CODEWITHSUNDEEP
javaalgorithmbooleanindexoutofboundsexception
user5022341
user5022341

Reputation:

Sieve of Eratosthenes, ArrayIndexOutOfBoundsException, output is composite number

I tried writing a Sieve of Eratosthenes algorithm, I am getting an ArrayIndexOutOfBoundsException but I don't seem to understand why, if I change the limits, upon printing it only displays composite numbers, the code's below help if you can.

public static Boolean[] solution(int N) {
        Boolean[] isPrime = new Boolean[N];
        isPrime[0] = false;
        for(int i = 1; i <= N; i++) {
            isPrime[i] = true;
        }

        for(int i = 2; i <= N; i++) {
            if(isPrime[i]== true) {
                System.out.println(i);
                for(int j = 2; (j * i) < N; j++) {
                    int k = j * i;
                    isPrime[k] = false;
                }

            }
        }

    return isPrime;

Upvotes: 1

Views: 65

Answers (2)

AlbertoD
AlbertoD

Reputation: 146

Boolean[N] creates an array of N elements, so, since the indexes start from 0, the last index is N-1. The error is caused by i<=N in the for loop

Upvotes: 1

Madhawa Priyashantha
Madhawa Priyashantha

Reputation: 9880

i <= N; cause the error

for(int i = 1; i <= N; i++) {
            isPrime[i] = true;
}

for example

if N=4 then you get error when i=4. isPrime[4] cause OutOfBounds exception because length is 4.arrays are zero index based. so maximum index you can access is 3.isPrime[3]

you can avoid this error by changing loop to for(int i = 1; i < N; i++) {

however i'm not sure what is Eratosthenes algorithem is .i hope you can change your code keep in mind arrays are zero index based

Upvotes: 1

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