Start index for iterating Python list

问题:

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?

回答1:

You can use slicing:

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

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

回答2:

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):
    ...

回答3:

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.

回答4:

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'])

回答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']
>>>

回答6:

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

回答7:

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 )