Random number between 1 and 4000?

[eksamor]

I can't seem to control the rand() function. I just makes an evil
humongeous number. How do I make it generate random numbers from 1 -
4000??

[Chris Dollin]

eksamor wrote:

I can't seem to control the rand() function. I just makes an evil
humongeous number. How do I make it generate random numbers from 1 -
4000??

You can't. But you can do something with the gynormous value you've
got.

--
Chris "x.f == f(x)" Dollin
Beware the ideas of March.

[eksamor]

I now made this:

int blop;

blop = rand();
while (blop > 4000 || blop < 1)
{
blop = rand();
}
printf("blop: %d\n",blop);

but it always prints "blop: 3722"

[Doc]

u can use like this:

rand()%4000 + 1

[Doc]

first,srand((unsigned)time(NULL)),
then,rand()

[eksamor]

that looks nice but it always prints 1384

[Ralph A. Moritz]

eksamor wrote:

I can't seem to control the rand() function. I just makes an evil
humongeous number. How do I make it generate random numbers from 1 -
4000??

You need to first seed rand(), using srand(). Below is a simplistic
example.

Regards,
Ralph
#include <stdio.h>
#include <stdlib.h>

int main(void)
{
int rn;
srand(time(NULL));
rn = rand() % 4000;

printf("%d\n", rn);
return 0;
}

--
Ralph Moritz

Laugh at your problems; everybody else does.

[Keith Thompson]

"eksamor" <ek*****@yahoo.com> writes:

I can't seem to control the rand() function. I just makes an evil
humongeous number. How do I make it generate random numbers from 1 -
4000??

The comp.lang.c FAQ is at <http://www.c-faq.com/>. Read questions
13.15, 13.16, and 13.17.

--
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.

[Anders Arnholm]

eksamor <ek*****@yahoo.com> skriver:

I can't seem to control the rand() function. I just makes an evil
humongeous number. How do I make it generate random numbers from 1 -
4000??

As stated befor you need to use srand() before using rand(), however
I would recoment to look into your operating system manual. Hopefully
you have someting better than the rand() interface for random numbers.
(However doing so takes you outside standard C.) I recomend
arc4random() if your system supports it.

/ Anders
--
http://anders.arnholm.nu/ Keep on Balping

[Martin Ambuhl]

eksamor wrote:

I can't seem to control the rand() function. I just makes an evil
humongeous number. How do I make it generate random numbers from 1 -
4000??

Beginners often seem to believe that they have discovered new problems,
but that is rarely so. In fact, most problems they run into have been
encountered many rimes in the past. This is the reason that many
newsgroups have "Frequently Asked Question" lists (FAQs). It is
normally expected that a poster will have checked the FAQ _before_
posting. Sometimes people will complain that they had no idea that
there was a FAQ or where it might be. But it is also expected behavior
to follow a newsgroup before posting. In almost every newsgroup with a
FAQ, the FAQ itself or at least a document pointing to it will be posted
about twice a month. Another objection often seen is that new posters
don't know about these simple guidelines of looking for the FAQ or
following the newsgroup. This is similar to claiming that there is no
need to check which side of the road people drive on when one goes to
another country. Unfortunately, for reasons known only to themselves,
people who provide newsreaders no longer provide them with subscriptions
already built in to <news:news.announce.newusers>. You need to do that
yourself.

In the case of <news:comp.lang.c>, the FAQ can be found at
<http://c-faq.com/>. In your case, the particular question you need to
check is
<http://c-faq.com/lib/randrange.html>
"Question 13.16: Q: How can I get random integers in a certain range?".

[Nils O. Selåsdal]

eksamor wrote:

that looks nice but it always prints 1384

You're either using/printing it wrongly,
or you are calling rand() once per invocation of your program,
and havn't set a seed (the srand function) that varies.
rand() yields pseudo random numbers, it will generate the same
sequence every time for a given seed.

