How to generate a random number? Post: 8621

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Top Forums Programming How to generate a random number? Post 8621 by PxT on Monday 15th of October 2001 12:35:15 PM

10-15-2001

Registered User

From the linux rand(3) man page:

Quote:

The versions of rand() and srand() in the Linux C Library use the same random number gen�
erator as random() and srandom(), so the lower-order bits should be as random as the
higher-order bits. However, on older rand() implementations, the lower-order bits are
much less random than the higher-order bits.

In Numerical Recipes in C: The Art of Scientific Computing (William H. Press, Brian P.
Flannery, Saul A. Teukolsky, William T. Vetterling; New York: Cambridge University Press,
1992 (2nd ed., p. 277)), the following comments are made:
"If you want to generate a random integer between 1 and 10, you should always do
it by using high-order bits, as in

j=1+(int)(10.0*rand()/(RAND_MAX+1.0));

and never by anything resembling

j=1+(rand() % 10);

(which uses lower-order bits)."

In other words, under Linux your algorithm is probably fine, on other architectures it may not be very random. For portability, use the noted syntax or check the man page for your particular rand implementation.

PxT

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LEARN ABOUT DEBIAN

ns_rand

ns_rand(3aolserver)					    AOLserver Built-In Commands 				       ns_rand(3aolserver)

__________________________________________________________________________________________________________________________________________________

NAME

       ns_rand - commands

SYNOPSIS

       ns_rand ?max?
_________________________________________________________________

DESCRIPTION

       ns_rand	generates  a  cryptographically-secure random number.  If max is not specified, the random number is a floating-point value n such
       that 0.0 <= n < 1.0.

       If max is specified, it must be a positive integer between 0 and 2147483647.  The return value is an integer value such that 0 <= n <= (max
       - 1).

       Internally  ns_rand is implemented with the drand48(3) and lrand48(3) standard library functions.  An internal random seed is generated the
       first time ns_rand is called after the server starts.

EXAMPLES

	nscp> ns_rand
	0.39938485692

	nscp> ns_rand 10
	7

SEE ALSO

       nsd(1), info(n), drand48(3), lrand48(3)

KEYWORDS

AOLserver								4.0						       ns_rand(3aolserver)

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