Project Euler Problem 7: 10001st prime

10001st prime

Problem 7

By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.

What is the 10 001st prime number?

C++:

#include <iostream>
#include <cmath>

using namespace std;

const int MAXN = 10001;

int prime[MAXN+1] = {2, 3};

bool isprime(int n)
{
int q=sqrt(n), i=1;

while(prime[i] <= q)
if(n % prime[i] == 0)
return false;
else
i++;

return true;
}

void maketable(int n)
{
int i = 2, val = 5;

while(i < n) {
if(isprime(val))
prime[i++] = val;

val += 2;
}
}

int main()
{
int n;

maketable(MAXN);

while(cin >> n)
cout << prime[n - 1] << endl;

return 0;
}

C++:

#include <iostream>
#include <cmath>

using namespace std;

const int MAXN = 10001;

int prime[MAXN+2] = {2, 3, 5};

bool isprime(int n)
{
int q=sqrt(n), i=1;

while(prime[i] <= q)
if(n % prime[i] == 0)
return false;
else
i++;

return true;
}

void maketable(int n)
{
int val1 = 1, val5 = 5;

for(int i=3; i<=n;) {
val1 += 6;
if(isprime(val1))
prime[i++] = val1;

val5 += 6;
if(isprime(val5))
prime[i++] = val5;
}
}

int main()
{
int n;

maketable(MAXN);

while(cin >> n)
cout << prime[n - 1] << endl;

return 0;
}