero@gmail.com> - 2023-01-08 18:22 +0100
              Re: Compute Unique Numbers in a Set Bart <bc@freeuk.com> - 2023-01-08 17:46 +0000
                Re: Compute Unique Numbers in a Set Bonita Montero <Bonita.Montero@gmail.com> - 2023-01-09 04:58 +0100
                Re: Compute Unique Numbers in a Set Bart <bc@freeuk.com> - 2023-01-09 11:26 +0000
                Re: Compute Unique Numbers in a Set Bonita Montero <Bonita.Montero@gmail.com> - 2023-01-09 15:57 +0100
            Re: Compute Unique Numbers in a Set Bart <bc@freeuk.com> - 2023-01-08 17:34 +0000
              Re: Compute Unique Numbers in a Set Ike Naar <ike@sdf.org> - 2023-01-08 21:45 +0000
                Re: Compute Unique Numbers in a Set Bart <bc@freeuk.com> - 2023-01-08 23:13 +0000
                Re: Compute Unique Numbers in a Set "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-01-08 15:18 -0800
          Re: Compute Unique Numbers in a Set Paavo Helde <eesnimi@osa.pri.ee> - 2023-01-08 21:19 +0200
          Re: Compute Unique Numbers in a Set "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2023-01-09 12:03 +0100
            Re: Compute Unique Numbers in a Set Bonita Montero <Bonita.Montero@gmail.com> - 2023-01-09 17:42 +0100
              Re: Compute Unique Numbers in a Set Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2023-01-09 14:48 -0800
              Re: Compute Unique Numbers in a Set Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-01-09 23:22 +0000
                Re: Compute Unique Numbers in a Set Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2023-01-09 18:28 -0800
                Re: Compute Unique Numbers in a Set Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2023-01-09 18:44 -0800
                Re: Compute Unique Numbers in a Set Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-01-10 03:08 +0000
                Re: Compute Unique Numbers in a Set Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2023-01-09 19:19 -0800
                Re: Compute Unique Numbers in a Set Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-01-10 17:30 +0000
                Re: Compute Unique Numbers in a Set Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2023-01-10 09:45 -0800
                Re: Compute Unique Numbers in a Set Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-01-10 02:57 +0000
              Re: Compute Unique Numbers in a Set Bonita Montero <Bonita.Montero@gmail.com> - 2023-01-13 08:26 +0100
                Re: Compute Unique Numbers in a Set Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2023-01-13 03:32 -0800
                Re: Compute Unique Numbers in a Set gazelle@shell.xmission.com (Kenny McCormack) - 2023-01-13 12:21 +0000
                Re: Compute Unique Numbers in a Set Bonita Montero <Bonita.Montero@gmail.com> - 2023-01-13 14:11 +0100
                Re: Compute Unique Numbers in a Set gazelle@shell.xmission.com (Kenny McCormack) - 2023-01-13 13:55 +0000
                Re: Compute Unique Numbers in a Set Bonita Montero <Bonita.Montero@gmail.com> - 2023-01-13 15:02 +0100
                Re: Compute Unique Numbers in a Set gazelle@shell.xmission.com (Kenny McCormack) - 2023-01-13 14:17 +0000
                Re: Compute Unique Numbers in a Set Bonita Montero <Bonita.Montero@gmail.com> - 2023-01-15 16:06 +0100
                Re: Compute Unique Numbers in a Set Bart <bc@freeuk.com> - 2023-01-15 16:27 +0000
                Re: Compute Unique Numbers in a Set Bonita Montero <Bonita.Montero@gmail.com> - 2023-01-15 17:42 +0100
                Re: Compute Unique Numbers in a Set gazelle@shell.xmission.com (Kenny McCormack) - 2023-01-15 17:11 +0000
        Re: Compute Unique Numbers in a Set tTh <tth@none.invalid> - 2023-01-08 10:13 +0100
          Re: Compute Unique Numbers in a Set Bonita Montero <Bonita.Montero@gmail.com> - 2023-01-08 10:25 +0100
            Re: Compute Unique Numbers in a Set gazelle@shell.xmission.com (Kenny McCormack) - 2023-01-08 13:20 +0000
    Re: Compute Unique Numbers in a Set "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-01-08 13:09 -0800
  Re: Compute Unique Numbers in a Set Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-01-09 02:37 +0000
  Re: Compute Unique Numbers in a Set John Forkosh <forkosh@panix.com> - 2023-01-15 02:49 +0000

csiph-web