Question

How to check if a number is circular prime (cyclically rotate left will also be prime) in Swift

Answer 1

import Foundation

func isPrime(_ n: Int) -> Bool {
    if n == 2 { return true }
    if n < 2 || n % 2 == 0 { return false }

    let limit = Int(sqrt(Double(n)))
    for i in stride(from: 3, through: limit, by: 2) {
        if n % i == 0 { return false }
    }
    
    return true
}

func cyclicallyRotateLeft(_ n: Int) -> Int {
    var m = n
    var p = 1
    
    while m >= 10 {
        p *= 10
        m /= 10
    }
    
    return (n % p) * 10 + m
}

func isCircularPrime(_ n: Int) -> Bool {
    if n < 2 { return false }

    var rotated = cyclicallyRotateLeft(n)
    while rotated != n {
        if rotated < n || !isPrime(rotated) {
            return false
        }
        rotated = cyclicallyRotateLeft(rotated)
    }
    
    return true
}

let testNumbers = [197, 1193, 23, 101, 119]

for n in testNumbers {
    let result = isCircularPrime(n) ? "Circular Prime" : "Not Circular Prime"
    print("\(n) → \(result)")
}

// 1193
// 1193 = prime
// 3119 = prime
// 9311 = prime
// 1931 = prime

/*
run:

197 → Circular Prime
1193 → Circular Prime
23 → Not Circular Prime
101 → Not Circular Prime
119 → Circular Prime

*/

 

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