random values

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2 total messages Started by keith.willis Thu, 20 Aug 1992 21:25

#913

Author: keith.willis
Date: Thu, 20 Aug 1992 21:25

43 lines
1965 bytes

        If there is a more appropriate group for this, perhaps a
        stats newsgroup, please let me know.

        An aquaintance has a requirement for a large quantity of
        truly random values.  I believe the application for which
        they are required is a genetic mutation simulator.

        The way he has gone about getting this random data is a method
        used in several encryption programs, namely asking the user to
        type keys, and recording the intervals between keystrokes with
        a hi-res timer.  In fact, I believe he threw together a TSR
        which trapped keystroke intervals over the course of a day or
        so, in order to get a sufficient quantity of data.

        This is fine as a random bitstream, but for his application,
        he needs a set of random _values_, in a defined range, with
        the constraint that there must be at least one of each
        possible value.  I am completely useless at stats, so could
        people suggest how this might be achieved?

        Thanks in advance.


---
 ~ PQ 2.15 194 ~ 

----- PGP v1.0 --- public key ------ keith.willis@almac.co.uk

section 1 of uuencode 5.15 of file willis.pub    by R.E.M.

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`
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sum -r/size 40780/274 section (from "begin" to "end")
sum -r/size 57997/179 entire input file
--
ALMAC BBS Ltd. 0324-665371
Dos based USENET access,  call for details!

Re: random values

#924

Author: Keith Lewis
Date: Fri, 21 Aug 1992 13:42

29 lines
1736 bytes

In article <1992Aug20.864.127@ALMAC>, keith.willis@almac.co.uk writes:
>        This is fine as a random bitstream, but for his application,
>        he needs a set of random _values_, in a defined range, with

You can convert from a random bitstream to a stream of random discrete
values (let's say, integers from 0..R) by the following algorithm.

    Determine N, the number of bits you need (lowest N such that 2**N > R)

    while (more bits available)
        construct integer X out of next N bits (X = b(1)+2*b(2)+4*b(3)+...)
        if X <= R
            output X to stream
        else
            do nothing (discard bad X)
    endwhile

For ranges that are exactly a power of two, this algorithm works perfectly. 
For other ranges, it may discard (waste) almost half your bitstream.

>        the constraint that there must be at least one of each
>        possible value.  

At least one of each possible value?  That's a weird requirement.  Could you
just add a list of all possible values onto the end of the stream?

Keith Lewis             klewis@mitre.org          "Mr. Cheap"
I don't dance to music; music dances to me.  Email me for my PGP key.
The above may not (yet) represent the opinions of my employer.

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