How to pass method output into an array element?

Question

The basis of my problem is here: https://github.com/experiencethebridge1/primeGap

Bottom line, I want to create an array in which the output of a method will populate the elements of the new array.

This is not homework.

package primenumbermethod;

import java.util.Scanner;

public class PrimeNumberMethod {

    public static void main(String[] args) {
        System.out.print("How many prime numbers do you want to work with? ");
        Scanner input = new Scanner(System.in);
        int arraySize = input.nextInt();

        // Invoke printPrimeNumbers method
        System.out.println("If I can ever get it to work, the number of the "
                + "elements in the array I want to build will be " + arraySize +".");

        System.out.println();
        printPrimeNumbers(arraySize);

        // How can I read parts of a method into elements of an array?
        int[] myList = new int[arraySize];


    }

    public static int printPrimeNumbers(int numberOfPrimes) {
        final int NUMBER_OF_PRIMES_PER_LINE = 10;  // Display 10 per line
        Scanner input = new Scanner(System.in);
        System.out.print("What number do you want to start from?  ");
        int number = input.nextInt();
        int count = 0; // Count the number of prime numbers
        // Repeatedly find prime numbers
        while (count < numberOfPrimes) {
            // Print the prime number and increase the count
            if (isPrime(number)) {
                count++; // Increase the count
                if (count % NUMBER_OF_PRIMES_PER_LINE == 0) {
                    // Print the number and advance to the new line
                    System.out.printf("%-15d\n", number);
                } else {
                    System.out.printf("%-15d", number);
                }
            }
            number++;
        }
        return 0;
    }
    // Method for checking if number is prime
    public static boolean isPrime(int number) {
        for (int divisor = 2; divisor <= number / 2; divisor++) {
            if (number % divisor == 0) {// If true, number is not prime 
                return false;  // Number is not a prime    
            }
        }
        return true; // Number is prime
    }
}

Tried using global variables, abstraction does not apply (but could).

The main method initiates the program, then traces to method printPrimeNumbers, then into method boolean isPrime. I want to return the output of that method into a new array...

The array size will be defined by the user input <"How many prime numbers do you want to work with? ">, and then <"What number do you want to start with?>

Problem, I can't seem to pass the output of a method into the elements of an array.

Thoughts?

You can pass a reference to the the array to the method. The method can write into the array whose reference was passed. For example, you could declare a method like this - myMethod( int[] array ) and call it like this: myMethod( myList ). — Andy Thomas
– Andy Thomas, Commented Feb 18, 2019 at 15:37
Thank you, that is very helpful. I've read that before too, don't know why I forgot. I appreciate the perspective. — Sean O'Brien
– Sean O'Brien, Commented Feb 18, 2019 at 15:48
I advice against changing the array that is passed in, it is bad practice to mutate the parameters. It is very non-obvious behaviour for the caller. — luk2302
– luk2302, Commented Feb 19, 2019 at 8:33

luk2302 · Accepted Answer · 2019-02-18 15:45:24Z

1

I would suggest you should restructure your code in the following way:

public static void main(String[] args) {
    int numberOfPrimes = readIntFromCommandLine...;
    int numberToStartWith = readIntFromCommandLine...;

    int[] primeNumbers = getPrimeNumbers(numberOfPrimes, numberToStartWith);

    // maybe extract this to another method as well
    for (int prime : primeNumbers) {
        // do whatever you want with prime, e.g. print it - or sum it, or multiply or whatever
    }
}

public static int[] getPrimeNumbers(int amount, int from) {
    int[] primes = new int[amount];
    int count = 0;

    /* now put your current prime logic here and whenever you 
       find a prime set primes[count] = newlyFoundPrime;  */
}

public static boolean isPrime(int number) { /* stays the same */ }

It is generally a good idea to only ask for user input at a well defined point in your code, not all over the place. Therefore I placed the two inputs at the front. Another generally good idea is to make every method (maybe except for the main method) only do one thing. Your isPrime is a good example of that. Moving the printing logic out of getPrimeNumbers simplifies that method and lets you handle the printing at another, dedicated place.

answered Feb 18, 2019 at 15:45

luk2302

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1 Comment

Sean O'Brien Over a year ago

I'll give that a shot. Thanks.

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