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 Posted: Sep 08, 2006 - 12:33 AM |
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Joined: Apr 19, 2006
Posts: 19
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| i need to have a random number between 0 and 7 generated. i am using an attiny2313. can any one give me a little snippet of code. |
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Posted: Sep 08, 2006 - 06:08 AM |
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Joined: Jan 15, 2002
Posts: 18
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mmmlinux wrote:
i need to have a random number between 0 and 7 generated. i am using an attiny2313. can any one give me a little snippet of code.
From the C FAQ:
Quote:
13.16: How can I get random integers in a certain range?
A: The obvious way,
rand() % N /* POOR */
(which tries to return numbers from 0 to N-1) is poor, because
the low-order bits of many random number generators are
distressingly *non*-random. (See question 13.18.) A better
method is something like
(int)((double)rand() / ((double)RAND_MAX + 1) * N)
If you're worried about using floating point, you could use
rand() / (RAND_MAX / N + 1)
Both methods obviously require knowing RAND_MAX (which ANSI
#defines in <stdlib.h>), and assume that N is much less than
RAND_MAX.
(Note, by the way, that RAND_MAX is a *constant* telling you
what the fixed range of the C library rand() function is. You
cannot set RAND_MAX to some other value, and there is no way of
requesting that rand() return numbers in some other range.)
If you're starting with a random number generator which returns
floating-point values between 0 and 1, all you have to do to get
integers from 0 to N-1 is multiply the output of that generator
by N.
References: K&R2 Sec. 7.8.7 p. 168; PCS Sec. 11 p. 172.
--
brunk |
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Posted: Jan 24, 2010 - 01:21 AM |
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Joined: Jan 17, 2010
Posts: 5
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this is a good fast integer generator
http://en.wikipedia.org/wiki/Linear_con ... _generator
I use this specific line for integers from 0 to 250
rand=(rand*109+89)%251;
init rand with some value. I start with 89. I think even 0 works. requires 16bit operations. might work with %256 too for 0-255 range. I just use primes.
short sequence of 40 numbers seems to have nice even distribution |
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Posted: Jun 24, 2010 - 10:24 AM |
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Joined: Apr 23, 2010
Posts: 1
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Kunne også bare gøres sådan
Dim I As Word ' dim variable
Do
I = Rnd(40) 'get random number (0-39)
Print I 'print the value
Wait 1 'wait 1 second
Loop |
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Posted: Jun 24, 2010 - 10:42 AM |
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Joined: Aug 21, 2002
Posts: 895
Location: Austria
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It would be nice to post in English language.
BTW, Bing translates it as
"Could also just be such" |
_________________
/Martin.
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Posted: Jun 24, 2010 - 12:15 PM |
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Joined: May 02, 2006
Posts: 321
Location: Norway
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| oh.. all people not speak norwegian! |
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Posted: Jun 24, 2010 - 12:55 PM |
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Joined: Aug 29, 2002
Posts: 790
Location: Muenster, Germany
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If you want a non-deterministic random generator (i.e. one not based on mathematical algorithms), just do some ADC samples of an open port line and use the low bit of consecutive samples to construct your random number.
-- Thilo |
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Posted: Jun 24, 2010 - 01:42 PM |
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Joined: May 24, 2004
Posts: 6029
Location: Tampere, Finland
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I also don't speak norwegian but can still understand good enough to translate "can also be made like this".
Some algorithms are better than others, but depending on what you are planning to do with the random number, we cannot know what is overkill and what is stupid way.
For normal dice games etc it is good enough to do something like this (pseudocode):
Code:
repeat
random=random+1;
if random==8 then random=0;
until key_pressed;
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Posted: Dec 29, 2011 - 02:37 AM |
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Joined: Jan 01, 2007
Posts: 1
Location: Norway
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| That's danish, not norwegian. |
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Posted: Jan 01, 2012 - 01:51 AM |
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Joined: Aug 03, 2001
Posts: 328
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A number from 0 to 7?
How random can 3 bits be?
There has been a pretty recent post about using the lower bit(s) of the ADC as input for a random number and a few years ago a guy wrote a nice article with several different sources for random bits in AVR processors. The ADC was one of the sources, phase noise between the avr main chrystal and a 32kHz watch chrystal was another. The article (in the pojects section?) also had (has) code examples and some analysis of the randomness of those different sources. Good read and highly recomended for danisch, norwegian and other people. |
_________________
Paul van der Hoeven.
