For-loops in Python

Question

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.

Answer 1

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.

• 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-0322

Answer 2

The 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.

• rbp: hughdbrown's version should be marked as the correct answer

Answer 3

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)]

• hughdbrown: -1: This has one too many items. It incorrectly includes n//2.

Answer 4

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 .

• 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.