tr1, random numbers

[WP]

Hello, I'm having some problems with the TR1 random number generators. I
need a function that takes a min and max value and generates a random
number within that range (inclusive). Here's my attempt:

int
random_number(int min, int max)
{
std::tr1::uniform_int<int>
distributor(min, max);
std::tr1::linear_congruential<unsigned long, 16807, 0, 2147483647>
generator;

generator.seed((unsigned long)time(NULL));

int n = distributor(generator);

cout << "Generated " << n << endl;

return n;
}

This function is called 50 times when the program is run with 3
different ranges and the problem is that it always returns the same
number for the same range. I am calling seed() on the generator so it's
not that. What am I doing wrong? I guess many of you won't be able to
run this code if you haven't got a very modern c++ compiler with tr1
implementation. This is my first time using tr1.

- WP

[Juha Nieminen]

WP wrote:

I am calling seed() on the generator so it's not that.

Incorrect. That's *exactly* the problem. You must *not* reseed the RNG
each time you want to get a value from it.

A RNG cannot be simply instantiated, a random value popped and then
destroyed. They don't work that way. You have to keep an instance
somewhere else, seed it only once, and then get values from it without
reseeding it.

[Pete Becker]

On 2008-07-06 10:32:27 -0400, WP <in*****@invalid.invalidsaid:

Hello, I'm having some problems with the TR1 random number generators.
I need a function that takes a min and max value and generates a random
number within that range (inclusive). Here's my attempt:

int
random_number(int min, int max)
{
std::tr1::uniform_int<intdistributor(min, max);
std::tr1::linear_congruential<unsigned long, 16807, 0, 2147483647>
generator;

generator.seed((unsigned long)time(NULL));

int n = distributor(generator);

cout << "Generated " << n << endl;

return n;
}

This function is called 50 times when the program is run with 3
different ranges and the problem is that it always returns the same
number for the same range. I am calling seed() on the generator so it's
not that. What am I doing wrong? I guess many of you won't be able to
run this code if you haven't got a very modern c++ compiler with tr1
implementation. This is my first time using tr1.

Create the generator and the distribution object once, and call them
each time you need a number.

--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)

[WP]

Juha Nieminen wrote:

WP wrote:

>I am calling seed() on the generator so it's not that.

Incorrect. That's *exactly* the problem. You must *not* reseed the RNG
each time you want to get a value from it.

A RNG cannot be simply instantiated, a random value popped and then
destroyed. They don't work that way. You have to keep an instance
somewhere else, seed it only once, and then get values from it without
reseeding it.

Yes, thanks alot for the quick reply!