Strong and weak primes: Difference between revisions
Added Swift solution
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Count of balanced primes <= 1,000,000: 2,994
Count of balanced primes <= 10,000,000: 21,837
</pre>
=={{header|Swift}}==
<lang swift>import Foundation
class PrimeSieve {
var composite: [Bool]
init(size: Int) {
composite = Array(repeating: false, count: size/2)
var p = 3
while p * p <= size {
if !composite[p/2 - 1] {
let inc = p * 2
var q = p * p
while q <= size {
composite[q/2 - 1] = true
q += inc
}
}
p += 2
}
}
func isPrime(number: Int) -> Bool {
if number < 2 {
return false
}
if (number & 1) == 0 {
return number == 2
}
return !composite[number/2 - 1]
}
}
func commatize(_ number: Int) -> String {
let n = NSNumber(value: number)
return NumberFormatter.localizedString(from: n, number: .decimal)
}
let limit1 = 1000000
let limit2 = 10000000
class PrimeInfo {
let maxPrint: Int
var count1: Int
var count2: Int
var primes: [Int]
init(maxPrint: Int) {
self.maxPrint = maxPrint
count1 = 0
count2 = 0
primes = []
}
func addPrime(prime: Int) {
count2 += 1
if prime < limit1 {
count1 += 1
}
if count2 <= maxPrint {
primes.append(prime)
}
}
func printInfo(name: String) {
print("First \(maxPrint) \(name) primes: \(primes)")
print("Number of \(name) primes below \(commatize(limit1)): \(commatize(count1))")
print("Number of \(name) primes below \(commatize(limit2)): \(commatize(count2))")
}
}
var strongPrimes = PrimeInfo(maxPrint: 36)
var weakPrimes = PrimeInfo(maxPrint: 37)
let sieve = PrimeSieve(size: limit2 + 100)
var p1 = 2, p2 = 3, p3 = 5
while p2 < limit2 {
if sieve.isPrime(number: p3) {
let diff = p1 + p3 - 2 * p2
if diff < 0 {
strongPrimes.addPrime(prime: p2)
} else if diff > 0 {
weakPrimes.addPrime(prime: p2)
}
p1 = p2
p2 = p3
}
p3 += 2
}
strongPrimes.printInfo(name: "strong")
weakPrimes.printInfo(name: "weak")</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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