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//Erin O'Toole//Prime Finder V4 (Use File IO to optimize)#include <iostream>#include <fstream>#include <cmath>int is_prime(int);using namespace std;int main(){	cout << "Prime Calculator 4.0" << endl;	cout << "Enter the max number you'd like to calculate" << endl;	int to_check, counter = 3;	cin >> to_check;		ofstream output;	output.open("PF4.txt");	if (output.fail())	{		cout << "file error" << endl;		return 0;	}			if (to_check >= 2)	{		cout << "2" << endl;		output << 2 << endl;	}		for (counter = 3; counter <= to_check; counter++)	{				if (is_prime(counter) == 1)		{			cout << counter << endl;			output << counter << endl;		}		/*else if (is_prime(counter) == 0)		{			cout << counter << ": is not prime" << endl;		}		/*else		{			cout << "error sqrt = " << sqrt(to_check) << endl;		}*/	}	cout << "done!" << endl;	cin >> to_check;	return 0;}	int is_prime(int viable){	int divisor;	double sqrt_viable = sqrt(viable);	sqrt_viable += 1;		ifstream input1;	input1.open("PF4.txt");	input1 >> divisor;	while ((input1.good()) && (divisor < sqrt_viable))	{		if ((viable % divisor) == 0)		{			return 0;		}		input1 >> divisor;	}		return 1;}