Smart people's game improvement: Chapter 3 Lesson 3 example: the secret of prime

#include <iostream> 
#include <cstdio>
using namespace std;
int n,i,j,k;
int t(int a)
{
	for (j=2;j*j<=a;j++)
		if (a%j==0)
			return 0;
	return 1;
}
int main()
{
	cin >>n;
	for (i=2;i<=n;i++)
	{
		if (t(i))
		{
			cout <<i <<" ";
			k++;
			if (k%5==0)
				cout <<endl;
		}
	}
	return 0;
}