列出质数

Question

我是一个业余的java学生，我的老师给了我这个作业，就是写一个列出前100个质数的代码。这是我到目前为止尝试过的。

for(int i =1; true; i++) {
    int k = 0;
    for(int j = 1; j <= i; j++) {
        if(i % j == 0){
            k ++;
        }

        if(k == 2) {
            System.out.println(i);
        }
    }
}

我遇到的问题是，即使我使用我熟悉的可能的机制，我也不能不停止控制台以在数到100后停止。例如：

for( int i =1; true; i++) {
    int k = 0;
    for(int j = 1; j <= i; j++) {
        if(i % j == 0) {
            k ++;
        }

        int m = 0;
        if(k == 2) {
            System.out.println(i);
            m++;
            if(m==100) {
                break;
            }
        }
    }
}

在计算了100个素数后，我能得到帮助来终止循环吗？

Answer 1

你需要跟踪你打印了多少质数，一旦达到100就停止打印。我已经在代码中使用注释解释了下面的代码。

public class Main
{
    public static void main(String[] args) {
        int k = 0;  // to keep track of how many primes you have prited
        int i = 2;  // number to check for prime, increases every loop
        while (k < 100){  // while you have printed less than 100 primes
            boolean isPrime = true;  // next few lines are checking i for prime and store it in this variable
            for(int divisor = 2; divisor <= i / 2; divisor++) {  // you should go with divisor <= Math.sqrt(i) in condition, I couldn't be bothered in import stuff.
                if (i % divisor == 0) {
                    isPrime = false;
                    break; // i is not a prime, no reason to continue checking
                }
            }
            if (isPrime){
                System.out.println(i);  // if i is prime, print it
                k ++; // increase k by when a print number is found
            }
            i ++; // increase i to check next number
        }
    }
}

Answer 2

您的代码会运行到无限大，因为您将break放在了错误的位置。您的break退出内部循环，即for(int j = 1; j <= i; j++)，尽管您应该以外部循环for( int i =1; true; i++)为目标，因为这是决定将打印多少数字的原因，内部循环只是检查数字是否为质数。

我已经在这段代码中更正了你的方法(注释中有解释)

int numbersPrinted = 0; // To keep track of how many numbers have been printed
for (int i = 2; true; i++) {
    boolean isPrime = true; // Assuming the number is prime
    for (int j = 2; j < i; j++) {
        // Checking if any number from 2 to i-1 completely divides i
        if (i % j == 0) {
            // If a number completely divides (gives 0 remainder)
            // The number is not prime
            isPrime = false;
            // You can use break here. I don't think it matters anyways.
        }
    }
    if (isPrime) {
        // Printing i only if it is prime
        System.out.print(i + ", ");
        numbersPrinted++; // Updating the numbersPrinted
    }
    // Checking if the numbers printed is grater than or equal to 100
    if (numbersPrinted >= 100)
        // This will break the outermost loop
        // This is where you messed up
        break;
}

输出(恰好100个质数)

2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317, 331, 337, 347, 349, 353, 359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 421, 431, 433, 439, 443, 449, 457, 461, 463, 467, 479, 487, 491, 499, 503, 509, 521, 523, 541, 

Answer 3

要回答你的问题如何在100停止：

for( int i =1; i<=100; i++){

for语句的第二个参数是继续的条件。如果此条件的计算结果为false，则for循环将停止迭代。