FAQ 4.10 Why aren't my random numbers random?
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From: PerlFAQ Server on 22 May 2010 12:00 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/>. -------------------------------------------------------------------- The perlfaq-workers, a group of volunteers, maintain the perlfaq. They are not necessarily experts in every domain where Perl might show up, so please include as much information as possible and relevant in any corrections. The perlfaq-workers also don't have access to every operating system or platform, so please include relevant details for corrections to examples that do not work on particular platforms. Working code is greatly appreciated. If you'd like to help maintain the perlfaq, see the details in perlfaq.pod. |