list - Getting an index error in a python fibonacci program -

i started "algorithmic toolbox" on coursera, , writing version of fibonacci program in python 2.7

def fibonacci(n):     f = []     f[0] = 0     f[1] = 1      in range(0,n):         f[i] = f[i-1] + f[i-2]      return f[n]  fibonacci(3) 

but, keep getting error:

traceback (most recent call last):   file "fibonacci.py", line 11, in <module>     fibonacci(3)   file "fibonacci.py", line 3, in fibonacci     f[0] = 0   indexerror: list assignment index out of range 

you can't create new elements in python list assigning non-existing indices. list empty, indices 0 , 1 don't exist.

use list.append() instead:

def fibonacci(n):     f = [0, 1]  # list 2 initial elements      in range(2, n + 1):         f.append(f[i-1] + f[i-2])  # add new value list      return f[n] 

note loop starts @ 2, not 0, because have values 0 , 1. stop argument range() not included if want find nth fibonacci number, need run range n + 1.

now code works:

>>> def fibonacci(n): ...     f = [0, 1] ...     in range(2, n + 1): ...         f.append(f[i-1] + f[i-2]) ...     return f[n] ... >>> fibonacci(10) 55 

note don't need store values in list; need track last 2. use 2 variables , swap these around each iteration; 1 stores

def fibonacci(n):     prev, curr = 0, 1  # 2 variables, 1 set 0, other 1     _ in range(n - 1):  # run n - 2 times         prev, curr = curr, prev + curr     return curr 

note doesn't boundary tests on n, n = 0 result going incorrect (1 returned rather 0). remedied if n < 1: return 0 line @ top.