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-   -   Why will this not generate a random number!? (http://www.velocityreviews.com/forums/t435611-why-will-this-not-generate-a-random-number.html)

 John Cassidy 10-27-2004 06:37 AM

Why will this not generate a random number!?

This has been driving me crazy. I've done basic C in school, but my
education is mainly based on object oriented design theory where Java
is our tool. For some reason, while helping a friend with a C
Programming lab. I cannot for the life of me generate a random number.

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

#define RAND_MAX 32768

int main(void)
{
float b;

b = ((float)rand()/RAND_MAX);

printf("Random Number: %f", b);

system("PAUSE");
return 0;
}

 Hans-Christian Egtvedt 10-27-2004 06:58 AM

Re: Why will this not generate a random number!?

John Cassidy wrote:
> This has been driving me crazy. I've done basic C in school, but my
> education is mainly based on object oriented design theory where Java
> is our tool. For some reason, while helping a friend with a C
> Programming lab. I cannot for the life of me generate a random number.

rand() will generate spesific "random", se my addition to the code below.

> #include <stdio.h>
> #include <stdlib.h>
> #include <math.h>
>
> #define RAND_MAX 32768
>
> int main(void)
> {
> float b;

// The function srand() is used to seed the random sequence generated by
// rand(). For any given seed, rand() will generate a specific "random"
// sequence over and over again.
srand(time(NULL));

> b = ((float)rand()/RAND_MAX);
>
>
> printf("Random Number: %f", b);
>
> system("PAUSE");
> return 0;
> }

If you're on a UNIX system you might want to consider using /dev/urandom
to generate random.

--
Regards
Hans-Christian Egtvedt

 Keith Thompson 10-27-2004 07:18 AM

Re: Why will this not generate a random number!?

johnwcassidy@gmail.com (John Cassidy) writes:
> This has been driving me crazy. I've done basic C in school, but my
> education is mainly based on object oriented design theory where Java
> is our tool. For some reason, while helping a friend with a C
> Programming lab. I cannot for the life of me generate a random number.
>
> #include <stdio.h>
> #include <stdlib.h>
> #include <math.h>
>
> #define RAND_MAX 32768
>
> int main(void)
> {
> float b;
>
> b = ((float)rand()/RAND_MAX);
>
>
> printf("Random Number: %f", b);
>
> system("PAUSE");
> return 0;
> }

Thank you for posting a complete program (too many people post
fragments or pseudo-code). But a little more detail on what behavior
you're actually seeing would be helpful. You say it doesn't generate
a random number; what does it do?

You don't need <math.h>; rand() is declared in <stdlib.h>

Don't define RAND_MAX yourself, it's defined for you in <stdlib.h>.

Your output doesn't end in a newline; on systems, this might cause
your output not to appear at all. system("PAUSE") won't work if the
system doesn't have a PAUSE command.

Your system should have documentation on the rand() and srand()
functions. Find it and read it.

I think your actual problem is probably answered by entry 13.17 of the
C FAQ, <http://www.eskimo.com/~scs/C-faq/q13.17.html>. Questions
13.15 through 13.20 deal with random numbers. Read the whole section.

If you call rand() without first calling srand(), it's equivalent to
calling rand() after calling srand(1). This always gives you the same
sequence of numbers. On one system I just tried, the first call to
rand() gave me 0.

--
Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

 Martin Ambuhl 10-27-2004 08:28 AM

Re: Why will this not generate a random number!?

John Cassidy wrote:

> This has been driving me crazy. I've done basic C in school, but my
> education is mainly based on object oriented design theory where Java
> is our tool. For some reason, while helping a friend with a C
> Programming lab. I cannot for the life of me generate a random number.

Before we get started here, please note that this is fully covered in
the FAQ, and it is normally accepted practice in newsgroups to check the
FAQ before posting. Before you shout, "But I didn't know there was a
FAQ," note also that it is normally accepted practice to lurk in
newsgroups before posting; many people here have links to the FAQ in
their sigs and, if you had lurked for anything like the expected period,
you would have seen one of the periodic postings of the FAQ itself.

>
> #include <stdio.h>
> #include <stdlib.h>
> #include <math.h>
>
> #define RAND_MAX 32768

No, you don't do this. RAND_MAX is a macro defined by your implementation.

>
> int main(void)
> {
> float b;
>

You ought to, unless you want the same sequence everytime you run the
program, seed the random number generator once with srand() before using
it. See the FAQ for details.

