How to get the previous element when using a for loop? [duplicate]

Question

Possible Duplicates:
Python - Previous and next values inside a loop
python for loop, how to find next value(object)?

I've got a list that contains a lot of elements, I iterate over the list using a for loop. For example:

li = [2, 31, 321, 41, 3423, 4, 234, 24, 32, 42, 3, 24, 31, 123]

for i in li:
    print(i)

But I want to get the element before i. How can I achieve that?

Answer 1

You can use zip:

for previous, current in zip(li, li[1:]):
    print(previous, current)

or, if you need to do something a little fancier, because creating a list or taking a slice of li would be inefficient, use the pairwise recipe from itertools

import itertools

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

for a, b in pairwise(li):
    print(a, b)

Answer 2

Normally, you use enumerate() or range() to go through the elements. Here's an alternative using zip()

>>> li = [2, 31, 321, 41, 3423, 4, 234, 24, 32, 42, 3, 24, 31, 123]
>>> list(zip(li[1:], li))
[(31, 2), (321, 31), (41, 321), (3423, 41), (4, 3423), (234, 4), (24, 234), (32, 24), (42, 32), (3, 42), (24, 3), (31, 24), (123, 31)]

the 2nd element of each tuple is the previous element of the list.