Python list: How to read the previous element when using for loop?

Question

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

Hi, everyone.

I've got a list contains a lot of elements, I iterate 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 previous element of i, how to achieve that?

I do not have to use a for loop, so feel free to change anything here. Thank you!

Answer 1

Use a loop counter as an index. (Be sure to start at one so you stay in range.)

for i in range(1,len(li)):
  print li[i], li[i-1]

Answer 2

Just keep track of index and get the previous item by index

li = range(10)

for i, item in enumerate(li):
    if i > 0:
        print item, li[i-1]

print "or..."   
for i in range(1,len(li)):
    print li[i], li[i-1]

output:

1 0
2 1
3 2
4 3
5 4
6 5
7 6
8 7
9 8
or...
1 0
2 1
3 2
4 3
5 4
6 5
7 6
8 7
9 8

another alternative is to remember the last item e.g.

last_item = None
for item in li:
    print last_item, item
    last_item = item

Answer 3

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

counter = 0
for l in li:
    print l
    print li[counter-1] #Will return last element in list during first iteration as martineau points out.
    counter+=1

Answer 4

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

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

Answer 5

you can use zip:

for a, b in zip(li, li[1:]):
  print a, b

or, if you need to do something a little fancier, because creating a list or taking a slice of li would be inefficient (for some reason...),

import itertools

readahead = iter(li)
next(readahead)

for a, b in itertools.izip(li, readahead)
    print a, b

Answer 6

j = None
for i in li:
    print j
    j = i

Answer 7

An option using the itertools recipe from here:

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

for i, j in pairwise(li):
    print i, j