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Full Discussion: How to generate a random number?
Top Forums Programming How to generate a random number? Post 8621 by PxT on Monday 15th of October 2001 12:35:15 PM
Old 10-15-2001
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.
 

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LEARN ABOUT HPUX

values

values(5)							File Formats Manual							 values(5)

NAME

       values - machine-dependent values

SYNOPSIS

DESCRIPTION

       This file contains a set of manifest constants, conditionally defined for particular processor architectures.

       The  model  assumed  for  integers  is binary representation (one's or two's complement), where the sign is represented by the value of the
       high-order bit.

	      The number of bits in a specified type (e.g., int).

	      The value of a short integer with only the high-order bit set
			     (in most implementations, 0x8000).

	      The value of a long integer with only the high-order bit set
			     (in most implementations, 0x80000000).

	      The value of a regular integer with only the high-order bit set
			     (usually the same as or

	      The maximum value of a signed short integer
			     (in most implementations, 0x7FFF == 32767).

	      The maximum value of a signed long integer
			     (in most implementations, 0x7FFFFFFF == 2147483647).

	      The maximum value of a signed regular integer
			     (usually the same as or

	      The maximum value of a single-precision floating-point number,
			     and its natural logarithm.

	      The maximum value of a double-precision floating-point number,
			     and its natural logarithm.

	      The minimum positive value of a single-precision floating-point number,
			     and its natural logarithm.

	      The minimum positive value of a double-precision floating-point number,
			     and its natural logarithm.

	      The number of significant bits in the mantissa of a single-precision
			     floating-point number.

	      The number of significant bits in the mantissa of a double-precision
			     floating-point number.

FILES

SEE ALSO

       intro(3), math(5).

STANDARDS CONFORMANCE

																	 values(5)
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