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

User · Answer

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

User · Answer

You can use slicing   for item in some list 2          do stuff   This will start at the third element and iterate to the end

User · Answer

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

User · Answer

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

User · Answer

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    gt  gt  gt  def rotated sequence seq  start index           n   len seq          for i in xrange n               yield seq  i   start index    n       gt  gt  gt  s    su m tu w th f sa  split    gt  gt  gt  list rotated sequence s  s index  m       m    tu    w    th    f    sa    su    gt  gt  gt

User · Answer

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

User · Answer

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

User · Answer

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

[python] Start index for iterating Python list

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