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Internet Message Format | 1996-08-05 | 1.9 KB

Path: Rezonet.net!news
From: [e-mail] (Ray Dunn)
Newsgroups: comp.lang.c
Subject: Re: swap high and low 16 bits?
Date: 18 Jan 1996 04:23:33 GMT
Organization: Ultimate Technographics Inc.
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In referenced article, Bill Simpson says...
>
>I wanted to obtain a "random" starting number each time a program
>is run.  The obvious choice is time(NULL).  The problem with it is
>that if the program runs and terminates fairly quickly, the number
>returned by time(NULL) is quite similar each run, differing only by
>the lowest bits.

If you use the number you get from time as the seed of the random
number generator i.e. srand((unsigned int)time(NULL)); then use rand()
to get a random number in the range 0 to RAND_MAX (which is usually the
maximum integer)  This should be exactly what you want. The rand
functions are defined in stdlib.h;

>One solution would be to swap the high 16 bits and the low 16 bits of
>the 32 bit unsigned long int.  Sorry this is dirty, not ANSI.

This would give you a number that was numerically very different each
time, but the low 16-bits would be the same every run.  Probably not
what you want, but you can do it easily in arithmetic:

  number = (number >> 16) | (number << 16);

If you don't currently understand bitwise operations, then try:

  number = number / 65536 + (number % 65536) * 65536;

[ % is the modulus operator - it produces the remainder, and is
required to avoid the * 65536 giving integer overflow, which may
cause a trap on you machine]

number must be unsigned for this to have the effect you want.
--
Ray Dunn (opinions are my own) | Phone: [phone]
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