How to iterate a list starting from different index, and also wrap around

Question

Similar question asked here (Start index for iterating Python list), but I need one more thing.

Assume I have a list [Sunday, Monday, ...Saturday], and I want to iterate the list starting from different position, wrap around and complete the loop. For example

a = [Sunday, Monday, ...Saturday]
for i in range(7):
    print("----")
    for j in (SOMETHING):
        print(j)

OUTPUT:
----
Sunday
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
----
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
Sunday
----
Tuesday
.
.
.
Friday

How could I approach this?

I got a eureka moment. ```python a[i:]+a[:i] ``` will do. I am not going to retract this post because I consider it might be helpful to someone else. — hcchiu1202, Sep 13 '19 at 08:23
then you should put this comment as an answer and mark it as accepted — olinox14, Sep 13 '19 at 08:24
Possible duplicate of [Efficient way to rotate a list in python](https://stackoverflow.com/questions/2150108/efficient-way-to-rotate-a-list-in-python) — olinox14, Sep 13 '19 at 08:26
Possible duplicate of [Circular list iterator in Python](https://stackoverflow.com/questions/23416381/circular-list-iterator-in-python) — Diptangsu Goswami, Sep 13 '19 at 08:31

yatu · Accepted Answer · 2019-09-13T09:13:16.953

3

One approach would be using collections.deque:

from collections import deque
from itertools import repeat

d = deque(['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'])

n = 7
for i in repeat(d, n):
    print(*i, sep='\n')
    print('-----')
    i.rotate(-1)

Sunday
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
-----
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
Sunday
-----
Tuesday
.
.
.

Though you might find more interesting to create a nested list:

n = 7
l = []
for i in repeat(d, n):
    sl = []
    for j in i:
        sl.append(j)
    l.append(sl)
    i.rotate(-1)

print(l)
# [['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'], 
#  ['Monday', 'Tuesday', 'Wednesday'...

edited Sep 13 '19 at 09:13

answered Sep 13 '19 at 08:29

yatu

86,083
12
84
139

Thank you for your answer. Does this approach create 7 lists in the memory? – hcchiu1202 Sep 13 '19 at 08:37
`repeat` is wimsply repeating the fed iterator 7 times, and here the result is simply printed, which seems to be what you trying to do @user12062120 – yatu Sep 13 '19 at 09:07

score 1 · Answer 2 · answered Sep 13 '19 at 08:25

1

It can be done by:

a[i:]+a[:i]

answered Sep 13 '19 at 08:25

hcchiu1202

61
6

It will cause a new list creation – Sawel Sep 13 '19 at 08:26

score 1 · Answer 3 · answered Sep 13 '19 at 08:34

1

You could pop the start item off and add it to the end.

days = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday']
for _ in range(7):
    print("----")
    print("\n".join(days))
    days.append(days.pop(0))

answered Sep 13 '19 at 08:34

Chris Doyle

10,703
2
23
42

score 1 · Answer 4 · answered Sep 13 '19 at 08:38

You can use collections.dequeue, which has a rotate method. However, if you want to make it on your own you can do it like this:

>>> a = ['a','b','c','d']
>>> counter = 0
>>> start_index=2
>>> while counter < len(a):
...     print(a[start_index])
...     start_index+=1
...     counter += 1
...     if start_index==len(a):
...             start_index=0
... 
c
d
a
b

This is quite optimal, because you do not need to make any copy or create a new list, just iterate.

score 0 · Answer 5 · answered Sep 13 '19 at 08:29

0

Use itertools.cycle

from itertools import cycle

counter = 1
days = ['Sunday', 'Monday', 'Tuesday']
for day in cycle(days):
    print(day)
    counter += 1
    if counter == 7:
        print('-----')
        counter = 1

answered Sep 13 '19 at 08:29

Diptangsu Goswami

5,554
3
25
36

score 0 · Answer 6 · answered Sep 13 '19 at 08:30

0

Use the following function:

def cycle_list(l, i):
 for element in l[i:]:
  yield element
 for element in l[:i]:
  yield element

answered Sep 13 '19 at 08:30

Sawel

929
7
17

score 0 · Answer 7 · answered Sep 13 '19 at 08:33

0

If you don't want to import any libraries.

DAYS = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday']
for i in range(7):
    print("----")
    for j in range(len(DAYS)):
        print(DAYS[(j+i) % len(DAYS)])

answered Sep 13 '19 at 08:33

Daniel Farhat

1
1

kederrac · Answer 8 · 2019-09-13T09:07:27.790

you can chain the elements starting from your current index (in your case the current index is i) with the elements before current index using generators, in this way you will not create new lists, will be memory-efficient:

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

from itertools import chain
for i in range(7):
    print("----")
    for j in chain((e for e in a[i:]), (e for e in a[:i])):
        print(j)

output:

----
Sunday
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
----
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
Sunday
----
Tuesday
Wednesday
Thursday
Friday
Saturday
Sunday
Monday
----
Wednesday
Thursday
Friday
Saturday
Sunday
Monday
Tuesday
----
Thursday
Friday
Saturday
Sunday
Monday
Tuesday
Wednesday
----
Friday
Saturday
Sunday
Monday
Tuesday
Wednesday
Thursday
----
Saturday
Sunday
Monday
Tuesday
Wednesday
Thursday
Friday

How to iterate a list starting from different index, and also wrap around

8 Answers8