How can I check a previous index when iterating through a for loop python

Question

I have a list that is somewhat random.

list  = [1,2,4,2,25,3,2,4,2,1,1,32,3,3,2,2,23,4,2,2,2,34,234,33,2,4,3,2,3,4,2,1,3,4,3]

I want to iterate through it and do something like this:

for item in list:
    while item >=5:
        if item = item_5_indexes_ago
            print "its the same as it was 5 entries ago!"

Obviously, item_5_indexes_ago is not valid python. What should I substitute here? I want to check if list[5]==list[1], if list[6]==list[2], ..... for every item in the list.

I don't know Python, but it seems like you can just calculate "index-5" and use that for the comparison. Example: if item[6] = item[6-5] .... — BrettFromLA, Jun 10 '14 at 19:55
What is the end goal? That may help come up with a proper solution. — MIke, Jun 10 '14 at 19:56
This question is about "this and the next", but perhaps it is also helpful for "this and 4 items before" http://stackoverflow.com/questions/5434891/iterate-a-list-as-pair-current-next-in-python — Jasper, Jun 10 '14 at 19:58
@MikeN: the end goal is the computation of a moving average. this is a simplified version of the part I am having trouble with — user3727514, Jun 10 '14 at 20:31

score 6 · Answer 1 · answered Jun 10 '14 at 19:56

6

You can also use list comprehension and enumerate function to get the elements:

[l for i, l in enumerate(list[:-5]) if l == list[i+5]]

answered Jun 10 '14 at 19:56

GHugo

2,584
13
14

Although the list comprehension is nice, there is a lot of information loss here - it's impossible to say for which indices there were matches. I'm not sure if it's what OP needs in this case. – Henrik Jun 11 '14 at 10:09

score 5 · Answer 2 · answered Jun 10 '14 at 19:56

You can loop thorugh indices instead:

for i in range(len(some_list)):
    # do something with
    # list[i]

and to access to a previous element, you can use:

if i >= 4 and list[i] == list[i - 4]:
    print "its the same as it was 4 entries ago!"

Notes:

Since you want to check list[5]==list[1] and if list[6]==list[2] it seems like you want to check for an element 4 indices before, not 5.
Don't use list as the name of a variable because it will hide its built-in implementation.

Henrik · Accepted Answer · 2020-07-20T06:30:16.183

5

A pythonic solution is to use the builtin enumerate to keep track of the index as well as the item

for index, item in enumerate(my_list):
    if index >= 5:
        item_5_indexes_ago = my_list[index-5]

edited Jul 20 '20 at 06:30

answered Jun 10 '14 at 19:57

Henrik

4,254
15
28

score 1 · Answer 4 · edited Jun 10 '14 at 21:11

1

The easiest method to do this:

for index, item in enumerate(list):
    if index>=5:
        if item == list[index-5]:
            print "It's the same as it was 5 entries ago!"

edited Jun 10 '14 at 21:11

Pavling

3,933
1
22
25

answered Jun 10 '14 at 19:57

lPlant

143
6

score 0 · Answer 5 · answered Jun 10 '14 at 19:55

0

A more convenient loop in this case would be to loop by index, like this:

for i in range(len(list)):
    if i >= 5 and list[i] == list[i-5]:
        print "it's the same as it was 5 entries ago!"

answered Jun 10 '14 at 19:55

Dave Yarwood

2,866
1
17
29

score 0 · Answer 6 · answered Jun 10 '14 at 20:00

Iterate over list[5:] and list[:-5] in parallel with zip:

for item, item_5_indices_ago in zip(list[5:], list[:-5]):
    do_whatever_with(item, item_5_indices_ago)

Since zip stops when the shortest input sequence does, you can replace list[:-5] with just list. Also, don't call your list list, or when you try to call the list built-in, you'll get a weird TypeError.

score -1 · Answer 7 · answered Jun 10 '14 at 20:03

-1

Just step through the list in groups of 6:

from itertools import izip_longest

def grouper(iterable, n, fillvalue=None):
    "Collect data into fixed-length chunks or blocks"
    # grouper('ABCDEFG', 3, 'x') --> ABC DEF Gxx
    args = [iter(iterable)] * n
    return izip_longest(fillvalue=fillvalue, *args)

for fives in grouper(biglist, 6):
    if fives[:-1].count(fives[-1]) > 1:
       print("its the same!")

answered Jun 10 '14 at 20:03

Burhan Khalid

169,990
18
245
284

This does something completely different from what the questioner is trying to do. – user2357112 Jun 10 '14 at 20:06

How can I check a previous index when iterating through a for loop python

7 Answers7