srand with big numbers

[Bill Cunningham]

I have read and studied and looked at references and can't find out how
to do this. Here's where some real knowledge comes in now lets see who in
clc really knows there stuff ;)

main(void) {
srand(time(NULL ));
printf("%i\\n", rand());
return 0;

Simple and forward now here's where I'm stumped. I want the random values
returned in the above way to be long doubles and the equivalent of
pow(2,128). But srand takes an unsigned 32 bit on my system and long double
is 96 bit on my system, or 12 bytes. I can find no conversion functions and
I don't know if I need casts. Can srand return random values of 2 to the 128
power? I know this can be done. Someone please show me.

Thanks

[Ian Collins]

Re: srand with big numbers

Bill Cunningham wrote:

I have read and studied and looked at references and can't find out how
to do this. Here's where some real knowledge comes in now lets see who in
clc really knows there stuff ;)
>
main(void) {
srand(time(NULL ));
printf("%i\\n", rand());
return 0;
>
Simple and forward now here's where I'm stumped. I want the random values
returned in the above way to be long doubles and the equivalent of
pow(2,128). But srand takes an unsigned 32 bit on my system and long double
is 96 bit on my system, or 12 bytes. I can find no conversion functions and
I don't know if I need casts. Can srand return random values of 2 to the 128
power? I know this can be done. Someone please show me.
>

srand() seeds a pseudo-random number generator, it does not return anything.

rand() an int the range of 0 to RAND_MAX. RAND_MAX is only guaranteed
to be 32767.

You will have to use a platform extension to get bigger random numbers.

--
Ian Collins.

[Bill Cunningham]

Re: srand with big numbers

You will have to use a platform extension to get bigger random numbers.
>

OK anything you might suggest?

Bill

[Ian Collins]

Re: srand with big numbers

Bill Cunningham wrote:

>You will have to use a platform extension to get bigger random numbers.
>>

OK anything you might suggest?
>

The drand48 family if you are using a Unix/Linux platform.

Try a platform specific programming group.

--
Ian Collins.

[Sjouke Burry]

Re: srand with big numbers

Bill Cunningham wrote:

I have read and studied and looked at references and can't find out how
to do this. Here's where some real knowledge comes in now lets see who in
clc really knows there stuff ;)
>
main(void) {
srand(time(NULL ));
printf("%i\\n", rand());
return 0;
>
Simple and forward now here's where I'm stumped. I want the random values
returned in the above way to be long doubles and the equivalent of
pow(2,128). But srand takes an unsigned 32 bit on my system and long double
is 96 bit on my system, or 12 bytes. I can find no conversion functions and
I don't know if I need casts. Can srand return random values of 2 to the 128
power? I know this can be done. Someone please show me.
>
Thanks
>
>

Google : long period random generator

[vippstar@gmail.com]

Re: srand with big numbers

On Apr 1, 3:58 am, Ian Collins <ian-n...@hotmail.co mwrote:

Bill Cunningham wrote:

I have read and studied and looked at references and can't find out how
to do this. Here's where some real knowledge comes in now lets see who in
clc really knows there stuff ;)

>

main(void) {

forgot int, but surprisingly not 'void'.

srand(time(NULL ));

no prototypes in scope

printf("%i\\n", rand());

it's not guaranteed to be written to stdout (ie no flush performed)
because instead of \n you had \\n, missing prototype of printf

return 0;

forgot } here

>

Simple and forward now here's where I'm stumped I want the random values

rand() is not truly random

returned in the above way to be long doubles and the equivalent of
pow(2,128).

ambiguous

But srand takes an unsigned 32 bit on my system and long double
is 96 bit on my system, or 12 bytes.

srand() ha nothing to do with the size of what rand() returns, so,
nonsense.

I can find no conversion functions and

Now baiting someone to explain him what a "conversion " is or what he
*might* mean with it

I don't know if I need casts. Can srand return random values of 2 to the 128
power? I know this can be done. Someone please show me.

First he asks then he claims he knows it can be done.
Baiting someone to post a long rant about *why* it cannot be done

conclusion: TROLL.
Everything I commented at is things regulars could argue for hours
with huge posts. A waste of time for everyone.
It's clear Bill Cunningham is a troll! Why cannot the community ignore
him like the other trolls?

>
srand() seeds a pseudo-random number generator, it does not return anything.
>
rand() an int the range of 0 to RAND_MAX. RAND_MAX is only guaranteed
to be 32767.

At *least* 32767, it could be 2^31-1 for example (which is the case on
my implementation) .
Don't reply to trolls.

[Bill Cunningham]

Re: srand with big numbers

Google : long period random generator

;) You got it. I am trying to learn by creating a UUID generator. I know I
can get one but how do you learn from that. Thanks

Bill

[Bill Cunningham]

Re: srand with big numbers


"Ian Collins" <ian-news@hotmail.co mwrote in message
news:65dg3iF2e7 qncU7@mid.indiv idual.net...

