Fibonacci with Python: IndexError: list index out of range

Question

I was trying to write a program to compute the Fibonacci number using Python:

n = int(input())

def fib(n):
    a = []
    if (n <=1):
        return n
    else:
        for i in range(2, n):
            a.append(a[-1] + a[-2])
        return a[i]
print (fib(n))

However, I can't print out the expected result. For example, after I typed in the number 8, the following message popped up:

Traceback (most recent call last):
  File "fibonacci.py", line 11, in <module>
    print (fib(n))
  File "fibonacci.py", line 9, in fib
    a.append(a[-1] + a[-2])
IndexError: list index out of range

What went wrong in the process? Thanks in advance.

IndexError: list index out of range your'e starting out with an empty array and trying to access index's on it. — Tom Slabbaert
– Tom Slabbaert, Commented May 15, 2019 at 8:22

Toni Sredanović · Accepted Answer · 2019-05-15 08:23:17Z

4

You need to populate your list with first 2 numbers of Fibonacci sequence:

a = [0, 1]

answered May 15, 2019 at 8:23

Toni Sredanović

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Comments

milanbalazs · Accepted Answer · 2019-05-15 08:41:29Z

2

-2 and -1 indexes are not available if you don't set them in "a" list before using.

n = int(input())

def fib(n):
    a = [0, 1]
    if n <=1:
        return n
    else:
        for i in range(2, n):
            a.append(a[-1] + a[-2])
        return a[i]
print (fib(n))

answered May 15, 2019 at 8:41

milanbalazs

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Comments

Chetan Vashisth · Accepted Answer · 2019-05-15 09:03:02Z

1

Make these changes in your code

n = int(input())

def fib(n):
    a = [0,1]  # add these values to your array to intialize
    if (n <=1):
        return n
    else:
        for i in range(2, n):
            a.append(a[-1] + a[-2])
    return a  # change this also so that you can get all the values returned at the same time

print (fib(n))

answered May 15, 2019 at 9:03

Chetan Vashisth

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Comments

Abhi · Accepted Answer · 2019-05-22 08:43:43Z

1

Call this function to compute any Fibonacci series number

def Fibonacci(n): 
    if n<0: 
        print("Incorrect input") 
# First Fibonacci number is 0 
    elif n==0: 
        return 0
# Second Fibonacci number is 1 
    elif n==1: 
        return 1
    else: 
        return Fibonacci(n-1)+Fibonacci(n-2)

Or you can use this formula the formula for nth term in the fibonacci series.

Fn = {[(√5 + 1)/2] ^ n} / √5

edited May 22, 2019 at 8:43

answered May 15, 2019 at 12:08

Abhi

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