Math Related Problem

[perito]

Im trying to solve this

Quote:

The prime 41, can be written as the sum of six consecutive primes:
41 = 2 + 3 + 5 + 7 + 11 + 13

This is the longest sum of consecutive primes that adds to a prime below one-hundred.

The longest sum of consecutive primes below one-thousand that adds to a prime, contains 21 terms, and is equal to 953.

Which prime, below one-million, can be written as the sum of the most consecutive primes?

I wrote this code, it should work but it doesnt !
It works correctly for number under 100, but it doesnt even work for numbers under 1000
it gives a wrong answer 281 (it must be 953)
can anyone tell me where im going wrong?

Code:

public class ConescutivePrimes 
{
	public static boolean isPrime(int nbr)
	{
		boolean isNPrime=true;
		for (int i = 2; i < nbr; i++)
			if ((nbr != i) && (nbr % i == 0))
			{
				isNPrime = false;
				break;
			}
		if (isNPrime == true) return true;
		else return false;
	}
	public static void main(String[] args) 
	{
		int maxValue=1000,sum=0,lastPrimeSum=0;
		for (int i=2;i<maxValue;i++)
		{
			if (isPrime(i) == true && sum+i<maxValue)
			{
				sum+=i;
				if (isPrime(sum)==true)
					lastPrimeSum=sum;
			}
		}
		System.out.println(lastPrimeSum);
	}
}

change the value of maxValue to 100 and see the correct answer,
change it to 1000 and see it go all wrong... WHY?

[CaptainMorgan]

Hi perito. I haven't had time to correct your code, but I ran a simple print test... 281 IS the last prime that your code finds - therefore it is output. You do not implement a check to correctly sum up the numbers. Here's the output from my print test:

Code:

2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 = 281

Here's a different test showing what sum is at each output:

Code:

2(2) 3(5) 5(10) 7(17) 11(28) 13(41) 17(58) 19(77) 23(100) 29(129) 31(160) 37(197) 41(238) 43(281) 47(328) 53(381) 59(440) 61(501) 67(568) 71(639) 73(712) 79(791) 83(874) 89(963) = 281

Since your code requires consecutive sums, you may wish to try re-initializing i once it's reached 1000 and the correct answer is still not found(how you do this I leave for you- maybe an outer surrounding loop?). For example, you start off with int i = 2; and since the sum of the above output is actually 963, it lead me to think you're only ten off... thus what would happen if i was started or restarted off at 7 instead of 2? It would be ten subtracted- hence, 953. Example, for int i = 7; we get:

Code:

7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 = 953