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Hi,

I'm implementating some algorithems with random data, so I must
initialisation the seed. But the code creates many objects of my class
so that the seed, which is set to
static_cast<std::time_t>(time(0)))
is in all objects the same and so I get in all objects the same "random
values".

My idea for solving this seed problem, ist to mix the time with the
object reference (*this).
Is this a good solution or do you know another algorithm? How can I
convert the reference to a numeric value?

Thanks

Phil

Philipp Kraus <>, on 22/08/2010 15:48:49, wrote:

> Hi,
>
> I'm implementating some algorithems with random data, so I must
> initialisation the seed. But the code creates many objects of my class
> so that the seed, which is set to
> static_cast<std::time_t>(time(0)))
> is in all objects the same and so I get in all objects the same "random
> values".
>
> My idea for solving this seed problem, ist to mix the time with the
> object reference (*this).
> Is this a good solution or do you know another algorithm? How can I
> convert the reference to a numeric value?

First of all,I don't see the need to cast the result of time(), as it
already is a time_t.

Further than that I cannot see the problem. If you need different seeds
just call time() again for each new instance of your classes (instead of
duplicating the first seed, as you seem to be doing).

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On 2010-08-22 16:00:17 +0200, Francesco S. Carta said:

> Philipp Kraus <>, on 22/08/2010 15:48:49, wrote:
>
>> Hi,
>>
>> I'm implementating some algorithems with random data, so I must
>> initialisation the seed. But the code creates many objects of my class
>> so that the seed, which is set to
>> static_cast<std::time_t>(time(0)))
>> is in all objects the same and so I get in all objects the same "random
>> values".
>>
>> My idea for solving this seed problem, ist to mix the time with the
>> object reference (*this).
>> Is this a good solution or do you know another algorithm? How can I
>> convert the reference to a numeric value?
>
> First of all,I don't see the need to cast the result of time(), as it
> already is a time_t.
>
> Further than that I cannot see the problem. If you need different seeds
> just call time() again for each new instance of your classes (instead
> of duplicating the first seed, as you seem to be doing).

This doesn't work, time() returns on every call the same value, so all
seed are equal. I need different seeds, so my idea I call time and mix
this value with my object reference, the object reference should be
different for every object.

Leigh Johnston <>, on 22/08/2010 14:58:03, wrote:

>
>
> "Philipp Kraus" <> wrote in message
> news:i4r9s2$5fs$...
>> Hi,
>>
>> I'm implementating some algorithems with random data, so I must
>> initialisation the seed. But the code creates many objects of my class
>> so that the seed, which is set to
>> static_cast<std::time_t>(time(0)))
>> is in all objects the same and so I get in all objects the same
>> "random values".
>>
>> My idea for solving this seed problem, ist to mix the time with the
>> object reference (*this).
>> Is this a good solution or do you know another algorithm? How can I
>> convert the reference to a numeric value?
>>
>> Thanks
>>
>> Phil
>>
>
> Just initialize the seed once at the top of your main function rather
> than in each object. Alternatively use something better than srand/rand
> such as the tr1/boost PRNGs.

He makes no mention about which PRNG he is using. I suggested him to get
different seeds because I thought he was using something different from
srand/rand.

Indeed, if he uses the standard C rand(), just setting the seed once
with srand() at the beginning of the program would be enough.

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Philipp Kraus <>, on 22/08/2010 16:14:17, wrote:

> On 2010-08-22 16:00:17 +0200, Francesco S. Carta said:
>
>> Philipp Kraus <>, on 22/08/2010 15:48:49,
>> wrote:
>>
>>> Hi,
>>>
>>> I'm implementating some algorithems with random data, so I must
>>> initialisation the seed. But the code creates many objects of my class
>>> so that the seed, which is set to
>>> static_cast<std::time_t>(time(0)))
>>> is in all objects the same and so I get in all objects the same "random
>>> values".
>>>
>>> My idea for solving this seed problem, ist to mix the time with the
>>> object reference (*this).
>>> Is this a good solution or do you know another algorithm? How can I
>>> convert the reference to a numeric value?
>>
>> First of all,I don't see the need to cast the result of time(), as it
>> already is a time_t.
>>
>> Further than that I cannot see the problem. If you need different
>> seeds just call time() again for each new instance of your classes
>> (instead of duplicating the first seed, as you seem to be doing).
>
> This doesn't work, time() returns on every call the same value, so all
> seed are equal. I need different seeds, so my idea I call time and mix
> this value with my object reference, the object reference should be
> different for every object.

No, time() returns different values at each call, assuming you let pass
at least one second from one call to the other

If you're using rand(), just call srand(time(0)) once at the beginning
of your program, although you would be better finding a better PRNG as
Leigh suggested.

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Leigh Johnston <>, on 22/08/2010 15:03:54, wrote:

>
>
> "Francesco S. Carta" <> wrote in message
> news:4c712d71$0$30905$...
>> Philipp Kraus <>, on 22/08/2010 15:48:49,
>> wrote:
>>
>>> Hi,
>>>
>>> I'm implementating some algorithems with random data, so I must
>>> initialisation the seed. But the code creates many objects of my class
>>> so that the seed, which is set to
>>> static_cast<std::time_t>(time(0)))
>>> is in all objects the same and so I get in all objects the same "random
>>> values".
>>>
>>> My idea for solving this seed problem, ist to mix the time with the
>>> object reference (*this).
>>> Is this a good solution or do you know another algorithm? How can I
>>> convert the reference to a numeric value?
>>
>> First of all,I don't see the need to cast the result of time(), as it
>> already is a time_t.
>>
>> Further than that I cannot see the problem. If you need different
>> seeds just call time() again for each new instance of your classes
>> (instead of duplicating the first seed, as you seem to be doing).
>>
>
> What if both objects are created within the same second resulting in
> time() returning the same for each? The traditional approach is to call
> srand just the once at the top of main() or similar. A better approach
> is to dump srand/time and use separate PRNG instances in each object
> (using tr1/boost PRNGs).