[Nick Keighley]

eksamor wrote:

1. include some context in your post (quote who you are replying to)

I now made this:

int blop;

blop = rand();
while (blop > 4000 || blop < 1)
{
blop = rand();
}
this may not terminate- and it is certainly ineffecient
printf("blop: %d\n",blop);

but it always prints "blop: 3722"

read up a pseudo-random number generators. In short they always
produce the same sequence unless you seed them.
--
Nick Keighley

Many astrologers think that this concentration on
[the sun-sign column] has done untold damage to serious astrology.
The Independent

[Nick Keighley]

Doc wrote:

u can use like this:

rand()%4000 + 1

1. put some context in your posts
2. its "you" not "u"
3. read the fact before you post rubbish like this
--
Nick Keighley

[Flash Gordon]

eksamor wrote:

that looks nice but it always prints 1384

Can *everyone* please provide context when replying. Google is *not*
Usenet and there is no guarantee that the post you are replying to has
been seen by other people. See
http://www.safalra.com/special/googlegroupsreply/ and
http://cfaj.freeshell.org/google/ for details.

Also see the link in my sig for more information about this group.

eksamor, rather than making people guess post a small *complete* program
showing your problem, copy and paste, do *not* retype. Since you
obviously don't know what the problem is you have no way of knowing
which line you haven't posted is the cause of the problem.
--
Flash Gordon, living in interesting times.
Web site - http://home.flash-gordon.me.uk/
comp.lang.c posting guidelines and intro:
http://clc-wiki.net/wiki/Intro_to_clc

[A. Sinan Unur]

"Ralph A. Moritz" <ra*******@gmail.com> wrote in
news:11**********************@u72g2000cwu.googlegr oups.com:

eksamor wrote:

I can't seem to control the rand() function. I just makes an evil
humongeous number. How do I make it generate random numbers from 1 -
4000??
You need to first seed rand(), using srand(). Below is a simplistic
example.

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

int main(void)
{
int rn;
srand(time(NULL));
rn = rand() % 4000;

printf("%d\n", rn);
return 0;
}

There are two problems with that code:

1) The OP wanted numbers between 1 and 4000. The code above will yield
zero occasionally.

2) The lower order bits may not be very random. See:

http://www.eskimo.com/~scs/c-faq.com...tveryrand.html

http://www.eskimo.com/~scs/c-faq.com/lib/randrange.html

Sinan

--
A. Sinan Unur <1u**@llenroc.ude.invalid>
(remove .invalid and reverse each component for email address)

[John Bode]

eksamor wrote:

I can't seem to control the rand() function. I just makes an evil
humongeous number. How do I make it generate random numbers from 1 -
4000??

Follow the link: http://www.eskimo.com/~scs/c-faq.com/lib/randrange.html

[Richard Bos]

Anders Arnholm <An*********@Arnholm.nu> wrote:

eksamor <ek*****@yahoo.com> skriver:

I can't seem to control the rand() function. I just makes an evil
humongeous number. How do I make it generate random numbers from 1 -
4000??

As stated befor you need to use srand() before using rand(), however
I would recoment to look into your operating system manual. Hopefully
you have someting better than the rand() interface for random numbers.
(However doing so takes you outside standard C.) I recomend
arc4random() if your system supports it.

What on earth is that? If it can be written in ISO C, perhaps that's a
better idea. Several pretty decent PRNGs can be written in ISO C; see,
for example, <S%******************@news1.rdc1.fl.home.com>. Or if you
want the sledgehammer approach, do a websearch on the Mersenne Twister.

Richard

[Jordan Abel]

On 2006-03-28, Nick Keighley <ni******************@hotmail.com> wrote:

read up a pseudo-random number generators. In short they always
produce the same sequence unless you seed them.

You can't "not seed" them. The issue is, rather, that the C standard
requires that it is always seeded with the same value at program start,
and if you want different sequences you need to re-seed it.

