Prime/Composite程序的问题

Question

我是Java新手，目前正在尝试创建一个程序，以确定输入的数字是质数还是复合数。我已经猜到了大部分，但有一部分让我感到困惑。

我希望代码打印一条消息，如果输入一个0或一个负数，说明“该数字需要大于1”，或者如果输入一个1，则"one既不是素数也不是复合数字“。

这是我的密码：

public class Main {

public static void main(String[] args) {
    
    int num;
    boolean prime = true;
    String answer;
    Scanner keyboard = new Scanner(System.in);
    
    do {
        
    System.out.print("Please enter a number ==> ");
    num = keyboard.nextInt();
    
    if (num < 1)
        System.out.println("The number entered needs to be greater than one.");
    else if (num == 1)
        System.out.println("One is neither a prime nor composite number.");
    else {
        for (int ii = 2; ii <= num/2; ii++) {
            if (num % ii == 0) {
                prime = false;
                break;
            }
        }
    }
    if (prime == true)
        System.out.println("The integer " + num + " is prime.");
    else {
        System.out.println("The integer " + num + " is composite.");
    }
    
    System.out.print("Would you like to go again? (yes/no) ");
    answer = keyboard.next();
    System.out.print("\n");
    } while (answer.equals("yes"));
}

}

我的问题是，当我运行代码并输入0、1或负数时，代码会打印正确的消息，但是添加“整数"0，1，-#”是素数。“

我不希望它这样做，只希望它打印相应的消息。我遗漏了什么？

Answer 1

只需在else中移动print语句就可以修复逻辑。

此外，还有更多的改进可以排除除2以外的偶数，并寻找素数，直到num的平方根实现为止。

if (num < 1)
    System.out.println("The number entered needs to be greater than one.");
else if (num == 1)
    System.out.println("One is neither a prime nor composite number.");
else {
    prime = num % 2 == 1 || num == 2;
    for (int ii = 3; prime && ii * ii <= num; ii += 2) {
        if (num % ii == 0) {
            prime = false;
        }
    }
    if (prime)
        System.out.println("The integer " + num + " is prime.");
    else {
        System.out.println("The integer " + num + " is composite.");
    }
}

Answer 2

无论num的值如何，此代码都在执行：

if (prime == true)
    System.out.println("The integer " + num + " is prime.");
else {
    System.out.println("The integer " + num + " is composite.");
}

要解决这个问题，只需将其移到the语句中即可。

    if (num < 1)
        System.out.println("The number entered needs to be greater than one.");
    else if (num == 1)
        System.out.println("One is neither a prime nor composite number.");
    else {
        for (int ii = 2; ii <= num/2; ii++) {
            if (num % ii == 0) {
                prime = false;
                break;
            }
        }
        if (prime == true)
            System.out.println("The integer " + num + " is prime.");
        else {
            System.out.println("The integer " + num + " is    composite.");
        }
    }

Answer 3

它打印语句的原因是，如果值小于1，则if代码将始终运行(因为您已经将素布尔值初始化为true )。对此进行修正，以将最后一条if语句移到inside部件中。

public class Main {

    public static void main(String[] args) {

        int num;
        boolean prime = true;
        String answer;
        Scanner keyboard = new Scanner(System.in);

        do {

            System.out.print("Please enter a number ==> ");
            num = keyboard.nextInt();

            if (num < 1)
                System.out.println("The number entered needs to be greater than one.");
            else if (num == 1)
                System.out.println("One is neither a prime nor composite number.");
            else {
                for (int ii = 2; ii <= num / 2; ii++) {
                    if (num % ii == 0) {
                        prime = false;
                        break;
                    }
                }
                if (prime == true)
                    System.out.println("The integer " + num + " is prime.");
                else {
                    System.out.println("The integer " + num + " is composite.");
                }
            }

            System.out.print("Would you like to go again? (yes/no) ");
            answer = keyboard.next();
            System.out.print("\n");
        } while (answer.equals("yes"));
    }
}

然后，它不会像前面那样给出额外的print语句。

Please enter a number ==> 1
One is neither a prime nor composite number.
Would you like to go again? (yes/no)