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Posted by PerlFAQ Server on May 2, 2008, 3:03 pm
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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 .
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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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