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Random numbers

Posted on 1998-10-18
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Last Modified: 2010-03-05
I need a section of code to generate a random number between 0 and some value n and then round it down
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Question by:jedimike
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b2pi earned 20 total points
ID: 1205474
'round it down' is a bit confusing.

Basically, though, you first need to set a seed if your perl is before
v 5.004 (If you don't, you'll get random numbers, but everytime you
run  the program, you could get about the same random numbers).  Just
to be safe, then


srand( time ^ $$ );

(There are many quibbles that the above might not be random enough.
Chances are, it is. If not, go to www.perl.com and search for
TrulyRandom)

Having seeded, you get a random value r (0 <= r < y ) by calling

$r = rand(y);

if n is a relatively large integer, then, you could do

$r = int(rand(n))

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Expert Comment

by:ozo
ID: 1205475
I would prefer ( time + $$ ) as a seed over ( time ^ $$ )
If $$ has a tendancy to be increasing, it could be easy for ( time ^ $$ ) to be tha same on subsequent runs.
(Using 5.004 may be even more preferred)
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by:b2pi
ID: 1205476
That requires that $$ is increasing at about the same rate as is time,
so $$ is approximately uniformly (approximately uniformly? Mea culpa,
mea maxima culpa) increasing by 1 per second.  If I remember my
combinatorics correctly (backed up by the perldoc page on srand) that
_still_ gives a 1 in 3 chance of being identical, nu? So if you're on
a machine that is averaging 1 pid per second (no more, no less), then
you've a 1 in 3 chance of problems.  If one's that worried about it,
best go to TrulyRandom, or, my all time favorite, make use of
http://lavarand.sgi.com 
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