Unequal random

[majidkamali1370]

Hi.
How to generate unequal probability random number.
I mean i.e when I want to generate a number between 1 and 10, these numbers does not have equal probability, this unequality can be an integer (i.e 2 has probability two times of other numbers) or it can be a float value (i.e 3 and 4 have probability 0.2 times of other values).
How can I achieve this?

Thanks :)

[Cubbi]

C++ has a special random number number distribution for this, it's called "discrete_distribution"

#include <iostream>
#include <map>
#include <random>
#include <iterator>
 
int main()
{
    std::random_device rd;
    std::mt19937 gen(rd());
    double weights[] =
            {0, // do not generate the number 0
             1, // number 1 at base probability
             2, // number 2 twice as often
             0.2, // number 3 at 1/5th probability
             0.2, // number 4 at 1/5th probability
             1, 1, 1, 1, 1, 1}; // 5..10 at base probability
    std::discrete_distribution<> d(std::begin(weights), std::end(weights));
    std::map<int, int> m;
    for(int n=0; n<10000; ++n) {
        ++m[d(gen)];
    }
    for(auto p : m) {
        std::cout << p.first << " generated " << p.second << " times\n";
    }
}

demo at ideone (with some dumbing down for older GCC): http://ideone.com/GH0WV

[majidkamali1370]

That was awesome. I compiled this with -std=c++0x option in G++.
Can you please explain your code? it's a little strange to me. What are random_device or mt19937?
Most strange part is last for. Please explain this.

Thanks Again ;)

[Cubbi]

std::random_device is the hardware random number generator (where hardware provides access to such). On Linux, std::random_device uses /dev/urandom by default.

I use it as a source of the seed for mt19937, but you could use it directly:

for(int n=0; n<10000; ++n)
        ++m[d(rd)];

mt19937 is the canonical Mersenne Twister pseudo random number generator. It has very good number-theoretical properties, so I usually use it by default (the drawback is that it's a bit slower than other PRNGs)

the last loop is just a range-for over the map, a C++11 syntax

[codekiddy]

Hi Cubbi,

Do you know if random_device works on windows and how do use it in my code?
Shall I connect some external device or what?

I can't fine a lot of info on the internet about these luxury stuffs so I don't have a clue ;_;

Do you have some references maybe?

Thanks!

[Cubbi]

This is standard C++, it works on Windows without modification. (well, you need VC11 beta for the range-for loop, but the <random> library has been around for many years)

MSDN on random_device: http://msdn.microsoft.com/en-us/library/bb982250.aspx

[codekiddy]

Thanks, Thats good, Visual beta 11 alredy instaled long time ago, but didn't notice that random_device can be used so easy.

i was confused after reading this wikipedia input:
http://en.wikipedia.org/wiki/Hardware_random_number_generator

Just currious hwat is the price for an good hardware based random generator like that from wikipedia.

[rollie]

Great explanation Cubbi, learned something new today :)

[majidkamali1370]

Thank you Cubbi ;)