You're right, I forgot to point out that "second granularity" issue. I
assumed some things about the original post and those things mislead my
reply.

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Francesco S. Carta <>, on 22/08/2010 16:16:59, wrote:

> Leigh Johnston <>, on 22/08/2010 14:58:03, wrote:
>
>>
>>
>> "Philipp Kraus" <> wrote in message
>> news:i4r9s2$5fs$...
>>> Hi,
>>>
>>> I'm implementating some algorithems with random data, so I must
>>> initialisation the seed. But the code creates many objects of my class
>>> so that the seed, which is set to
>>> static_cast<std::time_t>(time(0)))
>>> is in all objects the same and so I get in all objects the same
>>> "random values".
>>>
>>> My idea for solving this seed problem, ist to mix the time with the
>>> object reference (*this).
>>> Is this a good solution or do you know another algorithm? How can I
>>> convert the reference to a numeric value?
>>>
>>> Thanks
>>>
>>> Phil
>>>
>>
>> Just initialize the seed once at the top of your main function rather
>> than in each object. Alternatively use something better than srand/rand
>> such as the tr1/boost PRNGs.
>
> He makes no mention about which PRNG he is using.

Heck, the subject line reads "rand", so it seems that I was wrong. Sorry.

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Philipp Kraus <>, on 22/08/2010 15:48:49, wrote:

> Hi,
>
> I'm implementating some algorithems with random data, so I must
> initialisation the seed. But the code creates many objects of my class
> so that the seed, which is set to
> static_cast<std::time_t>(time(0)))
> is in all objects the same and so I get in all objects the same "random
> values".
>
> My idea for solving this seed problem, ist to mix the time with the
> object reference (*this).
> Is this a good solution or do you know another algorithm? How can I
> convert the reference to a numeric value?

As an aside from all the other replies, since it could be a good thing
to have different seeds in order to have different and reproducible
random sequences, you could, first of all, get a PRNG that you can
instantiate separately.

Once you have the first instance, set its seed from time(), and then
generate the further seeds from that first instance, seeds to be passed
to the further instances of the PRNG contained in your classes.

Doing things in this way you'll have a single originating seed for each
run of your program, a seed that you could eventually reuse to debug the
program or to simply reproduce past runs.

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Leigh Johnston <>, on 22/08/2010 15:20:40, wrote:

>
>
> "Leigh Johnston" <> wrote in message
> news:O4idndCEiKDQs-...
>>
>>
>> "Francesco S. Carta" <> wrote in message
>> news:4c712d71$0$30905$...
>>> Philipp Kraus <>, on 22/08/2010 15:48:49,
>>> wrote:
>>>
>>>> Hi,
>>>>
>>>> I'm implementating some algorithems with random data, so I must
>>>> initialisation the seed. But the code creates many objects of my class
>>>> so that the seed, which is set to
>>>> static_cast<std::time_t>(time(0)))
>>>> is in all objects the same and so I get in all objects the same "random
>>>> values".
>>>>
>>>> My idea for solving this seed problem, ist to mix the time with the
>>>> object reference (*this).
>>>> Is this a good solution or do you know another algorithm? How can I
>>>> convert the reference to a numeric value?
>>>
>>> First of all,I don't see the need to cast the result of time(), as it
>>> already is a time_t.
>>>
>>> Further than that I cannot see the problem. If you need different
>>> seeds just call time() again for each new instance of your classes
>>> (instead of duplicating the first seed, as you seem to be doing).
>>>
>>
>> What if both objects are created within the same second resulting in
>> time() returning the same for each? The traditional approach is to
>> call srand just the once at the top of main() or similar. A better
>> approach is to dump srand/time and use separate PRNG instances in each
>> object (using tr1/boost PRNGs).
>>
>> /Leigh
>
> I meant "srand/rand" not "srand/time", but I think I posted too quickly
> as OP didn't mention he was using srand/rand, I just assumed. xoring
> "this" pointer with the time() might be an acceptable approach if
> separate PRNG instances are being used.

We both posted quickly and we both assumed, but your guess was right. He
definitely seems to be using rand().

In any case, yes, the best way about randomizing stuff goes away from
rand() - shall anybody need a rationale for such "taking the distances"
from rand(), the problem is that rand() does not usually return good
random numbers, but moreover, its problem is about the fact that it
returns a single flow of random numbers, while having independent flows
can be very useful. This is achieved using a PRNG that can be
instantiated separately.

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On Aug 22, 8:48*am, Philipp Kraus <philipp.kr...@flashpixx.de> wrote:
> My idea for solving this seed problem, ist to mix the time with the
> object reference (*this).
> Is this a good solution or do you know another algorithm? How can I
> convert the reference to a numeric value?

There are some other options available to you. In my experience, the
"this" pointer isn't very random, though it'll help with the poor
resolution of time(). Instead of XOR-ing it (I assume this is what
you mean by 'mix'), concatenate the two values in an unsigned char
array and run some sort of hash on the result (ex., SHA-1). Then take
a word of the hash. Or use the two values as inputs to a decent
lagged fibonacci generator and run it for awhile (say, 1000
iterations)
then use that as your seed.

On *nix you have the option of reading from /dev/random or
/dev/urandom for a seed. Windows has some APIs for PRNGs.

--Jonathan