Question

I am new here. I am trying to solve this exercise Problem 18 just for reinforcing my solving skills. I’ve already coded the answer. The task asks for “How many of the primes below 1,000,000 have the sum of their digits equal to the number of days in a fortnight?” (a fortnight is 14 days). My answers is 16708, but it is wrong. I hope you can help me. I don’t know what my error is. I have 2 methods, 1 for generating the primes, and another for counting the digits of each prime.

This is my code:

import java.util.ArrayList;
import java.util.List;

public class Problema18 {

    public static void main(String args[]) {
        ArrayList<Integer> num = primes();
        System.out.println(num);
        count(primes());
    }

    public static ArrayList<Integer> primes() {
        List<Integer> primes = new ArrayList<Integer>();
        primes.add(2);

        for (int i = 3; i <= 1000000; i += 2) {
            boolean isPrime = true;

            int stoppingPoint = (int) (Math.pow(i, 0.5) + 1);
            for (int p : primes) {
                if (i % p == 0) {
                    isPrime = false;
                    break;
                }
                if (p > stoppingPoint) { break; }
            }
            if (isPrime) { primes.add(i); }
        }
        // System.out.println(primes);
        return (ArrayList<Integer>) primes;
        //System.out.println(primes.size());
    }

    public static void count(ArrayList<Integer> num) {
        int count = 0;

        for (int i = 0; i <= num.size() - 1; i++) {
            int number = num.get(i);
            String num1 = String.valueOf(number);
            int sum = 0;

            for (int j = 0; j < num1.length(); j++) {
                sum = Integer.parseInt(num1.charAt(j) + "") + sum;
                if (sum == 14) { count++; }
            }
            System.out.println(sum);
        }
        System.out.println(count);
    }
}

Answer 1

You should check whether sum == 14 outside the inner for loop. What happens now is that you also count those primes for which the sum of digits is larger than 14 but the sum of the digits in some prefix of the prime is equal to 14.