How do I properly develop a Probability System? - c++

Question

So I'm working on developing an online game, and one of the features of this game (like many other MMORPG's) is the drop system & upgrade system.
The drop system decides what items will drop from monsters when they are killed.
The upgrade system decides if an item will successfully upgrade to the next level or not.
They both need to be able to use probability to determine if:
An item Drops
An item upgrades successfully.
I've developed a system that generates a random number between 0 and 100000. In this system a 1% probability of either of the above happening would be represented by 1000. Similarly, a 0.5% would be 500... and 50% would be 50000.
Here is the guts of this code...
int RandomValueInRange(const int start, const int end)
{
std::random_device rd;
std::mt19937 generator(rd());
const int stable_end = ((end < start) ? start : end);
std::uniform_int_distribution<int> distribution(start, stable_end);
return distribution(generator);
}
Now in order to dermine if an item drops or upgrades sucecsfully, all I have to do is this...
const int random_value = RandomValueInRange(0, 100000);
const int probability = item.GetProbability();//This simply returns an integer stored in a config file which represents the probability of this item being dropped/upgraded.
if(random_value <= probability)
{
std::cout << "Probability Success!" << endl;
}
else
{
std::cout << "Probability Failed!" << endl;
}
I would expect the above to work, but for whatever reason it seems faulty... Players are able to get items that have a 0.1% probability with ease (something that should almost never happen!).
Does anyone know of a better system or how I can improve this system to truly follow the probability guidelines....

Answer 1

std::random_device rd;
std::mt19937 generator(rd());
...
return distribution(generator);
I think problem here, the std c++ library gives you uniform distribution
if you do reuse random_device and mt19937, but you recreate them each time,
it is not how that they should be used.
Save somewhere this std::random_device rd and this std::mt19937and this distribution

Answer 2

Ok, so the problem with your code is that you are choosing a random number between 0 and 100,000. Anyone can get between 1 and 100 with a bit of luck, because, if you think about it, 100 is a pretty big number and shouldn't be too hard to get.
Also, if you go back to Primary/Elementary (or whatever you want to call it) school maths books, you will see in the 'probability and chance' chapter, some questions like:
If there are 6 balls in a bag, 3 red, 1 green and 2 blue, then what is the chance of choosing a blue?
Of course, you would've answered 2/6 or 1/3. In C++, this can be changed to something like this:
#include <iostream>
#include <ctime>
#include <algorithm>
#include <random>
using namespace std;
// Be sure to have this in to get a truly random number
class MoreProbability {
// Be sure to have this in to get a truly random number
void GetProbability(int min, int max, int probability) {
const int arrayMax = max;
int probabilityArray[100000];
for (int i = 0; i < max; i++) {
if (i >= 0 && i <= probability) {
probabilityArray[i] = 1;
}
else {
probabilityArray[i] = 0;
}
}
// Arrays go from 0 to max-1 to account for the 0
std::random_shuffle(&probabilityArray[0], &probabilityArray[max - 1]);
// Check if the first element of the randomly shufffled array is equal to 1
if (probabilityArray[0] == 1) {
cout << "Probability Successful" << endl;
}
else {
cout << "Probability Failed" << endl;
}
}
int main() {
srand(time(0));
GetProbability(0, 100000, 100);
return 0;
}
};
It may give a StackOverflowException. To fix this, simply increase the 'Stack Reserve Size'.
EDIT:
After changing the code around a bit to return a 1 or a 0 based on the outcome, and putting it into a for loop which repeated itself 1000 times (I do NOT recommend trying this as it takes a while to complete), I got an output of 1, clearly showing that this piece of code works perfectly.