Random Prime Number Generator (C++)


2 min read


To create a random number engine that generates prime number on the go! Here the link GitHub repo

Plan of Attack:

  • Since finding prime number in constant time is impossible by the math at the moment, So We are going to use a database-based approach. Lets store all the prime number in primeDB file
  • Since C++ already have random engines, Let us create a wrapper around that using templates


  • Container that takes in engine class and override all the engine functions, also takes in filepath for database
  • Functions to override
    • () -> returns random prime number
    • min() -> returns min prime number
    • max() -> returns max prime number
    • << -> save the state to std::stringstream
    • >> -> restores the state from std::stringstream


  • Private Members:
    • template <typename Engine> Engine eng;
    • int min_prime;
    • int max_prime;
    • char const *db_filepath;
  • Public Members:
    • Constructor
    • int operator()();
    • friend stringstream &operator<<
    • friend stringstream &operator>>

Constructor: takes in seed and filepath

#define DB_FILEPATH "primeDB"

template <typename Engine>
random_prime_engine<Engine>::random_prime_engine(unsigned long rand_seed, char const *db_filepath=DB_FILEPATH)
    // Get DB path
    this->db_filepath = db_filepath;
// Usage

ra::random_prime_engine<std::default_random_engine> rpe_2(rand_seed);

() operator,

#define DB_SIZE 10000

template <typename Engine>
int random_prime_engine<Engine>::operator()()
    int idx = eng() % DB_SIZE; // to get a index within file
    int rand_prime;

    // Open the file
    std::ifstream file;

    // update rand_prime till it gets the value at index
    for (int i = 0; i <= idx; i++)
        file >> rand_prime;

    return (rand_prime);
// Usage

std::cout << rpe()

min and max

# pseudo code
# initialize min/max with 0, for lazy initialization

    # return min or max if once found
    if bool(min/max):
        return min/max

    min = file.first_line
    max = file.last_line

    return min/max

>> and <<

// Friend Functions
#define stringstream SS
function SS &operator<>(SS&in/out, rpe){
    in/out >> rpe.eng;
    return in/out
// Usage

std::stringstream state;
state << rpe; //save current state
state >> rpe; //restore old state

All the code can be found at the below link: github.com/ra101/Random-Prime-Number-Genera..

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