How to loop in a list more times than the list size?

Question

Is there a function or native lib in Python that allows me to loop in a list more times than the number of elements in the list? In other words, when my index of interest is greater than the list size, the next element is the first of the list.

For example, I have this list:

abc = ['a', 'b', 'c', 'd', 'e']

So, if I have an index with value 10, the result will be 'a'. If the index is with value 18, the result will be 'd'.

You can create infinite sequence from you `abc` array with the function `cycle(seq)` from `itertools` module and then you can cut this sequence at some point with `islice(seq, count)` function from the same module. — Nikita Sivukhin, May 10 '18 at 14:09

wim · Accepted Answer · 2018-04-26T01:39:09.613

13

Simplest: wrap the index with modulo

>>> abc = ['a', 'b', 'c', 'd', 'e' ]
>>> abc[18 % len(abc)]
'd'

You can wrap it up in a helper class if you want:

>>> class ModList(list):
...     def __getitem__(self, item):
...         if isinstance(item, slice):
...             return super().__getitem__(item)
...         return super().__getitem__(item % len(self))
...     
>>> abc = ModList('abcde')
>>> abc[18]
'd'
>>> abc[-5]
'a'
>>> abc[-6]
'e'

You might want to implement __setitem__ and __delitem__ similarly.

edited Apr 26 '18 at 01:39

answered Apr 26 '18 at 01:33

wim

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2

what about abc[18:20]? :P – khachik Apr 26 '18 at 01:45

score 1 · Answer 2 · answered Apr 26 '18 at 02:47

itertools.cycle() works if you want to iterate sequentially, repeatedly through the list

from itertools import cycle


abc = ['a', 'b', 'c', 'd', 'e' ]

alfs = ''

for n, e in enumerate(cycle(abc)):  # lazy enumeration?
    alfs += e
    if n >= 18:  # must have stopping test to break infinite loop
        break
alfs
Out[30]: 'abcdeabcdeabcdeabcd'

How to loop in a list more times than the list size?

2 Answers2