What is the best way of doing this in Python?
for (v = n / 2 - 1; v >= 0; v--)
I actually tried Google first, but as far as I can see the only solution would be to use while.
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4 Answers
I would do this:
for i in reversed(range(n // 2)):
# Your code
pass
It's a bit clearer that this is a reverse sequence, what the lower limit is, and what the upper limit is.
4 Comments
hughdbrown
@ ΤΖΩΤΖΙΟΥ: true, but this does not have the benefit you may think. range() in pre-3.0 pythons pulls all the data at once. xrange() acts like a generator, pulling only one item at a time. But if you are going to reverse a sequence, you need all the data at once, so reversed(range()) and reversed(xrange()) will work in the same way: they'll each have all the data pulled before it is reversed.
Markus Hedlund
I actually ended up using this, big thanks! A follow-up question: I know I've seen it, but how do I print a
range() (an iterator I guess)? Edit: thanks hughdbrown for the performance note, I'll consider that in more complex applications
tzot
@hughdbrown: it does have the benefit you think I might be thinking. Check the
dir(xrange) output and note the __reversed__ special method.hughdbrown
@ ΤΖΩΤΖΙΟΥ: The things I learn. docs.python.org/library/functions.html#reversed explains that
reversed can consume __reversed__ method if present -- so that xrange() could be implemented optimally, as ΤΖΩΤΖΙΟΥ suggests. See also: python.org/dev/peps/pep-0322The way to do it is with xrange():
for v in xrange(n // 2 - 1, -1, -1):
(Or, in Python 3.x, with range() instead of xrange().) // is flooring division, which makes sure the result is a whole number.
1 Comment
rbp
hughdbrown's version should be marked as the correct answer
for v in range(n//2, -1, -1)
However, in 90% of the cases when you would have used a for loop in C/Java/C#/VB, what you really want is list comprehension:
listOfStuff = [doSomethingWith(v) for v in range(n//2, -1, -1)]
answered Apr 12, 2010 at 21:25
BlueRaja - Danny Pflughoeft
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1 Comment
hughdbrown
-1: This has one too many items. It incorrectly includes
n//2.for v in xrange(n/2 - 1, 0, -1):
#your code here
Where v and n are ints or treated as ints. This means that the division will be an integer division, i.e., 1/2 == 0 is True.
Note: This is for Python 2.x .
1 Comment
hughdbrown
-1: This has one too few items. It incorrectly omits
0. Separately, in python3 it fails because n/2 generates a float (TypeError: 'float' object cannot be interpreted as an integer). But maybe that's why you called out that it is for python 2.x.