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FindingPrimes.java
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45 lines (41 loc) · 1.27 KB
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/**
* The Sieve of Eratosthenes is an algorithm use to find prime numbers,
* up to a given value.
* Illustration: https://upload.wikimedia.org/wikipedia/commons/b/b9/Sieve_of_Eratosthenes_animation.gif
* (This illustration is also in the github repository)
*
* @author Unknown
*
*/
public class FindingPrimes{
/**
* The Main method
*
* @param args Command line arguments
*/
public static void main(String args[]){
SOE(20); //Example: Finds all the primes up to 20
}
/**
* The method implementing the Sieve of Eratosthenes
*
* @param n Number to perform SOE on
*/
public static void SOE(int n){
boolean sieve[] = new boolean[n];
int check = (int)Math.round(Math.sqrt(n)); //No need to check for multiples past the square root of n
sieve[0] = false;
sieve[1] = false;
for(int i = 2; i < n; i++)
sieve[i] = true; //Set every index to true except index 0 and 1
for(int i = 2; i< check; i++){
if(sieve[i]==true) //If i is a prime
for(int j = i+i; j < n; j+=i) //Step through the array in increments of i(the multiples of the prime)
sieve[j] = false; //Set every multiple of i to false
}
for(int i = 0; i< n; i++){
if(sieve[i]==true)
System.out.print(i+" "); //In this example it will print 2 3 5 7 11 13 17 19
}
}
}