Getting index Error for function

Question

def doubles(lst):
    for i in range(len(lst)):
        if lst[i]*2==lst[i+1]:
            print(lst[i+1],end=' ')

What I get:

8 -24 6 12 24 Traceback (most recent call last):
  File "<pyshell#68>", line 1, in <module>
    doubles( [4,8,-12,-24,48,3,6,12,24,2])
    line 32, in doubles
    if lst[i+1]:
IndexError: list index out of range

The output is what I want 8 -24 6 12 24 , but I'm not sure how to get rid of the index error for this function. Any ideas?

Answer 1

Loop until the one but last element :

def doubles(lst):
    for i in range(len(lst) - 1):
        if lst[i] * 2 == lst[i + 1]:
            print(lst[i + 1], end=' ')

because otherwise you try to access lst[i + 1] which is guaranteed to be 1 index further than there are elements in the list.

You can also loop directly over lst , together with enumerate() to produce the indices:

def doubles(lst):
    for i, elem in enumerate(lst[:-1]):
        if elem * 2 == lst[i + 1]:
            print(lst[i + 1], end=' ')

or you can use zip() to pair up elements with the next element:

def doubles(lst):
    for i, j in zip(lst, lst[1:]):
        if i * 2 == j:
            print(j, end=' ')

which you could put into a generator expression:

def doubles(lst):
    print(*(j for i, j in zip(lst, lst[1:]) if i * 2 == j), sep=' ')

Demo of all of these:

>>> [4,8,-12,-24,48,3,6,12,24,2]
[4, 8, -12, -24, 48, 3, 6, 12, 24, 2]
>>> lst = [4,8,-12,-24,48,3,6,12,24,2]
>>> for i in range(len(lst) - 1):
...     if lst[i] * 2 == lst[i + 1]:
...         print(lst[i + 1], end=' ')
... 
8 -24 6 12 24 >>> 
>>> for i, elem in enumerate(lst[:-1]):
...     if elem * 2 == lst[i + 1]:
...         print(lst[i + 1], end=' ')
... 
8 -24 6 12 24 >>> 
>>> for i, j in zip(lst, lst[1:]):
...     if i * 2 == j:
...         print(j, end=' ')
... 
8 -24 6 12 24 >>> 
>>> print(*(j for i, j in zip(lst, lst[1:]) if i * 2 == j), sep=' ')
8 -24 6 12 24

Answer 2

You can use the pairwise recipe from the itertools docs :

from itertools import tee, izip

def pairwise(iterable):
    "s -> (s0,s1), (s1,s2), (s2, s3), ..."
    a, b = tee(iterable)
    next(b, None)
    return izip(a, b)

def doubles(lst):    
    for x,y in pairwise(lst):
        if x * 2 == y:
            print(y, end=' ')