How to loop backwards in python? [duplicate]

I'm talking about doing something like:

for(i=n; i>=1; --i) { //do something with i } 

I can think of some ways to do so in python (creating a list of range(1,n+1) and reverse it, using while and --i, ...) but I wondered if there's a more elegant way to do it. Is there?

EDIT: Some suggested I use xrange() instead of range() since range returns a list while xrange returns an iterator. But in Python 3 (which I happen to use) range() returns an iterator and xrange doesn't exist.

5 Answers

range() and xrange() take a third parameter that specifies a step. So you can do the following.

range(10, 0, -1) 

Which gives

[10, 9, 8, 7, 6, 5, 4, 3, 2, 1] 

But for iteration, you should really be using xrange instead. So,

xrange(10, 0, -1) 

Note for Python 3 users: There are no separate range and xrange functions in Python 3, there is just range, which follows the design of Python 2's xrange.

for x in reversed(whatever): do_something() 

This works on basically everything that has a defined order, including xrange objects and lists.

All of these three solutions give the same results if the input is a string:

1.

def reverse(text): result = "" for i in range(len(text),0,-1): result += text[i-1] return (result) 

2.

text[::-1] 

3.

"".join(reversed(text)) 

def reverse(text): reversed = '' for i in range(len(text)-1, -1, -1): reversed += text[i] return reversed print("reverse({}): {}".format("abcd", reverse("abcd"))) 

To reverse a string without using reversed or [::-1], try something like:

def reverse(text): # Container for reversed string txet="" # store the length of the string to be reversed # account for indexes starting at 0 length = len(text)-1 # loop through the string in reverse and append each character # deprecate the length index while length>=0: txet += "%s"%text[length] length-=1 return txet