NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part1/1739

Text File | 1996-08-05 | 1.9 KB | 56 lines

Path: mail2news.demon.co.uk!genesis.demon.co.uk
From: Lawrence Kirby <[e-mail]>
Newsgroups: comp.lang.c
Subject: Re: swap high and low 16 bits?
Date: Tue, 16 Jan 96 17:06:29 GMT
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In article <[e-mail]>
           [e-mail] "Bill Simpson" writes:

>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.
>
>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.

OK, I assume then you have something like :

#include <time.h>

    unsigned long val;

    val = time(NULL);

That's not strictly portable (for instance time_t could have type double
and the conversion to unsigned long could trap) but works on many
implementations.

One simple approach would be to call your random funciton (rand() or whatever)
a few times and discard the values. The 2 random number sequences should
diverge quickly for a reasonable RNG algorithm. Another approach might be

    ...
    val *= N;

where N is a large odd number. Unsigned arithmetic is modulo arithmetic - it
can't overflow. Alternatively you could do something like:

    val ^= val << 16;

which will cause variation in higher order bits for small changes of val.

--
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Lawrence Kirby | [e-mail]
Wilts, England | [e-mail]
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