30 lines
1.0 KiB
Java
30 lines
1.0 KiB
Java
/*
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* The Sieve of Eratosthenes is an algorithm used to find prime numbers, up to a given value.
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* Illustration: https://upload.wikimedia.org/wikipedia/commons/b/b9/Sieve_of_Eratosthenes_animation.gif
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*/
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public class FindingPrimes{
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public static void main(String args[]){
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SOE(20); //Example: Finds all the primes up to 20
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}
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public static void SOE(int n){
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boolean sieve[] = new boolean[n];
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int check = (int)Math.round(Math.sqrt(n)); //No need to check for multiples past the square root of n
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sieve[0] = false;
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sieve[1] = false;
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for(int i = 2; i < n; i++)
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sieve[i] = true; //Set every index to true except index 0 and 1
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for(int i = 2; i< check; i++){
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if(sieve[i]==true) //If i is a prime
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for(int j = i+i; j < n; j+=i) //Step through the array in increments of i(the multiples of the prime)
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sieve[j] = false; //Set every multiple of i to false
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}
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for(int i = 0; i< n; i++){
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if(sieve[i]==true)
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System.out.print(i+" "); //In this example it will print 2 3 5 7 11 13 17 19
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}
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}
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} |