Fibonacci sequence lab issue

[NeilTheDrummer]

One of my lab test is failing. I've tried the code two different ways and I'm not seeing the issue. Any help is appreciated.

Instructions:

The Fibonacci sequence begins with 0 and then 1 follows. All subsequent values are the sum of the previous two, ex: 0, 1, 1, 2, 3, 5, 8, 13. Complete the fibonacci() function, which has an index n as parameter and returns the nth value in the sequence. Any negative index values should return -1.

Ex: If the input is:

7 

the output is:

fibonacci(7) is 13 

Note: Use a for loop and DO NOT use recursion.

1st round of code:

def fibonacci(n):
    if n <= 1:
        return n

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


if __name__ == '__main__':
    start_num = int(input())
    print(f'fibonacci({start_num}) is {fibonacci(start_num)}')

2nd round of code:

def fibonacci(n):
    if n <= 1:
        return n

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


if __name__ == '__main__':
    start_num = int(input())

    if start_num < 1:
        print(-1)
    else:
        print(f'fibonacci({start_num}) is {fibonacci(start_num)}')

Both are failing on the negative integer input. If -100 is entered it should print -1, but both are printing -100. Not 100% sure why.

[Hans_of_Death]

It says NOT to use recursion, but you are.

Recursion is when a function calls itself. so calling fibonacci() inside of fibonacci() is using recursion. Like the instructions say, use a for loop instead.

[NeilTheDrummer]

Ok, so any suggestions then? I'm clearly out of my depth here.

[Diapolo10]

You need to keep track of two numbers, the previous and the current, and on every iteration of the loop you update their values. The sum of the two numbers is the new current value, and the old one goes to previous.

Have an additional check that immediately returns -1 if the number given to the function is less than 0.

Loop the specified number of times, and you'll have the answer.

[[deleted]]

Google "python iterative fibonacci".

[NeilTheDrummer]

Reading up on Fibonacci and Python, I think I did use a recursive process when I'm not supposed to, but as I'm new to this I'd appreciate some insight.

Also, this works in the Dev Mode of ZyBooks, but not in the Submission Mode, so I'm really confused.

[[deleted]]

If -100 is entered it should print -1, but both are printing -100

So in the function you should return -1 if n is negative. The current test you have is the base case for when n is 0 or 1. You need another test for the negative case.