Java 中的素数（return 下一个素数为 false）

Question

public class PrimeNumbers {
    public static void main(String[] args) {
        int num = 6;
        boolean isPrime = true;

        for (int i = 2; i <= num - i; i++) {
            if (num % i == 0) {
                isPrime = false;
                break;
            }
        }
        int nextPrime = num++;
        {
            while (isPrime = false) {


            }
            if (isPrime)
                System.out.println("Is a prime number");
            else
                System.out.println("Is not a prime number" + "the next prime number is" + nextPrime(num));
        }
    }
}

我曾尝试在 Java 中编码以找到下一个质数，而之前返回的是假数，但我真的很困惑如何继续“while”代码。这是我到目前为止尝试过的方法：

Answer 1

为了找到质数，您基本上需要一个嵌套循环。所以我建议在 while 循环中使用 for 循环。或者你可以在外面有一个 do-while 循环，一直循环直到你找到一个素数。

您的代码应如下所示：

public class PrimeNumbers {

    public static boolean check_prime(int num){
        
        for (int i = 2; i <= num-i ; i++) { 
            if (num % i == 0) {             
                return false;
            }
        } 
        return true;
    }

    public static void main(String[] args){
        int num = 6;

        boolean isPrime= true;

        if(!check_prime(6)){                        
            int nextPrime = num;
            do{
                nextPrime++;
                if (check_prime(nextPrime)) {             
                    isPrime = true;
                    break;
                 }else{
                    isPrime = false;
                 }
            }while(!isPrime);
        }
    
        if (isPrime)
            System.out.println("Is a prime number");
        else
            System.out.println("Is not a prime number" +"the next prime number is" + nextPrime);
    }}}

Answer 2

您可以创建两个函数：一个（例如，isPrime）检查数字是否为素数，第二个（例如，nextPrime）查找下一个素数。第二个函数中的逻辑将非常简单：继续将数字递增 1 直到 isPrime returns true.

请注意，您只需检查数字的平方根即可确定它是否为质数。检查 https://en.wikipedia.org/wiki/Primality_test 以了解更多信息。

演示：

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter an integer: ");
        int n = scanner.nextInt();
        if (isPrime(n)) {
            System.out.println("It is a prime number");
        } else {
            System.out.println("It is not a prime number. The next prime number is " + nextPrime(n));
        }
    }

    static boolean isPrime(int n) {
        if (n == 1 || n == 0) {
            return false;
        }
        for (int i = 2; i <= Math.sqrt(n); i++) {
            if (n % i == 0) {
                return false;
            }
        }
        return true;
    }

    static int nextPrime(int n) {
        while (!isPrime(n)) {
            n++;
        }
        return n;
    }
}

样本运行:

Enter an integer: 10
It is not a prime number. The next prime number is 11

另一个样本运行:

Enter an integer: 11
It is a prime nmber

Answer 3

这段代码怎么样？

public static void main(String[] args) {
    int number= 14; // will print the second SOUT statement with 17
    if (isPrime(number)) {
        System.out.println(number+ " is a prime number");
    } else {
        System.out.println(number+ " is not a prime number, " + 
                "the next prime number is " + nextPrime(number));
    }
}

/**
 *  Check if the number is a prime number
 *  For 0 or 1 you can return false, for the rest if is divisible by each number
 *  You might want to throw an exception for the negative input 
 */
static boolean isPrime(int number) {
    if (number==0 || number==1) {
        return false;
    }
    for (int i=2; i<=number/2; i++) {
        if (number%i==0) {
            return false;
        }
    }
    return true;
}

/**
 *  Iterate the number by one until you find a prime number using the while cycle
 */
static int nextPrime(int number) {
    int prime;
    while (true) {
        boolean isPrime = isPrime(++number);
        if (isPrime) {
            prime = number;
            break;
        }
    }
    return prime;
}

Java 中的素数（return 下一个素数为 false）

Prime Numbers in Java (return next prime when false)

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