Debugging my recursive Fibonacci Java code and correcting the logic

Question

I'm unable to understand why the output is wrong.

I've tried to write a recursive code of printing Fibonacci numbers and got the expected output and a stream of unexpected values.

public class FibonacciSeries {

    static int limitNum = 10;  //the desired number of Fibonacci values

    public static void main(String[] args) {
        FibonacciSeries series = new FibonacciSeries();
        series.printRecursiveFibonacci(0,1,1);
    }

    public void printRecursiveFibonacci(int a, int b, int count)
    {
        while(count<=limitNum)
        {
          if(count==1||count==2)
          {
            System.out.println(count-1);
            count++;
            continue;
          }

        int k=a+b;
        a=b;
        b=k;
        System.out.println(b);
        count++;
        printRecursiveFibonacci(a, b, count);   
        }
    }

}

The expected output is 0 1 1 2 3 5 8 13 21 34

But I got - 0 1 1 2 3 5 8 13 21 34 34 21 34 34 13 21 34 34 21 34 34 8 13 21 34 34 21 34 34 13 21 34 34 21 34 34 5 8 13 21 34 34 21 34 34 13 21 34 34 21 34 34 8 13 21 34 34 21 34 34 13 21 34 34 21 34 34 3 5 8 13 21 34 34 21 34 34 13 21 34 34 21 34 34 8 13 21 34 34 21 34 34 13 21 34 34 21 34 34 5 8 13 21 34 34 21 34 34 13 21 34 34 21 34 34 8 13 21 34 34 21 34 34 13 21 34 34 21 34 34 2 3 5 8 13 21 34 34 21 34 34 13 21 34 34 21 34 34 8 13 21 34 34 21 34 34 13 21 ...

What's the purpose of your while(count<=limitNum) loop in your recursive function? That loop is occurring on every recursive call which doesn't make much sense. If you're using recursion, you don't need a loop. — lurker
– lurker, Commented Aug 24, 2019 at 20:14
"Please help me in understand the output of this recursive Fibonacci java code ..." --- Please read: Why is “Can someone help me?” not an actual question? --- "... and correct the logic" - How is that supposed to help? If we solve it for you, we take away every chance for you learning something. — Turing85
– Turing85, Commented Aug 24, 2019 at 20:19
@lurker, Thank you very much for pointing that out. I was practicing recursion and got lost. after replacing the while with if, it worked. Thank you again, I learnt my lesson. — AzharKhaji
– AzharKhaji, Commented Aug 24, 2019 at 23:13
@Turing85, I knew there is a mistake in my code and tried seeking help here to identity it as I was lost with it. Now, because of you, I learnt a better way to phrase my question and thanks a lot to your suggestion, really appreciate it. and I also spotted the mistake in my code. — AzharKhaji
– AzharKhaji, Commented Aug 24, 2019 at 23:26

cameron1024 · Accepted Answer · 2019-08-24 20:26:12Z

2

I'm not sure what exactly is going on in your code, a lot of weird things are going on:

while and recursion in the same method
if and continue instead of if/else
printing count - 1 sometimes and b other times.

I suspect it has something to do with your loop not terminating as you expect. In general, try to stick to either recursion or loops. Here is an example implementation:

public static void printFibLimit(int a, int b, int count) {
    if (count >= EXTERNALLY_DEFINED_LIMIT_VARIABLE) return;  // if true, we're done, no more work has to be done. 
// Since this is a tail-recursive function, return will terminate the function

    System.out.println(a);  // print our first value

    printFibLimit(b, a + b, count + 1);
    // a -> b
    // b -> a + b
    // count -> count + 1

}

answered Aug 24, 2019 at 20:26

cameron1024

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2 Comments

AzharKhaji Over a year ago

Thank you, I got it right now after replacing the Loop with If condition. I learnt a lot from your answer. Thank you for helping this newbie, really appreciate it.

cameron1024 Over a year ago

No problem, glad I could help :)

mentallurg · Accepted Answer · 2019-08-24 20:51:08Z

1

The problem is that you have mixed 3 (!) different tasks within a single method:

Calculation of the next Fibonacci number
Printing Fibonacci number
Limiting the number of Fibonacci numbers you are going to find

There is a good principle: separation of concerns. Each method should perform exactly one task.

Besides, if you refer recusrsion, probably you meant following:

fibonacci(n) = fibonacci(n-1) + fibonacci(n-2)

Split your method:

In one method keep only calculation the next Fibonacci number.
In another method organize a loop and printing.

Here is a possible solution:

public class Fibonacci {

    private static final int LIMIT = 10;

    private static int fibonacci(int n) {
        if (n == 1) {
            // The 1st Fibonacci number is 0
            return 0;
        }

        if (n == 2) {
            // The 2nd Fibonacci number is 1
            return 1;
        }

        return fibonacci(n - 1) + fibonacci(n - 2);
    }

    public static void main(String[] args) {
        for (int i = 1; i <= LIMIT; i++) {
            int fibo = fibonacci(i);
            System.out.println(fibo);
        }
    }
}

edited Aug 24, 2019 at 20:51

answered Aug 24, 2019 at 20:36

mentallurg

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3 Comments

AzharKhaji Over a year ago

Thank you for suggesting the design pattern. I was trying to practice recursion and got lost with the code.

mentallurg Over a year ago

See above with recursion.

AzharKhaji Over a year ago

@mentalurg, Thank you, I got it right after replacing the Loop with If condition. I learnt a lot from your answer. Thank you for helping this newbie, really appreciate it.

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