// =============================== // CodeName : Fair coin trial.cpp // Author : Babak Sheykhan // Date : Apr 04, 2018 // =============================== #include #include #include #include #include using namespace std; // Motivation behind this program /* You probably know that pseudorandom number generators (PRNG) that we are using regularly in our programs are unfair, and they won't produce a sequence of numbers with good distribution. sometimes they generate some numbers more than the others which when the recurrence period is short (e.g 1-100) it becomes worse. In the following code, I'm going to share with you a simple but practical simulation regarding flipping a coin. Suppose you think a coin is unfair. You don't know the probabilities of getting heads or tails, but you suspect the probabilities are not 0.5. What you could do about that? Let's see! */ /*-----------------------------*/ /* */ /* INTERFACE */ /* */ /*-----------------------------*/ // // ____________________________ // | FairCoin (UML) | // +----------------------------+ // | - flip_1 : Boolean | // | - flip_2 : Boolean | // | - ss : Stringstream | // | - resultLog : string | // | - MIN : UInteger = 1 | // | - MID : UInteger = 50 | // | - MAX : UInteger = 100 | // |____________________________| // | + checkFairness() : String | // | - flipping() | // | - randomizing() : Integer | // |____________________________| class FairCoin { public: FairCoin(); string checkFairness(); private: void flipping(); int randomizing(); bool flip_1, flip_2; stringstream ss; string resultLog; static const unsigned MIN = 1; static const unsigned MID = 50; static const unsigned MAX = 100; }; /*-----------------------------*/ /* */ /* IMPLIMENTATION */ /* */ /*-----------------------------*/ FairCoin::FairCoin() { ss << "The coin is going to flip twice...\n\n"; flipping(); } string FairCoin::checkFairness() { string stat_1; string stat_2; do { stat_1 = flip_1 ? "Heads" : "Tails"; stat_2 = flip_2 ? "Heads" : "Tails"; ss << "The result is (" << stat_1 << ", " << stat_2 << ")\n"; if (flip_1 == flip_2) { ss << "Result is unfair! Trying again...\n\n"; flipping(); } else { ss << "Result is fair! " << stat_1 << " goes first.\n"; break; } } while (true); resultLog = ss.str(); return resultLog; } void FairCoin::flipping() { // true for Heads -- between 1 and 50 inclusive // false for Tails -- between 51 and 100 inclusive flip_1 = randomizing() <= MID ? true : false; flip_2 = randomizing() <= MID ? true : false; } int FairCoin::randomizing() { return MIN + rand() % (MAX - MIN + 1); } int main() { time_t currentTime = time(NULL); srand(currentTime); FairCoin coinTest; cout << coinTest.checkFairness(); }