Question

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

Answer 1

import kotlin.math.sqrt

fun isPrime(n: Int): Boolean {
    if (n == 2) return true
    
    if (n < 2 || n % 2 == 0) return false
    
    val limit = sqrt(n.toDouble()).toInt()
    for (i in 3..limit step 2) {
        if (n % i == 0) return false
    }
    
    return true
}

fun cyclicallyRotateLeft(n: Int): Int {
    var m = n
    var p = 1

    while (m >= 10) {
        p *= 10
        m /= 10
    }

    return (n % p) * 10 + m
}

fun isCircularPrime(n: Int): Boolean {
    if (n < 2) return false
    
    var rotated = cyclicallyRotateLeft(n)
    
    while (rotated != n) {
        if (rotated < n || !isPrime(rotated)) return false
        rotated = cyclicallyRotateLeft(rotated)
    }
    
    return true
}

fun main() {
    val testNumbers = listOf(197, 1193, 23, 101, 119)
    
    for (n in testNumbers) {
        val result = if (isCircularPrime(n)) "Circular Prime" else "Not Circular Prime"
        println("$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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