[Keith Thompson]

"John Bode" <jo*******@my-deja.com> writes:

eksamor wrote:

I can't seem to control the rand() function. I just makes an evil
humongeous number. How do I make it generate random numbers from 1 -
4000??

Follow the link: http://www.eskimo.com/~scs/c-faq.com/lib/randrange.html

That link works, but the FAQ is now in its own domain. See
<http://www.c-faq.com/lib/randrange.html>.

--
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.

[Blarney]

Martin Ambuhl wrote:

eksamor wrote:

I can't seem to control the rand() function. I just makes an evil
humongeous number. How do I make it generate random numbers from 1 -
4000??

Beginners often seem to believe that they have discovered new problems,
but that is rarely so. In fact, most problems they run into have been
encountered many rimes in the past. This is the reason that many
newsgroups have "Frequently Asked Question" lists (FAQs). It is
normally expected that a poster will have checked the FAQ _before_
posting. Sometimes people will complain that they had no idea that
there was a FAQ or where it might be. But it is also expected behavior
to follow a newsgroup before posting. In almost every newsgroup with a
FAQ, the FAQ itself or at least a document pointing to it will be posted
about twice a month. Another objection often seen is that new posters
don't know about these simple guidelines of looking for the FAQ or
following the newsgroup. This is similar to claiming that there is no
need to check which side of the road people drive on when one goes to
another country.

That's a poor analogy.

Most severe consequence of posting to a newsgroup without knowing the
simple guidelines: mental anguish

Most severe consequence of driving on the wrong side of the road in a
foreign country: death

--
jay

[Arndt Jonasson]

"A. Sinan Unur" <1u**@llenroc.ude.invalid> writes:

"Ralph A. Moritz" <ra*******@gmail.com> wrote in
news:11**********************@u72g2000cwu.googlegr oups.com:

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

int main(void)
{
int rn;
srand(time(NULL));
rn = rand() % 4000;

printf("%d\n", rn);
return 0;
}

There are two problems with that code:

1) The OP wanted numbers between 1 and 4000. The code above will yield
zero occasionally.

2) The lower order bits may not be very random. See:

http://www.eskimo.com/~scs/c-faq.com...tveryrand.html
http://www.eskimo.com/~scs/c-faq.com/lib/randrange.html

A third possible problem is that it makes the numbers (very) slightly
biased, unless the maximum number that rand() returns is a multiple of
4000. Of course, for many practical purposes, this may not be
important, and if it is, a better generator should be used anyway.

[Anders Arnholm]

In comp.lang.c, you wrote:

Anders Arnholm <An*********@Arnholm.nu> wrote:

eksamor <ek*****@yahoo.com> skriver:

> I can't seem to control the rand() function. I just makes an evil
> humongeous number. How do I make it generate random numbers from 1 -
> 4000??

As stated befor you need to use srand() before using rand(), however
I would recoment to look into your operating system manual. Hopefully
you have someting better than the rand() interface for random numbers.
(However doing so takes you outside standard C.) I recomend
arc4random() if your system supports it.

What on earth is that? If it can be written in ISO C, perhaps that's a
better idea. Several pretty decent PRNGs can be written in ISO C; see,
for example, <S%******************@news1.rdc1.fl.home.com>. Or if you
want the sledgehammer approach, do a websearch on the Mersenne Twister.

A good random function can unfortunalty not yet be written i ISO C.
Without getting into system dependent things you can't get a good
entropy data from any place. If you can precent a good way to get
strong entropy data within ISO C, I stand corrected, but I don't
know of any interface to such stuff without knowing about the OS.
arc4random(), entropy data from any place. If you can precent a good way
to get strong entropy data within ISO C, I stand corrected, but I don't
know of any interface to such stuff without knowing about the OS.
arc4random(), is a good interface for this, rand() isn't.

