Question

Possible Duplicate:
Accessing the index in Python for loops

I wonder: does Python have something like?

for (i=0; i < length; i += 1){ .... }

Of course, I might say

i = 0
for item in items:
 #.....
 i += 1

but I think there should be something similar to for(i = 0;...), shouldn’t it?

Answer 1

Use the enumerate() function:

for i, item in enumerate(items):
     print i, item

or use range():

for i in range(len(items)):
     print i

(On python 2 you’d use xrange() instead).

range() let’s you step through i in steps other than 1 as well:

>>> list(range(0, 5, 2))
[0, 2, 4]
>>> list(range(4, -1, -1))
[4, 3, 2, 1, 0]

You usually don’t need to use an index for sequences though; not with the itertools library or the reversed() function; most usecases for ‘special’ index value ranges are covered:

>>> menu = ['spam', 'ham', 'eggs', 'bacon', 'sausage', 'onions']
>>> # Reversed sequence
>>> for dish in reversed(menu):
...     print(dish)
... 
onions
sausage
bacon
eggs
ham
spam
>>> import itertools
>>> # Only every third
>>> for dish in itertools.islice(menu, None, None, 3):
...     print(dish)
... 
spam
bacon
>>> # In groups of 4
>>> for dish in itertools.izip_longest(*([iter(menu)] * 4)):
...     print(dish)
... 
('spam', 'ham', 'eggs', 'bacon')
('sausage', 'onions', None, None)