# Resolved: Fibonacci Series Step Through

I am having trouble grasping how python steps through the Fibonacci series. The code in the lectures defines the function as:

def fib(n):

a = 0

b = 1

for i in range(n):

a,b = b,b+a

return a

However, if I try to step through the fist 3 values in the series, I derive an incorrect value, which cannot be the case.

a = 1

b = 1+0 = 1

a = 2

b = 1 + 1 = 2

a = 4

b = 2 + 2 = 4

What is python actually doing to obtain the correct value?

Thank you!

Hey Patrick,

Thank you for your question!

Let's perform the first couple of iterations. We start with a = 0 and b = 1. Then, we enter the `for`

-loop.

For `i = 0`

:

a, b = 1, 1 + 0

=> a, b = 1, 1

For `i = 1`

:

a, b = 1, 1 + 1 = 2

=> a, b = 1, 2

For `i = 2`

:

a, b = 2, 2 + 1 = 3

=> a, b = 2, 3

and so on..

Hope this helps!

Kind regards,

365 Hristina