怎么先求素数的总和，再放下面的素数

Question

I'm currently using this code, and I'm trying to output the total sum of how many prime numbers first and then the prime numbers list

Here is my code:

import java.util.*;

public class PrimeTime {

    public static void main(String[] args) {
        
        Scanner sc = new Scanner(System.in);
        
        int r, s = 0;
        
        System.out.print("Input number: ");
        r = sc.nextInt();
        
        System.out.println();
        
        for (int i = 2; i < r; i++) {
            
            int f = 0;
            
            for (int j = 2; j < i; j++) {
                
                if (i%j == 0)
                    f = 1;
            }
            
            if (f == 0) {
                
                System.out.print(i+ " ");
                s = s +1;
            }
            
        }
        System.out.println("N =" +s);
    }
    
}

This is the result I get:

Input number: 10

2 3 5 7 N = 4

But what I'm looking for is this result:

Input number: 10

N = 4
2 3 5 7

I don't know how, I tried putting the SOP in different places and I can't figure out how.

Answer 1

If you want to display N = 4 before 2 3 5 7 , you can append the numbers (ie 2 3 5 7 ) to a StringBuilder and print the same after printing N = 4 eg

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int r, s = 0;
        StringBuilder numbers = new StringBuilder();

        System.out.print("Input number: ");
        r = sc.nextInt();

        System.out.println();

        for (int i = 2; i < r; i++) {
            int f = 0;

            for (int j = 2; j <= Math.sqrt(i); j++) {

                if (i % j == 0)
                    f = 1;
            }

            if (f == 0) {
                numbers.append(i).append(' ');// Append to StringBuilder instead of printing
                s = s + 1;
            }
        }
        System.out.println("N =" + s);
        System.out.println(numbers);
    }
}

A sample run:

Input number: 10

N =4
2 3 5 7 

Note: for checking the primality , checking up to Math.sqrt(i) is sufficient ie you should replace j < i with j <= Math.sqrt(i) .

Answer 2

You can store the numbers in a list and then print them after the loop is over
Don't worry if you have not studied lists yet cause you will have to soon anyways.

here's the code:

 public static void main(String[] args) {

    Scanner sc = new Scanner(System.in);

    int r, s = 0;
    LinkedList<Integer> list = new LinkedList<>();

    System.out.print("Input number: ");
    r = sc.nextInt();

    System.out.println();

    for (int i = 2; i < r; i++) {

        int f = 0;

        for (int j = 2; j < i; j++) {

            if (i%j == 0)
                f = 1;
        }

        if (f == 0) {

            //System.out.print(i+ " ");
            list.add(i);
            s = s +1;
        }

    }
    System.out.println("N = " +s);

    for(int i = 0; i<list.size(); i++ ) {
        System.out.print(list.get(i) + " ");
    }
}