> b = ((float)rand()/RAND_MAX);

OK, so you want b in the range [0,1]. Generally, however, you really
want b in the range [0,1):
b = rand()/(RAND_MAX+1.);

> printf("Random Number: %f", b);
>
> system("PAUSE");

You are getter off not relying on the host system having a command named
"PAUSE".
getchar();
would accomplish the same thing, wouldn't it?
> return 0;
> }

 Stuart Gerchick 10-27-2004 12:16 PM

Re: Why will this not generate a random number!?

johnwcassidy@gmail.com (John Cassidy) wrote in message news:<6f109565.0410262237.43310d88@posting.google. com>...
> This has been driving me crazy. I've done basic C in school, but my
> education is mainly based on object oriented design theory where Java
> is our tool. For some reason, while helping a friend with a C
> Programming lab. I cannot for the life of me generate a random number.
>
> #include <stdio.h>
> #include <stdlib.h>
> #include <math.h>
>
> #define RAND_MAX 32768
>
> int main(void)
> {
> float b;
>
> b = ((float)rand()/RAND_MAX);
>
>
> printf("Random Number: %f", b);
>
> system("PAUSE");
> return 0;
> }

Try to seed with srand()

 Dan Pop 10-27-2004 05:28 PM

Re: Why will this not generate a random number!?

In <6f109565.0410262237.43310d88@posting.google.com > johnwcassidy@gmail.com (John Cassidy) writes:

>This has been driving me crazy. I've done basic C in school, but my
>education is mainly based on object oriented design theory where Java
>is our tool. For some reason, while helping a friend with a C
>Programming lab. I cannot for the life of me generate a random number.
>
>#include <stdio.h>
>#include <stdlib.h>
>#include <math.h>

What did you include <math.h> for?

>#define RAND_MAX 32768

a better one. If it did, you have absolutely no excuse for having
ignored it!

>int main(void)
>{
> float b;
>
> b = ((float)rand()/RAND_MAX);
>
>
> printf("Random Number: %f", b);

Why do you think this number is not random enough? If you want different
numbers on different runs, open your favourite C book and read the
specification of the rand() and srand() functions. Or read the FAQ,
which is a MUST before posting for the first time, anyway.

> system("PAUSE");

You don't need this crap. If you're using a poorly designed IDE that
closes the program console as soon as the program terminates, getchar()
is the function you want to call here. It is also supposed to fix the
bug in your printf call, which is not terminating the line properly.

> return 0;
>}

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Dan.Pop@ifh.de
Currently looking for a job in the European Union

 Dan Pop 10-27-2004 05:33 PM

Re: Why will this not generate a random number!?

In <2u94htF27g57qU1@uni-berlin.de> Martin Ambuhl <mambuhl@earthlink.net> writes:

>John Cassidy wrote:
>
>> #include <stdio.h>
>> #include <stdlib.h>
>> #include <math.h>
>>
>> #define RAND_MAX 32768

>
>No, you don't do this. RAND_MAX is a macro defined by your implementation.
>
>>
>> int main(void)
>> {
>> float b;
>>

>
>You ought to, unless you want the same sequence everytime you run the
>program, seed the random number generator once with srand() before using
>it. See the FAQ for details.
>
>> b = ((float)rand()/RAND_MAX);

>
>OK, so you want b in the range [0,1]. Generally, however, you really
>want b in the range [0,1):
> b = rand()/(RAND_MAX+1.);

Have a closer look at his definition of RAND_MAX. If his implementation
is using a 15-bit generator (like the one shown in the C standard), he is
effectively dividing by RAND_MAX+1 :-)

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Dan.Pop@ifh.de
Currently looking for a job in the European Union

 CBFalconer 10-27-2004 05:50 PM

Re: Why will this not generate a random number!?

Stuart Gerchick wrote:
> johnwcassidy@gmail.com (John Cassidy) wrote in message
>
>> This has been driving me crazy. I've done basic C in school, but
>> my education is mainly based on object oriented design theory
>> where Java is our tool. For some reason, while helping a friend
>> with a C Programming lab. I cannot for the life of me generate a
>>
>> #include <stdio.h>
>> #include <stdlib.h>
>> #include <math.h>
>>
>> #define RAND_MAX 32768
>>
>> int main(void)
>> {
>> float b;
>>
>> b = ((float)rand()/RAND_MAX);
>> printf("Random Number: %f", b);
>> system("PAUSE");
>> return 0;
>> }

>
> Try to seed with srand()

Maybe because he should be using double, not float?

--
Chuck F (cbfalconer@yahoo.com) (cbfalconer@worldnet.att.net)
Available for consulting/temporary embedded and systems.

 Old Wolf 10-27-2004 11:06 PM

Re: Why will this not generate a random number!?

> John Cassidy wrote:
>
> > system("PAUSE");

>
> You are getter off not relying on the host system having a
> command named "PAUSE".
> getchar();
> would accomplish the same thing, wouldn't it?

Not entirely: "pause" waits for any keypress, whereas getchar()
usually requires the Enter key.

 Martin Ambuhl 10-28-2004 12:00 AM

Re: Why will this not generate a random number!?

Old Wolf wrote:

>
>>John Cassidy wrote:
>>
>>
>>> system("PAUSE");

>>
>>You are getter off not relying on the host system having a
>>command named "PAUSE".
>> getchar();
>>would accomplish the same thing, wouldn't it?

>
>
> Not entirely: "pause" waits for any keypress, whereas getchar()
> usually requires the Enter key.

Can you really predict with confidence that "pause" works that way on
any platform?

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