Re: Randomly selecting an element from an STL collection

From:

"Victor Bazarov" <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++,comp.sources.d

Date:

Thu, 28 Jun 2007 15:30:42 -0400

Message-ID:

<f61293$160$1@news.datemas.de>

Generic Usenet Account wrote:

[..] I would really appreciate if you
could tell me how I can avoid the "bias towards smaller elements". I
have always used the "modulo division by random number" trick to
restrict my pseudo-random numbers to a certain range. I am keen to
avoid this mistake at some future date. A sample code snippet would
be great. Although I do not expect anywhere close to 32767 entries in
my collection, it is certainly going to be much more than 10 or 20.

When I need a random number in a certain range, I use

    int rand_n(int lower, int upper)
    {
        return int(rand() * (upper - lower + 1.0) / (RAND_MAX + 1.0))
                 + lower;
    }

That essentially subdivides the 0..RAND_MAX range into (upper-lower+1)
subranges and gives you the index of the subrange in which the random
number you get from 'rand' fits, offset by 'lower'. I am sure that
this doesn't work very well for (upper-lower) larger than RAND_MAX/2.

V
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