FAQ 4.10 Why aren't my random numbers random?

Discussion in 'Perl Misc' started by PerlFAQ Server, Feb 12, 2011.

  1. This is an excerpt from the latest version perlfaq4.pod, which
    comes with the standard Perl distribution. These postings aim to
    reduce the number of repeated questions as well as allow the community
    to review and update the answers. The latest version of the complete
    perlfaq is at http://faq.perl.org .

    --------------------------------------------------------------------

    4.10: Why aren't my random numbers random?

    If you're using a version of Perl before 5.004, you must call "srand"
    once at the start of your program to seed the random number generator.

    BEGIN { srand() if $] < 5.004 }

    5.004 and later automatically call "srand" at the beginning. Don't call
    "srand" more than once--you make your numbers less random, rather than
    more.

    Computers are good at being predictable and bad at being random (despite
    appearances caused by bugs in your programs :). see the random article
    in the "Far More Than You Ever Wanted To Know" collection in
    <http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz>, courtesy of Tom
    Phoenix, talks more about this. John von Neumann said, "Anyone who
    attempts to generate random numbers by deterministic means is, of
    course, living in a state of sin."

    If you want numbers that are more random than "rand" with "srand"
    provides, you should also check out the "Math::TrulyRandom" module from
    CPAN. It uses the imperfections in your system's timer to generate
    random numbers, but this takes quite a while. If you want a better
    pseudorandom generator than comes with your operating system, look at
    "Numerical Recipes in C" at <http://www.nr.com/>.



    --------------------------------------------------------------------

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    so please include as much information as possible and relevant in any
    corrections. The perlfaq-workers also don't have access to every
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    corrections to examples that do not work on particular platforms.
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    perlfaq.pod.
    PerlFAQ Server, Feb 12, 2011
    #1
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