Project Euler Solution 7

Problem Statement

Find the 10001st prime.

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 10001st prime number?

Solution

I have written a program for checking primality in problem 3 and I just used it to solve this problem.

I used a ‘count’ to keep track of the prime number. Also I have considered only the odd numbers for checking primality.

Implementation

#include <stdio.h>#include <math.h>
int isPrime(long long int num){    int j, num_sqrt;    num_sqrt = sqrt(num);    for (j = 2; j <= num_sqrt; j++)    {        if ((num % j) == 0) return 0;    }    return 1;}
int main(){    int count;    long long int number;    count = 2;    number = 5;    while (number != 0)    {        if (isPrime(number)) count++;        if (count == 10001) break;        number += 2;    }    printf("%d", number);    return 0;}

Sample Output