How do I loop through a list by twos

Question

I want to loop through a Python list and process 2 list items at a time  Something like this in another language   for int i   0  i  lt  list length    i  2          do something with list i  and list i   1      What s the best way to accomplish this

User · Answer

If you have control over the structure of the list, the most pythonic thing to do would probably be to change it from:

l=[1,2,3,4]

to:

l=[(1,2),(3,4)]

Then, your loop would be:

for i,j in l:
    print i, j

User · Answer

If you re using Python 2 6 or newer you can use the grouper recipe from the itertools module   from itertools import izip longest  def grouper n  iterable  fillvalue None        grouper 3   ABCDEFG    x   -- gt  ABC DEF Gxx      args    iter iterable     n     return izip longest fillvalue fillvalue   args    Call like this   for item1  item2 in grouper 2  l         Do something with item1 and item2   Note that in Python 3 x you should use zip longest instead of izip longest

User · Answer

You can use for in range with a step size of 2   Python 2  for i in xrange 0 10 2     print i    Python 3  for i in range 0 10 2     print i    Note  Use xrange in Python 2 instead of range because it is more efficient as it generates an iterable object  and not the whole list

User · Answer

You can also use this syntax  L start stop step     mylist    1 2 3 4 5 6 7 8 9 10  for i in mylist   2       print i    prints 1 3 5 7 9  for i in mylist 1  2       print i    prints 2 4 6 8 10   Where the first digit is the starting index  defaults to beginning of list or 0   2nd is ending slice index  defaults to end of list   and the third digit is the offset or step

User · Answer

The simplest in my opinion is just this  it   iter  1 2 3 4 5 6   for x  y in zip it  it       print x  y  Out  1 2      3 4      5 6  No extra imports or anything  And very elegant  in my opinion

User · Answer

nums   range 10  for i in range 0  len nums -1  2       print nums i    Kinda dirty but it works

User · Answer

This might not be as fast as the izip longest solution  I didn t actually test it   but it will work with python  lt  2 6  izip longest was added in 2 6    from itertools import imap  def grouper n  iterable        grouper 3   ABCDEFG   -- gt    A  B   C      D   E   F      G  None None       args    iter iterable     n      return imap None   args    If you need to go earlier than 2 3  you can substitute the built-in map for imap  The disadvantage is that it provides no ability to customize the fill value

[python] How do I loop through a list by twos?

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