Velocity Reviews > C++ > Random number generator and seeding

# Random number generator and seeding

Joe
Guest
Posts: n/a

 11-19-2004
Hi, I have been working on some code that requires a high use of random
numbers within. Mostly I either have to either:
1) flip a coin i.e. 0 or 1, or
2) generate a double between 0 and 1.

I have utilised the following random number source code
http://www.agner.org/random/

What I have found is that there is a problem with seeding. The code
generates a seed based on time(0). I have found that I need to increment
seed and

A snippet of my code is below. I basically generate a Population object that
contains various individuals. I want to randomly choose two individuals from
this population. I wish I could choose these individuals by the following:

// get two parents
ind1 = pool.RouletteWheel();
ind2 = pool.RouletteWheel();

Chris Theis
Guest
Posts: n/a

 11-19-2004

"Joe" <(E-Mail Removed)> schrieb im Newsbeitrag
news:(E-Mail Removed)...
> Hi, I have been working on some code that requires a high use of random
> numbers within. Mostly I either have to either:
> 1) flip a coin i.e. 0 or 1, or
> 2) generate a double between 0 and 1.
>
> I have utilised the following random number source code
> http://www.agner.org/random/
>
> What I have found is that there is a problem with seeding. The code
> generates a seed based on time(0). I have found that I need to increment
> seed and
>
> A snippet of my code is below. I basically generate a Population object

that
> contains various individuals. I want to randomly choose two individuals

from
> this population. I wish I could choose these individuals by the following:
>

[SNIP]

I guess it's necessary to clarify some things about random number
generators. In principle they work the following way that you have to pass a
starting seed which is the starting point of your sequence of (pseudo)random
numbers. Passing the same seed to the RNG will result in the same sequence.
This is useful if you need random numbers but simultaneously have the
requirement of reproducing the behavior of your algorithm.

However, normally the seed is passed only at the construction time and
subsequent calls to the RNG's function which actually returns the numbers,
should modify the internally stored seed of the RNG object. You should be
aware that seeds are not numbers which will continuously increase or
decrease. Furthermore, the seed has a great influence on the quality of the
random sequence but this would lead too far here.

In summary you should pass the seed only once to your random number object
and also this object should be created only once.

int Population :: RouletteWheel ( int seed )
{
TRandomMersenne rg(seed);
return rg.Random();
}

This implementation creates a RNG object whenever RouletteWheel is called. I
don't know about the details of your population class but you could put the
TRandomMersenne object there as a member which is initialized with the seed
in the constructor of "population".

HTH
Chris

Howard
Guest
Posts: n/a

 11-19-2004

"Joe" <(E-Mail Removed)> wrote in message news:(E-Mail Removed)...
> Hi, I have been working on some code that requires a high use of random
> numbers within. Mostly I either have to either:
> 1) flip a coin i.e. 0 or 1, or
> 2) generate a double between 0 and 1.
>
> I have utilised the following random number source code
> http://www.agner.org/random/
>
> What I have found is that there is a problem with seeding. The code
> generates a seed based on time(0). I have found that I need to increment
> seed and
>
> A snippet of my code is below. I basically generate a Population object
> that contains various individuals. I want to randomly choose two
> individuals from this population. I wish I could choose these individuals
> by the following:
>
> // get two parents
> ind1 = pool.RouletteWheel();
> ind2 = pool.RouletteWheel();
> .
> .
> int Population :: RouletteWheel ( )
> {
> int seed = time(0);
> TRandomMersenne rg(seed);
> return rg.Random();
> }
>
> In the above case all the random functions are contained in the Population
> class. But when i do this it passes back two identical individuals. So
> nstead I had to implement passing an incremented seed to the RouletteWheel
> member function as shown below
>

<snip>
>
> I dont like this solution as it enforced defining random functions in the
> main.cpp class - which i dont want. Is there any way to implement my
> desired approach (the former). I dont like the idea of passing incremented
> seeds .. even though it does appear to introduce randomness in my results.
>
> Any ideas greatly appreciated!
>
> Cheers,
> Joe
>

You only want to seed a random-number generator once during a given run of a
program. You then call the RNG multiple times to get "random" values.
Seeding the RNG multiple times destroys the "randomness" of the RNG.

-Howard