Java-为什么我的素数程序不会显示最后一个数字？

Question

I'm supposed to write a program for a Java Intro class that lists the prime numbers up to the number a user inputs. However, with the code I have, if the user inputs a prime number, my program won't list back that number, even though it's supposed to do so when it's prime. Could someone give me a hint as to why it's not working?

    int userNum=kbd.nextInt();
    if (userNum<2){
        System.out.println("Not a valid number.");
        System.exit(1);
    }
    System.out.println("The prime numbers up to your integer are:");
    for (int num1 = 2; num1<userNum; num1++) {   
        boolean prime = true;


        for (int num2 = 2; num2 < num1; num2++) {
            if (num1 % num2 == 0) {
                prime = false;
                break; 

            }
        }

        // Prints the number if it's prime.
        if (prime) {
            System.out.print(num1 + " ");
        }
    }

For example, when I input "19," the program prints all prime numbers up to and including "17." Thanks!

Answer 1

You need num1 to take the value userNum as the last value to check.

Therefore, you need to replace num1 < userNum with num1 <= userNum .

Answer 2

Your loop has a condition of num1 < userNum . This means that it allows all numbers below userNum . What you appear to want is all numbers below and including userNum , so you want num1 <= userNum .

The start of your loop would thus be this:

    for (int num1 = 2; num1 <= userNum; num1++) {   

Answer 3

Change this

num1<userNum

to

num1<=userNum

At the moment you stop before the two numbers are equal in your for loop.

Answer 4

将for语句的结束条件更改为num1 <= userNum。