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Why will this not generate a random number!?

P: n/a
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.
i don't know what is wrong. please help.

#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;
}
Nov 14 '05 #1
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15 Replies


P: n/a
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.
i don't know what is wrong. please help.
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
Nov 14 '05 #2

P: n/a
jo**********@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.
i don't know what is wrong. please help.

#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.
Read the whole FAQ.

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) ks***@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.
Nov 14 '05 #3

P: n/a
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.
i don't know what is wrong. please help.
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;
}

Nov 14 '05 #4

P: n/a
jo**********@gmail.com (John Cassidy) wrote in message news:<6f**************************@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.
i don't know what is wrong. please help.

#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()
Nov 14 '05 #5

P: n/a
In <2u*************@uni-berlin.de> Martin Ambuhl <ma*****@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: Da*****@ifh.de
Currently looking for a job in the European Union
Nov 14 '05 #6

P: n/a
In <6f**************************@posting.google.com > jo**********@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.
i don't know what is wrong. please help.

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
What did you include <math.h> for?
#define RAND_MAX 32768
If your compiler didn't complain about the redefinition of RAND_MAX, get
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: Da*****@ifh.de
Currently looking for a job in the European Union
Nov 14 '05 #7

P: n/a
Stuart Gerchick wrote:
jo**********@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
random number. i don't know what is wrong. please help.

#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 (cb********@yahoo.com) (cb********@worldnet.att.net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home.att.net> USE worldnet address!

Nov 14 '05 #8

P: n/a
Martin Ambuhl <ma*****@earthlink.net> 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.
Nov 14 '05 #9

P: n/a
Old Wolf wrote:
Martin Ambuhl <ma*****@earthlink.net> 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?
Nov 14 '05 #10

P: n/a

"Dan Pop" <Da*****@cern.ch> wrote in message
news:cl***********@sunnews.cern.ch...
In <6f**************************@posting.google.com > jo**********@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.
i don't know what is wrong. please help.

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


What did you include <math.h> for?
#define RAND_MAX 32768


If your compiler didn't complain about the redefinition of RAND_MAX, get
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.

Please elaborate further on the last line. What is printf supposed to do?
how should printf terminate the line?
Thanks
Mike
return 0;
}


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

Nov 14 '05 #11

P: n/a
>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() does not generate random numbers. It generates pseudo-random
numbers. The difference may not mean much to you in your application,
but for serious users of cryptography (e.g. spies), it can mean the
difference between life and death.
i don't know what is wrong. please help.

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

#define RAND_MAX 32768
You do not get to define RAND_MAX. The implementation does that.

int main(void)
{
float b;

b = ((float)rand()/RAND_MAX);
printf("Random Number: %f", b);
How do you know the result you got is NOT pseudo-random?

rand() is defined to generate consistent sequences if you
are concerned with getting the same result on repeated invocations.
In that case, look up srand().

system("PAUSE"); PAUSE: command not found return 0;
}


Gordon L. Burditt
Nov 14 '05 #12

P: n/a


Michael wrote:
"Dan Pop" <Da*****@cern.ch> wrote in message
news:cl***********@sunnews.cern.ch...
In <6f**************************@posting.google.com >


jo**********@gmail.com (John Cassidy) writes:

[snip]
printf("Random Number: %f", b); printf("Random Number: %f\n", b);

[snip] 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.


Please elaborate further on the last line. What is printf supposed to do?
how should printf terminate the line?


The last output of your program should throw out a linebreak ('\n',
see the above correction of the original code).
Otherwise, it may be that you will not see the output (which also
could be achieved by playing with the prompt) nor actually get it.
Cheers
Michael
--
E-Mail: Mine is a gmx dot de address.

Nov 14 '05 #13

P: n/a
In <84**************************@posting.google.com > ol*****@inspire.net.nz (Old Wolf) writes:
Martin Ambuhl <ma*****@earthlink.net> 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.


Is it really *any* key, or only *some* keys? If the latter, then a
function waiting for a well defined key is preferable.

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Da*****@ifh.de
Currently looking for a job in the European Union
Nov 14 '05 #14

P: n/a
CBFalconer <cb********@yahoo.com> writes:
Stuart Gerchick wrote:
jo**********@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
random number. i don't know what is wrong. please help.

#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?


Um, no. Double would only give you more precision (with a number that
has only 15 bits of precision in the first place). The argument to
printf() is promoted to double anyway, so there's no type conflict
with "%f" format.

--
Keith Thompson (The_Other_Keith) ks***@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.
Nov 14 '05 #15

P: n/a
Keith Thompson wrote:
CBFalconer <cb********@yahoo.com> writes:
Stuart Gerchick wrote:
jo**********@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
random number. i don't know what is wrong. please help.

#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?


Um, no. Double would only give you more precision (with a number that
has only 15 bits of precision in the first place). The argument to
printf() is promoted to double anyway, so there's no type conflict
with "%f" format.


You're right. I missed the fact that he is generating only one
random value from the initial state.

--
Chuck F (cb********@yahoo.com) (cb********@worldnet.att.net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home.att.net> USE worldnet address!
Nov 14 '05 #16

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