Method to find prime numbers in a given array of integers by user

Question

i am trying to write a method in JAVA that receives an array of integers from the user , and returns an ArrayList of prime numbers that were in that array. i tried to first verify if each element of array is a prime number or not. and then i added the ones that were prime, in an ArrayList and in the end return the ArrayList. But, when i run the code it returns all the numbers, even though the logic looks correct to me.

Here is my code :

package numeriPrimi;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Scanner;

public class NumeriPrimi {

    public static ArrayList arrayPrimi(int[] number) {
        ArrayList<Integer> listaPrimi = new ArrayList<>(); 
        boolean ris = false ;
        for(int i=0 ; i<number.length ; i++) {
            if (number[i] == 2) {
                ris = true;
            } 
            else { 
                if (number[i] != 2 && number[i] % 2 == 0 ) {
                    ris = false ; 
                } 
                else {
                    if(number[i]<=3) {
                        for(int j=3 ; j < number[i]; j++) {
                            if(number[i] % j == 0 ) {
                               ris = false ; }
                            }
                        }
                    }
                }   
                if(ris=true) {
                    listaPrimi.add(number[i]);
                }
            }
            return listaPrimi; 
        }

    public static void main(String[] args) {
        Scanner input= new Scanner(System.in);
        System.out.println("how many elements does the array have?");
        int l= Integer.parseInt(input.nextLine());
        int[] numbers = new int[l]; 
        
        System.out.println("enter the array of numbers?");
        
        for(int i=0 ; i< numbers.length ; i++) {
            numbers[i] = input.nextInt();
        }
        System.out.println("the array is : " + Arrays.toString(numbers));
        ArrayList<int[]> lista = NumeriPrimi.arrayPrimi(numbers);
        System.out.println("the Prime numbers in the array "+ Arrays.toString(numbers) + " are as follows : \n" + NumeriPrimi.arrayPrimi(numbers));
    }

}

Answer 1

ris = true makes the value of ris equal to true and the entire expression then resolves to that value. In other words:

if (ris = true) System.out.println("hello");

is identical to:

ris = true;
System.out.println("hello");

You should never write == true in java, for any reason. It's always pointless. Only true is true, so let's say you have a variable x which is a boolean , don't write if (x == true) , just write if (x) . If you have a calculation, same deal. Write if (x < 5) , not if ((x < 5) == true) .

Doing that = foo trick pretty much at any other time (ie with non-booleans) wouldn't suffer from this problem: if (x = 5) doesn't compile ( if (x == 5) does).

Fix this style error (of writing == true ) - not just here but in general in how you program. In passing this bug will go away.

Answer 2

It's a bit hard to follow, but you never set the "ris" value back to false, so you will end up adding everything after the first ris = true. Either start the for loop by setting it to false, or set it false after you added the current element to the list.