Question

How to find the first 18 circular prime numbers (cyclically rotate left will also be prime) in Dart

Answer 1

import 'dart:io';

bool is_prime(int n) {
    if (n == 2) {
        return true;
    }
    if (n < 2 || n % 2 == 0) {
        return false;
    }
    for (var i = 3; i * i <= n; i += 2) {
        if (n % i == 0) {
            return false;
        }
    }
    return true;
}
    
int cyclically_rotate_left(int n) {
    var m = n;
    var p = 1;
    
    while (m >= 10) {
        p *= 10;
        m = m ~/ 10;
    }
    return (m + 10 * (n % p));
}
    
bool is_circular_prime(int n) {
    if (!is_prime(n)) {
        return false;
    }
    
    var rotated_n = cyclically_rotate_left(n);
    
    while (rotated_n != n) {
        if (rotated_n < n || !is_prime(rotated_n)) {
            return false;
        }
        rotated_n = cyclically_rotate_left(rotated_n);
    }
    return true;
}

void main() {
    
    // 3779 = prime
    // 7793 = prime
    // 7937 = prime
    // 9377 = prime

    int n = 2;
 
    for (int i = 0; i < 18; n++) {
        if (is_circular_prime(n)) {
            if (i > 0) {
                stdout.write(", ");
            }
            stdout.write(n);
            i++;
        }
    }
}




/*
run:

2, 3, 5, 7, 11, 13, 17, 37, 79, 113, 197, 199, 337, 1193, 3779, 11939, 19937, 193939

*/

 

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