Fibonacci generating algorithm

Question

I have written an algorithm that generates a Fibonacci series to the \$Nth\$ number. The code below works fine, but as a beginner I know I must be writing pretty ugly code.

For example I think it is probably not good practice to declare and initialize the variables where I have, but I am confused about what would be the best place.

All comments and advice on code style etc. are welcome.

package algorithms;

import java.util.Scanner;

/**
* This class will generate a list of fibonacci numbers
* @author Richard
*
*/
public class FibonacciGenerator {

/**
 * This method will generate a list of fibonacci numbers
 * @param args
 */
public static void main(String[] args) {
    //declare vars and set initial values
    int number = 0, previousNumber = 0, twoNumbersAgo = 1;
    Scanner input = new Scanner(System.in);

    //prompt user for nth number
    System.out.println("Enter the n'th number for your Fibonacci series: ");

    //set loop counter equal to given nth number
    int n = input.nextInt();

    //loop through as many times as n
    for (int counter=1;counter<=n;counter++){

        //setting each new number in list equal to the sum of the previous two
        number = (previousNumber) + (twoNumbersAgo);

        //print out number
        System.out.print(number+" ");

        //change vars to be next two numbers
        twoNumbersAgo = previousNumber;
        previousNumber = number;    
    }
    //close resources
    input.close();
}
}

coderodde · Accepted Answer · 2017-10-23 11:25:43Z

Advice 1

One very useful principle is that each method should solve only one problem and not more. In this context, I suggest that you roll a static method that simply computes a desired Fibonacci sequence and returns it as an array/list. Then, another method can call the previous and output the numbers.

Advice 2

Instead of closing a Scanner explicitly, you could use a try with resources. Any Autocloseable can do and will be closed implicitly.

try(Scanner scanner = new Scanner(System.in)) {
    // Use scanner here.
}
// Scanner automatically closed here.

The above will close Scanner when exiting the block, or in the case of exception.

Advice 3

Fibonacci sequence grows exponentially. For that reason you might want to use BigInteger.

Summa summarum

An alternative implementation may look like this:

import java.math.BigInteger;
import java.util.Arrays;
import java.util.Scanner;

public class FibonacciGenerator {

    public static BigInteger[] getFibonacciSequencePrefix(int n) {
        BigInteger[] result = new BigInteger[n];

        BigInteger a = BigInteger.ONE;
        BigInteger b = BigInteger.ONE;

        for (int i = 0; i < n; ++i) {
            result[i] = a;
            BigInteger tmp = a;
            a = b;
            b = b.add(tmp);
        }

        return result;
    }

    public static void main(String[] args) {
        try (Scanner scanner = new Scanner(System.in)) {
            int n = scanner.nextInt();
            System.out.println(Arrays.toString(getFibonacciSequencePrefix(n)));
        }
    }
}

Just note that the original program prints the first N Fibonacci numbers, it does not just print the N'th number. – Judging from the first sentence in the question, that seems to be intentional. — Martin R
– Martin R, Commented Oct 22, 2017 at 15:41
I didn’t get the try part. Can you elaborate on it a little? — piepi
– piepi, Commented Oct 22, 2017 at 22:32
@piepi Done! If there is more content I could add, let me know! — coderodde
– coderodde, Commented Oct 23, 2017 at 11:26

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