A C++ script written to generate sequences from the Collatz Conjecture, an open problem in mathematics.

collatz.cpp

#include <iostream>
#include <fstream>
#include <cstdlib>
#include <ctime>


int main(int argc, char** argv)
{
	int time_initial = 0,
        time_final = 0,
        time_delta = 0;
    long long int limit = 10000;

    std::ofstream output;
	output.open("collatz_sequences.txt");

	output << "**Collatz Conjecture Sequences**\n\n";

    if (argc != 2)
    {
        std::cout << "Enter the number of sequences to generate: ";
        for (;;)
        {
            if (std::cin >> limit) { break; }
            std::cout << "Invalid input, retry (enter another limit): ";
            std::cin.sync();
            std::cin.clear();
        }
    }
    else
    {
        if (atoi(argv[1]) > 0 && atoi(argv[1]) <= 100000000)
        {
            limit = atoi(argv[1]);
        }
    }

	time_initial = time(0);
	for (int i = 2; i <= limit; i++)
	{
		output << "** Number: " << i << " **\n\n";

        int sequence_num = i, sequence_length = 0;
        char arrow[] = " -> ";

        output << sequence_num << arrow;
        for (int sequence_num = i; sequence_num != 1; sequence_length++)
        {
            if (sequence_num % 2 == 0)
            {
                sequence_num /= 2;
            }
            else
            {
                sequence_num = ((sequence_num * 3) + 1);
            }
            if (sequence_num != 1)
            {
                output << sequence_num << arrow;
            }
        }
        output << "1\nCompleted in " << sequence_length << " steps.\n\n";
	}

	time_final = time(0);
	time_delta = time_final - time_initial;
	output << "\n\n** Generating these sequences took "
           << time_delta
           << " seconds. **";

	output.close();

    std::cout << "Done.\n";

    return 0;
}