Anonymous user
Strong and weak primes: Difference between revisions
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=={{header|Java}}==
<lang java>
public class StrongAndWeakPrimes {
private static int MAX = 10_000_000 + 1000;
private static boolean[] primes = new boolean[MAX];
public static void main(String[] args) {
sieve();
System.out.println("First 36 strong primes:");
displayStrongPrimes(36);
for ( int n : new int[] {1_000_000, 10_000_000}) {
System.out.printf("Number of strong primes below %,d = %,d%n", n, strongPrimesBelow(n));
}
System.out.println("First 37 weak primes:");
displayWeakPrimes(37);
for ( int n : new int[] {1_000_000, 10_000_000}) {
System.out.printf("Number of weak primes below %,d = %,d%n", n, weakPrimesBelow(n));
}
}
private static int weakPrimesBelow(int maxPrime) {
int priorPrime = 2;
int currentPrime = 3;
int count = 0;
while ( currentPrime < maxPrime ) {
int nextPrime = getNextPrime(currentPrime);
if ( currentPrime * 2 < priorPrime + nextPrime ) {
count++;
}
priorPrime = currentPrime;
currentPrime = nextPrime;
}
return count;
}
private static void displayWeakPrimes(int maxCount) {
int priorPrime = 2;
int currentPrime = 3;
int count = 0;
while ( count < maxCount ) {
int nextPrime = getNextPrime(currentPrime);
if ( currentPrime * 2 < priorPrime + nextPrime) {
count++;
System.out.printf("%d ", currentPrime);
}
priorPrime = currentPrime;
currentPrime = nextPrime;
}
System.out.println();
}
private static int getNextPrime(int currentPrime) {
int nextPrime = currentPrime + 2;
while ( ! primes[nextPrime] ) {
nextPrime += 2;
}
return nextPrime;
}
private static int strongPrimesBelow(int maxPrime) {
int priorPrime = 2;
int currentPrime = 3;
int count = 0;
while ( currentPrime < maxPrime ) {
int nextPrime = getNextPrime(currentPrime);
if ( currentPrime * 2 > priorPrime + nextPrime ) {
count++;
}
priorPrime = currentPrime;
currentPrime = nextPrime;
}
return count;
}
private static void displayStrongPrimes(int maxCount) {
int priorPrime = 2;
int currentPrime = 3;
int count = 0;
while ( count < maxCount ) {
int nextPrime = getNextPrime(currentPrime);
if ( currentPrime * 2 > priorPrime + nextPrime) {
count++;
System.out.printf("%d ", currentPrime);
}
priorPrime = currentPrime;
currentPrime = nextPrime;
}
System.out.println();
}
private static final void sieve() {
// primes
for ( int i = 2 ; i < MAX ; i++ ) {
primes[i] = true;
}
for ( int i = 2 ; i < MAX ; i++ ) {
if ( primes[i] ) {
for ( int j = 2*i ; j < MAX ; j += i ) {
primes[j] = false;
}
}
}
}
}
</lang>
{{out}}
<pre>
First 36 strong primes:
11 17 29 37 41 59 67 71 79 97 101 107 127 137 149 163 179 191 197 223 227 239 251 269 277 281 307 311 331 347 367 379 397 419 431 439
Number of strong primes below 1,000,000 = 37,723
Number of strong primes below 10,000,000 = 320,991
First 37 weak primes:
3 7 13 19 23 31 43 47 61 73 83 89 103 109 113 131 139 151 167 181 193 199 229 233 241 271 283 293 313 317 337 349 353 359 383 389 401
Number of weak primes below 1,000,000 = 37,780
Number of weak primes below 10,000,000 = 321,750
</pre>
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