Thread: Finding prime numbers in a loop

I think you need to learn how to write and use a function.

Functions in C and C++ - Cprogramming.com

Tim S.

For starters, you need to format, especially indent, your code properly. For example:

Code:

#include <iostream>
#include <stdlib.h>
#include <math.h>

using namespace std;

//This program is to determine how many times a player wins by rolling two dice adding up to a prime number 500 times
main()
{
    int n = 0, die1 = 0, die2 = 0, roll_count = 0, playerwin = 0, i;
    while (roll_count <= 500)//Number of times dice will roll
    {
        srand(time(NULL));
        die1 = rand() % 6 + 1; //randomizing the first die
        srand(time(NULL));
        die2 = rand() % 6 + 1; //randomizing the second die
        n = die1 + die2; //Adding the randomized values together
        for (i = 2; i <= sqrt(n); i++) //i is the integer being tested on n
        {
            if (n % i == 0) //if they get even, playerwin gets added 1 and return 1
            {
                playerwin++;
                return 1;
            }
            if (i == 11) //If integer exceeds 11 which eliminates any possible 
            even divisible between 2-11, it is considered not prime
            and returns 0
            {
                return 0;
            }
        }
        roll_count++; //After the sequence is done, roll_count gets counted
        the and loop starts again
    }
    cout << "Number of times player has won is: " << playerwin << endl;             // Counted playerwin will be displayed
    return 0;
}

A few things to note:

• <stdlib.h> is a C header. The C++ header is <cstdlib>
• <math.h> is a C header. The C++ header is <cmath>
• You forgot the return type for the main function, i.e., it should be int main
• You called srand twice, and moreover called it with a loop body! srand should only be called once, near the start of the main function.
• You used time, which means that you should #include <ctime>

You are apparently simulating two six sided dice on each iteration, so the range of their sum is [2, 12]. The primes within that range are well known: 2, 3, 5, 7, 11. These are few enough that you can check them directly. Alternatively, you can "test each integer I receive from the dice between 1 and sqrt(n) using modulus" by pre-computing the maximum prime divisor other than the prime itself for integers in that range, i.e., 3. This way, you just check if the number is greater than 3 and is divisible by 2 or 3: if so, it is composite, otherwise it is prime. An explicit computation of the square root is definitely unnecessary (i.e., you don't need <cmath> at all).

It is also possible to create an array with all the primes between 1 and 12 inclusive. Then simply check if a value is in that array to determine if it is prime.