Program that takes an int and returns an array of all the prime numbers smaller than that int

[mark donner]

I am not sure what is wrong with my code

Basically, for whatever int I put in, I get the biggest prime number (that is smaller than that int), print the amount of times that there are prime numbers in the array.

I very much appreciate anyone who can help me out.

import java.util.Scanner;

public class aa
{

public static void main (String[]args)
{
System.out.println("enter a number");
Scanner keyboard = new Scanner (System.in);
int ex = keyboard.nextInt();
for (int i=0; i<firstPrimeNumbers(ex).length; i++)
	{System.out.println(firstPrimeNumbers(ex)[i]);}

}

public static int[] firstPrimeNumbers(int n)
{
	int count = 0;
	for (int s=0; s<=n; s++)
		{
		if (isPrime(s))
			{count ++;}
		}

	int[] number = new int[count];
	for (int i=0; i<count; i++)
		{
		for (int s=2; s<=n; s++)
			{
			if (isPrime(s))
				{number[i] = s;}
			}
		}
	return number;
}
	



public static boolean isPrime (int k)
{
	if (k<=1)
	{return false;}

	for (int i = 2; i<k; i++)
	{if (k%i == 0) {return false;}}
	
	return true;
	

}
	
	
}

[Stefaan Dutry]

The problem you have is that, when you put the primes into the array, that you, for each of the array indexes keep walking through every value and keep overwriting the previously found one
=> ending up with an array where each index contains the maximum prime below the value entered.




I strongly suggest you have a look at ArrayList.

List<Integer> primes = new ArrayList<Integer>();

This way you can add every Integer (or int via autoboxing) you come across to this arrayList instead of having to go through them for the size first and then again for adding them.

If you change your method to return a List of integers, you can also directly loop through all values of the List with an enhanced for loop with autounboxing.

private static List<Integer> firstPrimeNumbers(int n) {

for (int prime : firstPrimeNumbers(ex)) { // loop through values in the list and assign current value to the variable named "prime"
//code for doing stuff in loop goes here
}

Regards,

Stefaan

[mark donner]

Thank you. Very helpful