eratosthenes.cpp File Reference

The Sieve of Eratosthenes More...

#include <cassert>
#include <chrono>
#include <iostream>
#include <string>
#include <vector>

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Namespaces
namespace	math
	for IO operations

Functions
void	math::sieve (std::vector< bool > *vec)
	Performs the sieve.
void	math::print_primes (std::vector< bool > const &primes)
	Prints all the indexes of true values in the passed std::vector.
static void	test ()
	Self-tests the sieve function for major inconsistencies.
int	main (int argc, char *argv[])
	Main function.

Detailed Description

The Sieve of Eratosthenes

Store an array of booleans where a true value indicates that it's index is prime. For all the values in the array starting from 2 which we know is prime, we walk the array in multiples of the current outer value setting them to not prime. If we remove all multiples of a value as we see it, we'll be left with just primes.

Pass "print" as a command line arg to see the generated list of primes

Author

Keval Kapdee

Function Documentation

main()

int main	(	int	argc,
		char *	argv[] )

Main function.

Parameters

argc	commandline argument count
argv	commandline array of arguments

Returns

0 on exit

                                 {
    test();  // run self-test implementations
 
    // The largest prime we will check for
    auto max = 10000;
 
    // Store a boolean for every number which states if that index is prime or
    // not
    auto primes = std::vector<bool>(max, true);
 
    // Store the algorithm start time
    auto start = std::chrono::high_resolution_clock::now();
 
    // Run the sieve
    math::sieve(&primes);
 
    // Time difference calculation
    auto time = std::chrono::duration_cast<
                    std::chrono::duration<double, std::ratio<1>>>(
                    std::chrono::high_resolution_clock::now() - start)
                    .count();
 
    // Print the primes if we see that "print" was passed as an arg
    if (argc > 1 && argv[1] == std::string("print")) {
        math::print_primes(primes);
    }
 
    // Print the time taken we found earlier
    std::cout << "Time taken: " << time << " seconds" << std::endl;
 
    return 0;
}

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test()

static void test ( )

static

Self-tests the sieve function for major inconsistencies.

Returns

void

                   {
    auto primes = std::vector<bool>(10, true);
    math::sieve(&primes);
    assert(primes[0] == false);
    assert(primes[1] == false);
    assert(primes[2] == true);
    assert(primes[3] == true);
    assert(primes[4] == false);
    assert(primes[5] == true);
    assert(primes[6] == false);
    assert(primes[7] == true);
    assert(primes[8] == false);
    assert(primes[9] == false);
 
    std::cout << "All tests have successfully passed!\n";
}

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