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;
}