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Java

Posted by Diego Murray
Diego Murray
2,515 Points

Factorials and BigInteger Code - Bug - JAVA

I am not receiving the correct solution. If you find what is wrong, can you explain how you broke it down and found the bug. Thanks so much.

package euler20;

import java.math.BigInteger;

public class sumDigitsIn100Factorial {

        public static final int n = 100;

        public static void main(String[] args) {

            long startTime = System.currentTimeMillis();

            System.out.println("100! = " + factorial(n));
            System.out.println("Sum of all digits in 100! = " + sumOfDigits(factorial(n)));

            long endTime = System.currentTimeMillis();
            System.out.println("Time: " + (endTime - startTime) + " ms");

        }

        public static BigInteger factorial(int n1) {

            BigInteger n = BigInteger.ONE;

            for (int i = 1; i <= n1; i++) {
                n = n.multiply(BigInteger.valueOf(i));
            }
            return n;
        }

        public static int sumOfDigits(BigInteger n1) {

            String bigIntStr = factorial(n).toString();
            int sum = 0;

            for (int i = 1; i <= bigIntStr.length(); i++) {
                sum += Integer.parseInt(String.valueOf(i));
            }
            return sum;
        }
}

1 Answer

Kourosh Raeen
Kourosh Raeen
23,733 Points

Hi Diego - It looks like that the issue is in the sumOfDigits function. You've converted the return value of factorial(n) to a String, but instead of adding the digits, the individual characters in that String, you're adding up the numbers from 1 thru the length of the String. For example, 6! is 720. The sum of digits should be 7+2+0=9. But you're adding 1+2+3 and you end up with 6. This is what I did:

public static int sumOfDigits(BigInteger n1) {
    String bigIntStr = factorial(n).toString();
    System.out.println(bigIntStr);
    int sum = 0;

    for (char digit : bigIntStr.toCharArray()) {
        sum += Integer.parseInt(digit + "");    
    }    
    return sum;
}
Diego Murray
Diego Murray
2,515 Points

Thanks. What is the purpose of the " " marks after digit??

Kourosh Raeen
Kourosh Raeen
23,733 Points

Glad I could help! The parseInt() method expects a String so I had to convert digit, which is a char, to a String by concatenating an empty String to it.