C++ - prime numbers

I was reading about algorithms the other day and came to know of a faster algorithm to find prime numbers. It is called the Sieve of Eratosthenes. When I compared the execution times of this algorithm to the one I coded, the times were remarkably faster. A very simple and impressive algorithm to find prime numbers less than a number n.

	#include <iostream>
	#include <ctime>
	using namespace std;
	//interesting method of finding primes
	// saw this one in one of the algorithm books
	void sieve_of_eratosthenes(int n)
	{
	int a[n + 1];
	for (int i = 0; i <= n; i++) {
	a[i] = 1;
	}
	for (int i = 2; i <= n / 2; i++)
	for (int j = 2; j <= n / i; i++)
	a[i * j] = 0;
	for (int i = 2; i <= n; i++)
	if (a[i]) {
	cout << i << " is prime" << endl;
	}
	}
	// the one I coded
	void sieve_of_harpreet(int n)
	{
	bool p;
	for (int i = 2; i < n; i++) {
	p = true;
	//optimized by checking only until sqrt of the num
	for (int j = 2; j * j < i; j++) {
	if (i % j == 0) {
	p = false;
	break;
	}
	}
	if (p) {
	cout << i << " is prime" << endl;
	}
	}
	}
	int main()
	{
	unsigned int start = clock();
	sieve_of_harpreet(30000000);
	std::cout << "Time taken: " << clock() - start << endl;
	start = clock();
	sieve_of_eratosthenes(30000000);
	std::cout << "Time taken: " << clock() - start << endl;
	return 0;
	}

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