Why does a mt19937 generator, generate the same result everytime when used in a while loop

Question

I've been writing this game to just type in the amount of humans and skeletons with a random attack chance. If you enter in the same amount when running the program over and over it generates the same amount. Why isn't the hitChance/random attack amount changing everytime the while loop restarts? When I print the attackChance(randGen) in the while loop it changes. Why isn't the randGen changing the output of the winner of the battle?

   #include <iostream>
#include <string>
#include <random>
#include <ctime>

using namespace std;

int getNumSkeletons();
int getNumHumans();
void finishedBattleStats(int numHumans, int startHumans, int numSkeletons, int startSkeletons);
void simulatedBattle(int &numSkeletons, int &numHumans, int &startHumans, int &startSkeletons);

int main() {

    int startHumans;
    int startSkeletons;

    int numHumans;
    int numSkeletons;


    cout << "------------------------------------\nSkeletons V Humans\n\n";

    //Gets Number of Humans
    numHumans = getNumHumans();
    startHumans = numHumans;
    //Gets Number of Skeletons
    numSkeletons = getNumHumans();
    startSkeletons = numSkeletons;

    //Simulates Battle
    simulatedBattle(numSkeletons, numHumans, startHumans, startSkeletons);

    //End Battle Stats
    finishedBattleStats(numHumans, startHumans, numSkeletons, startSkeletons);
    cin.get();

    return 0;
}

int getNumHumans() {
    int numHumans;

    cout << "Enter number of Humans: \n";
    cin >> numHumans;
    return numHumans;
}

int getNumSkeletons() {
    int numSkeletons;

    cout << "Enter number of Skeletons: \n";
    cin >> numSkeletons;
    return numSkeletons;
}

void simulatedBattle(int &numSkeletons, int &numHumans, int &startHumans, int &startSkeletons) {

    static mt19937 randGen(time(NULL));
    uniform_real_distribution<float> attackChance(0.0f, 1.0f);

    //HumanProperties
    float humanDamage = 30.0f;
    float humanHitChance = 0.6f;
    float humanCritChance = 0.2f;
    float humanHealth = 50.0f;
    float startHumanHealth = humanHealth;
    float currentHuman = startHumanHealth;

    //Skeleton Properties
    float skeletonDamage = 20.0f;
    float skeletonHitChance = 0.7f;
    float skeletonCritChance = 0.2f;
    float skeletonHealth = 40.0f;
    float startSkeletonHealth = skeletonHealth;
    float currentSkeleton = startSkeletonHealth;

    char turn = 'H';

    float hitChance;

    while (numSkeletons > 0 && numHumans > 0) {
        hitChance = attackChance(randGen);
        if (turn == 'H') {
            if (hitChance > humanHitChance) {
                currentSkeleton -= humanDamage;
                turn = 'S';
                if (currentSkeleton < 0) {
                    numHumans--;
                    currentSkeleton = startSkeletonHealth;
                    turn = 'S';
                }
            }
        }
        else {
            if (hitChance > skeletonHitChance) {
                currentHuman -= skeletonDamage;
                turn = 'H';
            }
            if (currentHuman < 0) {
                numSkeletons--;
                currentHuman = startHumanHealth;
                turn = 'H';
            }
        }
    }

}

void finishedBattleStats(int numHumans, int startHumans, int numSkeletons, int startSkeletons) {
        if (numHumans == 0) {
            cout << "Skeletons won! \n\n";
            cout << "Skeletons left: " << numSkeletons << endl;
            cout << "Skeleton Casualties: " << startSkeletons - numSkeletons << endl;
            cout << "All " << startHumans << " humans are dead! \n\n";
            cout << "Game Over!";
            cin.get();
        }
        else {
            cout << "Humans Won! \n\n";
            cout << "Humans left: " << numHumans << endl;
            cout << "Human Casualties: " << startHumans - numHumans << endl;
            cout << "All " << startSkeletons << " skeletons are dead! \n\n";
            cout << "Game Over!";
            cin.get();
        }
    }

Answer 1

IMHO opinion you have a flaw in the battle calculations. That is, when a human's healths goes < 0 you subtract a skeleton and not a human and vice versa. Below is the correction. With a demo. I tried the demo a number of times and gives different results (ENJOY):

void simulatedBattle(int &numSkeletons, int &numHumans, int &startHumans, int &startSkeletons) {
    uniform_real_distribution<float> attackChance(0.0f, 1.0f);
    static mt19937 randGen(time(NULL));

    //HumanProperties
    float humanDamage = 25.0f;
    float humanHitChance = 0.2f;
    float humanCritChance = 0.2f;
    float humanHealth = 50.0f;
    float startHumanHealth = humanHealth;
    float currentHuman = startHumanHealth;

    //Skeleton Properties
    float skeletonDamage = 20.0f;
    float skeletonHitChance = 0.8f;
    float skeletonCritChance = 0.2f;
    float skeletonHealth = 40.0f;
    float startSkeletonHealth = skeletonHealth;
    float currentSkeleton = startSkeletonHealth;

    char turn = 'H';

    float hitChance;

    while (numSkeletons > 0 && numHumans > 0) {
        hitChance = attackChance(randGen);
        if (turn == 'H') {
            if (hitChance > humanHitChance) {
                currentSkeleton -= humanDamage;
                if (currentSkeleton < 0) {
                    --numSkeletons;
                    currentSkeleton = startSkeletonHealth;
                }
                turn = 'S';
            }
        }
        else {
            if (hitChance > skeletonHitChance) currentHuman -= skeletonDamage;
            if (currentHuman < 0) {
                --numHumans;
                currentHuman = startHumanHealth;
            }
            turn = 'H';
        }
    }
}

LIVE DEMO

Answer 2

You have to seed a mersenne twister before use, or you'll always have the same output. It is common practice to seed it with the internal clock:

// obtain a seed from the timer
myclock::duration d = myclock::now() - beginning;
unsigned seed2 = d.count();
generator.seed (seed2);

ref: std::mersenne_twister_engine::seed