40+ projects with AVR's:
http://www.hoevendesign.com
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Posted: Jan 01, 2012 - 10:09 AM |
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Joined: Jan 08, 2009
Posts: 1202
Location: Lund, Sweden
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Paulvdh wrote:
A number from 0 to 7?
How random can 3 bits be?
That's a very good question. Here's a sequence of 20 independent random numbers from 0 to 7
Code:
3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3
No, I'm not kidding, it really is. It's just that 3 have a 99.99% probability to be drawn and thus a very low probability for the other numbers to share.
Here's another sequence from 0 to 7 generated with a completely different method
Code:
3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3
In this case I have a start number choosen with equal probability and next number be the same as the previous with 99.99% probability and with 0.01% probability be a new one choosen uniformly from 0 to 7.
These two methods will give completely different sequences in the long run, with completely different statistical properties. But they're both random sequences of 3 bit random numbers.
So how random can 3 bits be? Well they can be random in very many ways. Usually one want something more than just "random". |
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Posted: Jan 10, 2012 - 04:08 PM |
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Joined: Aug 03, 2001
Posts: 328
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@sniggelen:
You're complely wrong of course, it's not "3" but it's "4".
Code:
int getRandomNumber()
{
return 4; // Chosen by fair dice roll.
// Guaranteed to be random.
}
Reference:
http://xkcd.com/221/
Anyway, I always did suck at statistics at school.
Quote:
Usually one want something more than just "random".
A very good example of that is the shuffle function of a cd player. I once had a cd player with a true "random" button and it was pretty annoying when it played the same song 2 or 3 times in a row. |
_________________
Paul van der Hoeven.
40+ projects with AVR's:
http://www.hoevendesign.com
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Posted: Feb 23, 2012 - 09:46 AM |
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Joined: Feb 23, 2012
Posts: 3
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There are a variety of well-studied techniques for this. Google "pseudorandom number generation" or "pseudorandom number generator".
I believe that modern Intel chips have an instruction or somesuch that actually produces true random numbers (it uses two interfering oscillators plugged into a voltage meter, or something) but I do not know whether any programming libraries actually tap into this. |
Last edited by Quatar on Jan 10, 2013 - 06:35 AM; edited 2 times in total
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Posted: Feb 23, 2012 - 10:42 AM |
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Joined: Jul 18, 2005
Posts: 62944
Location: (using avr-gcc in) Finchingfield, Essex, England
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Quote:
I believe that modern Intel chips have an instruction or somesuch that actually produces true random numbers (it uses two interfering oscillators plugged into a voltage meter, or something) but I do not know whether any programming libraries actually tap into this.
What relevance would that have to an avr-gcc programmer anyway?
In the meantime the Tutorial forum here has at least two good articles about random numbers on AVRs. |
_________________
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Posted: Feb 23, 2012 - 02:48 PM |
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Joined: Nov 11, 2003
Posts: 4042
Location: Chicago Illinois USA
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| One I tried in my younger days was to read the program's code byte by byte. It was terrible. |
_________________
Discursive design,
Torby
Some days, it's just not worth chewing through the restraints.
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Posted: Feb 23, 2012 - 02:50 PM |
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Joined: Jul 18, 2005
Posts: 62944
Location: (using avr-gcc in) Finchingfield, Essex, England
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Quote:
It was terrible.
Not only not random but presumably totally deterministic. At least if you have a number of variables in SRAM I suppose there'd be a chance if you pointed to the area (maybe the SFRs too?) then hashed/CRCd the whole lot you mioght get something a bit random? |
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Posted: Feb 14, 2013 - 04:00 AM |
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Joined: Mar 23, 2012
Posts: 20
Location: Brisbane
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| Thanks worked a treat. I know it's not purely random, but it's varied enough to suit my needs. |
_________________
Australian Entrepreneur, using the AVR technology for prototyping and market testing.
http://brisrocket.wordpress.com
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Posted: Feb 14, 2013 - 02:49 PM |
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Joined: Nov 11, 2003
Posts: 4042
Location: Chicago Illinois USA
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TPE wrote:
oh.. all people not speak norwegian!
What a surprise. |
_________________
Discursive design,
Torby
Some days, it's just not worth chewing through the restraints.
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