Start index for iterating Python list

Question

What is the best way to set a start index when iterating a list in Python. For example, I have a list of the days of the week - Sunday, Monday, Tuesday, ... Saturday - but I want to iterate through the list starting at Monday. What is the best practice for doing this?

Do you just want to loop until Saturday, or do you want it to wrap around and print Sunday last? — juanchopanza, May 27 '11 at 07:09
I just wanted to loop until Saturday. I hadn't realized until now that you can use slicing in Python lists. — Vincent Catalano, May 27 '11 at 15:49
is there a solution that deals with generators/iterables too and not only lists? Or really large lists? — Charlie Parker, Aug 09 '17 at 17:03

Björn Pollex · Accepted Answer · 2018-02-23T07:30:04.207

227

You can use slicing:

for item in some_list[2:]:
    # do stuff

This will start at the third element and iterate to the end.

edited Feb 23 '18 at 07:30

answered May 27 '11 at 06:31

Björn Pollex

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Isn't this inefficient for large lists? I believe this slice operation has to copy the list elements that are being referenced into a new list. – UndeadKernel May 13 '14 at 13:41
6

Yes this is inefficient for large lists. See gnibblers answer below for a solution that doesn't copy. – Björn Pollex May 13 '14 at 14:20
how do u do this though if u are looping using a generators/iterables? – Charlie Parker Aug 09 '17 at 17:02
2

The you should use `islice`, as suggested in John La Rooy's answer. – Björn Pollex Aug 09 '17 at 17:39
@CharlieParker For more functions supporting generators/iterables, take a look at [toolz](http://toolz.readthedocs.io), specifically [`drop`](http://toolz.readthedocs.io/en/latest/api.html#toolz.itertoolz.drop). – Björn Pollex Aug 09 '17 at 17:40
TypeError: 'type' object is not subscriptable – Sanket Parchande Feb 22 '18 at 20:12
@SanketParchande I've renamed the variable to avoid the name-clash with the builtin `list`. Don't name your list `list`. – Björn Pollex Feb 23 '18 at 07:31

John La Rooy · Answer 2 · 2017-08-09T22:23:54.417

58

islice has the advantage that it doesn't need to copy part of the list

from itertools import islice
for day in islice(days, 1, None):
    ...

edited Aug 09 '17 at 22:23

answered May 27 '11 at 06:49

John La Rooy

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score 22 · Answer 3 · answered Jul 14 '20 at 20:05

22

Why are people using list slicing (slow because it copies to a new list), importing a library function, or trying to rotate an array for this?

Use a normal for-loop with range(start, stop, step) (where start and step are optional arguments).

For example, looping through an array starting at index 1:

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

answered Jul 14 '20 at 20:05

Charlie Su

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This should be the real answer – ryanulit Oct 07 '21 at 19:52

score 13 · Answer 4 · answered May 27 '11 at 06:35

You can always loop using an index counter the conventional C style looping:

for i in range(len(l)-1):
    print l[i+1]

It is always better to follow the "loop on every element" style because that's the normal thing to do, but if it gets in your way, just remember the conventional style is also supported, always.

score 11 · Answer 5 · edited Jan 13 '14 at 21:09

11

stdlib will hook you up son!

deque.rotate():

#!/usr/local/bin/python2.7

from collections import deque

a = deque('Monday Tuesday Wednesday Thursday Friday Saturday Sunday'.split(' '))
a.rotate(3)
deque(['Friday', 'Saturday', 'Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday'])

edited Jan 13 '14 at 21:09

Björn Pollex

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answered May 27 '11 at 09:16

synthesizerpatel

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juanchopanza · Answer 6 · 2011-05-27T07:27:26.080

5

If all you want is to print from Monday onwards, you can use list's index method to find the position where "Monday" is in the list, and iterate from there as explained in other posts. Using list.index saves you hard-coding the index for "Monday", which is a potential source of error:

days = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday']
for d in days[days.index('Monday'):] :
   print d

edited May 27 '11 at 07:27

answered May 27 '11 at 07:17

juanchopanza

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really nice solution! – aderchox Jan 28 '20 at 19:21

score 5 · Answer 7 · answered May 27 '11 at 08:52

5

Here's a rotation generator which doesn't need to make a warped copy of the input sequence ... may be useful if the input sequence is much larger than 7 items.

>>> def rotated_sequence(seq, start_index):
...     n = len(seq)
...     for i in xrange(n):
...         yield seq[(i + start_index) % n]
...
>>> s = 'su m tu w th f sa'.split()
>>> list(rotated_sequence(s, s.index('m')))
['m', 'tu', 'w', 'th', 'f', 'sa', 'su']
>>>

answered May 27 '11 at 08:52

John Machin

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Yes - and would be easy to extend to generate an infinite recurring sequence. – slothrop May 27 '11 at 09:30
can't help thanking @JohnMachin: great work for someone dead these 264 years – jjon Sep 22 '15 at 00:53

slothrop · Answer 8 · 2011-05-27T08:25:06.687

If you want to "wrap around" and effectively rotate the list to start with Monday (rather than just chop off the items prior to Monday):

dayNames = [ 'Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 
            'Friday', 'Saturday',  ]

startDayName = 'Monday'

startIndex = dayNames.index( startDayName )
print ( startIndex )

rotatedDayNames = dayNames[ startIndex: ] + dayNames [ :startIndex ]

for x in rotatedDayNames:
    print ( x )

Andrei .F · Answer 9 · 2022-02-02T20:00:15.787

0

Loop whole list (not just part) starting from a random pos efficiently:

import random
arr = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
cln = len(arr)
start = random.randint(0, cln-1)
i = 0
while i < cln:
    pos = i+start
    print(arr[pos if pos<cln else pos-cln])
    i += 1

edited Feb 02 '22 at 20:00

answered Feb 02 '22 at 19:52

Andrei .F

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Start index for iterating Python list

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