However as this goes outside ISO C it has to be considerd for
portability. Desinging the same bad stuff over and over again
to keep inside ISO C have no place in good programing. As little
and using platform specific non ISO C stuff on things that ISO C
is as good at.

/ Anders
--
http://anders.arnholm.nu/ Keep on Balping

[Purush]

If you are using a DOS based compiler use the function randomize().
This will reinitialise the random number generator. So that you will
not have the same set of random numbers every time.

Thanks

[Mr John FO Evans]

In article <11*********************@j33g2000cwa.googlegroups. com>, "Purush"
<pu*********@gmail.com> wrote:

If you are using a DOS based compiler use the function randomize().
This will reinitialise the random number generator. So that you will
not have the same set of random numbers every time.

On my system I initialise the random number generator with the system time
this gives me a satisfactory new start on each run.

--
_ _________________________________________
/ \._._ |_ _ _ /' Orpheus Internet Services
\_/| |_)| |(/_|_|_> / 'Internet for Everyone'
_______ | ___________./ http://www.orpheusinternet.co.uk

[CBFalconer]

Purush wrote:

If you are using a DOS based compiler use the function randomize().
This will reinitialise the random number generator. So that you
will not have the same set of random numbers every time.

Please don't give such misinformation. There is no such standard C
function as randomize. However, srand() does exist. Use it when
you don't want the generator to always produce the same sequence.

In addition, ensure you include adequate context for your replies.
For the means to do this on the broken google interface to usenet,
see below.

--
"If you want to post a followup via groups.google.com, don't use
the broken "Reply" link at the bottom of the article. Click on
"show options" at the top of the article, then click on the
"Reply" at the bottom of the article headers." - Keith Thompson
More details at: <http://cfaj.freeshell.org/google/>
Also see <http://www.safalra.com/special/googlegroupsreply/>

[Richard Bos]

Anders Arnholm <An*********@Arnholm.nu> wrote:

In comp.lang.c, you wrote:

Anders Arnholm <An*********@Arnholm.nu> wrote:

As stated befor you need to use srand() before using rand(), however
I would recoment to look into your operating system manual. Hopefully
you have someting better than the rand() interface for random numbers.
(However doing so takes you outside standard C.) I recomend
arc4random() if your system supports it.
What on earth is that? If it can be written in ISO C, perhaps that's a
better idea. Several pretty decent PRNGs can be written in ISO C; see,
for example, <S%******************@news1.rdc1.fl.home.com>. Or if you
want the sledgehammer approach, do a websearch on the Mersenne Twister.

A good random function can unfortunalty not yet be written i ISO C.

Which is why I wrote PRNGs instead. For most applications, a good PRNG
is good enough, and those of us who do need a _real_ RNG know it.
arc4random(), entropy data from any place.

Impossible to implement reliably on many systems. Keyboard timing? You
get into trouble with CLI plumbing. Network timing? Not all machines are
networked. Brownian motion? Needs a TTC interface, and those are rare.

Richard

[Richard Bos]

"Purush" <pu*********@gmail.com> wrote:

[ Learn to quote! I'm led to believe that there is a button on Google
Broken Beta somewhere which will let you do so. ]

If you are using a DOS based compiler use the function randomize().

And what guarantee do you have that all DOS (_whose_ DOS, anyway?) based
compilers have this function? And that it works correctly with rand() in
all those compilers?

Richard

[Randy Howard]

Richard Bos wrote
(in article <44****************@news.xs4all.nl>):

Impossible to implement reliably on many systems. Keyboard timing? You
get into trouble with CLI plumbing. Network timing? Not all machines are
networked. Brownian motion? Needs a TTC interface, and those are rare.

Of course, on some current operating systems, the arrival time
of unsolicited spyware and virii could be used as an entropy
source, unless you can't sample them fast enough.

--
Randy Howard (2reply remove FOOBAR)
"The power of accurate observation is called cynicism by those
who have not got it." - George Bernard Shaw