[c++]소수 찾기

#include <iostream>
int main() {
	int max_value;
	std::cout << "Display primes up to what value? ";
	std::cin >> max_value;
	for (int value = 2; value <= max_value; value++) {
		//가장 작은 소수는 2
		bool is_prime = true; 

		for (int trial_factor = 2;
			is_prime && trial_factor < value;
			trial_factor++)
			is_prime = (value % trial_factor != 0);
		if (is_prime)
			std::cout << value << " "; 
	}
	std::cout << '\n'; 
}

 

#include<iostream>
using namespace std;
int main() {
	int max_value;
	cout << "Display primes up to what value?";
	cin >> max_value;

	int value = 2; //가장 작은 소수가 2임
	while (value <= max_value) {
		bool is_prime = true;
		int trial_factor = 2;
		while (trial_factor < value) {
			if (value % trial_factor == 0) {
				is_prime = false;
				break;
			}
			trial_factor++;
		}
		if (is_prime)
			cout << value << " ";
		value++;
	}
	cout << endl;
}

 

#include <iostream>
#include <cmath>

bool is_prime(int n) {
	bool result = true; //소수가정
	double r = n, root = sqrt(r);

	for (int trial_factor = 2;
		result && trial_factor <= root; trial_factor++)
		result = (n % trial_factor != 0);
	return result;
}

int main() {
	int max_value;
	std::cout << "Display primes up to what value? ";
	std::cin >> max_value;
	for (int value = 2; value <= max_value; value++)
		if (is_prime(value))
			std::cout << value << " ";
	std::cout << '\n';
}

 

 

벡터로 접근해서, 범위의 최소까지 정해줄 수 있다.

#include<iostream>
#include<vector>
#include<cmath>
using namespace std;

void print(const vector<int>& v) {
	for (int elem : v)
		cout << elem << " ";
	cout << endl;
}
bool is_prime(int n) {
	if (n < 2)
		return false;
	else {
		bool result = true;
		double r = n, root = sqrt(r);
		for (int trial = 2;result & trial <= root;trial++)
			result = (n % trial != 0);
		return result;
	}
}

vector<int> primes(int begin, int end) {
	vector<int> result;
	for (int i = begin; i <= end; i++)
		if (is_prime(i))
			result.push_back(i);
	return result;
}

int main() {
	int low, high;
	cout << "please enter lowest and highest values in the range:";
	cin >> low >> high;
	vector<int> prime_list = primes(low, high);
	print(prime_list);
}