The drand48 family if you are using a Unix/Linux platform.
>
Try a platform specific programming group.
>

This is probably written in C so C can do this. It's just in the std
library. THanks Ian. I'm seeing erand and jrand too.

Bill

[Bartc]

Re: srand with big numbers


"Ian Collins" <ian-news@hotmail.co mwrote in message
news:65dfkoF2e7 qncU5@mid.indiv idual.net...

Bill Cunningham wrote:

> I have read and studied and looked at references and can't find out
>how
>to do this. Here's where some real knowledge comes in now lets see who in
>clc really knows there stuff ;)

srand() seeds a pseudo-random number generator, it does not return
anything.
>
rand() an int the range of 0 to RAND_MAX. RAND_MAX is only guaranteed
to be 32767.
>
You will have to use a platform extension to get bigger random numbers.

I just use something like this:

#define bigrand (rand()<<16 + rand())

Any particular problem with that? The randomness is not critical.

--
Bart

[Philip Potter]

Re: srand with big numbers

Bartc wrote:

>rand() an int the range of 0 to RAND_MAX. RAND_MAX is only guaranteed
>to be 32767.
>>
>You will have to use a platform extension to get bigger random numbers.

>
I just use something like this:
>
#define bigrand (rand()<<16 + rand())
>
Any particular problem with that? The randomness is not critical.

Suppose for a moment that the period of rand() is 65536. The period of
your aggregate generator bigrand will be 32768 - that is, 32768 samples
after the first random sample, you will get the same sequence out again.
Aggregating the outputs together cannot increase the period.

If the randomness is not critical, then perhaps that's good enough for
you. I can't answer that for you.

Philip

[Ben Bacarisse]

Re: srand with big numbers

"Bartc" <bc@freeuk.comw rites:

"Ian Collins" <ian-news@hotmail.co mwrote in message
news:65dfkoF2e7 qncU5@mid.indiv idual.net...

>Bill Cunningham wrote:

>> I have read and studied and looked at references and can't find out
>>how
>>to do this. Here's where some real knowledge comes in now lets see who in
>>clc really knows there stuff ;)

>

>srand() seeds a pseudo-random number generator, it does not return
>anything.
>>
>rand() an int the range of 0 to RAND_MAX. RAND_MAX is only guaranteed
>to be 32767.
>>
>You will have to use a platform extension to get bigger random numbers.

>
I just use something like this:
>
#define bigrand (rand()<<16 + rand())
>
Any particular problem with that? The randomness is not critical.

Yes. You probably meant ((rand()<<16) + rand()). Frequent UB otherwise.

As for the crrected version... tt depends. If RAND_MAX is only 32767
you get two "dead bits" in the result. Worse, if the system has 16
bit ints the shift is undefined. (unsigned long)rand() << 16 is safer.

--
Ben.

[Bill Cunningham]

Re: srand with big numbers

Yes. You probably meant ((rand()<<16) + rand()). Frequent UB otherwise.
>
As for the crrected version... tt depends. If RAND_MAX is only 32767
you get two "dead bits" in the result. Worse, if the system has 16
bit ints the shift is undefined. (unsigned long)rand() << 16 is safer.
>
--

Is this an example of what's called a "macro expansion" in C ? I haven't
got there yet but I will try this.

Bill

[CBFalconer]

Re: srand with big numbers

Bill Cunningham wrote:

>
I have read and studied and looked at references and can't find
out how to do this. Here's where some real knowledge comes in now
lets see who in clc really knows there stuff ;)
>
main(void) {
srand(time(NULL ));
printf("%i\\n", rand());
return 0;
>
Simple and forward now here's where I'm stumped. I want the random
values returned in the above way to be long doubles and the
equivalent of pow(2,128). But srand takes an unsigned 32 bit on my
system and long double is 96 bit on my system, or 12 bytes. I can
find no conversion functions and I don't know if I need casts. Can
srand return random values of 2 to the 128 power? I know this can
be done. Someone please show me.

7.20.2.1 The rand function

Synopsis
[#1]
#include <stdlib.h>
int rand(void);

.... snip ...

Returns

[#4] The rand function returns a pseudo-random integer.

Environmental limits

[#5] The value of the RAND_MAX macro shall be at least
32767.

7.20.2.2 The srand function

Synopsis
[#1]
#include <stdlib.h>
void srand(unsigned int seed);

Description

[#2] The srand function uses the argument as a seed for a
new sequence of pseudo-random numbers to be returned by
subsequent calls to rand. If srand is then called with the
same seed value, the sequence of pseudo-random numbers shall
be repeated. If rand is called before any calls to srand
have been made, the same sequence shall be generated as when
srand is first called with a seed value of 1.

[#3] The implementation shall behave as if no library
function calls the srand function.

Returns

[#4] The srand function returns no value.

--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home .att.net>
Try the download section.



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