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)
Answer from Chinmay Kanchi on Stack OverflowNote for Python 3 users: There are no separate
rangeandxrangefunctions in Python 3, there is justrange, which follows the design of Python 2'sxrange.
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
rangeandxrangefunctions in Python 3, there is justrange, which follows the design of Python 2'sxrange.
for x in reversed(whatever):
do_something()
This works on basically everything that has a defined order, including xrange objects and lists.
python - What's "better" the reverse method or the reversed built-in function? - Stack Overflow
Differences in iteration through a loop in reverse order - Python
loops - Traverse a list in reverse order in Python - Stack Overflow
iterate over a string in reverse
Videos
foo.reverse() actually reverses the elements in the container. reversed() doesn't actually reverse anything, it merely returns an object that can be used to iterate over the container's elements in reverse order. If that's what you need, it's often faster than actually reversing the elements.
There seems to be a great difference. I really thought it's the other way round. Why is rearranging the values in a list faster than creating a new one from an iterator ?
from decorators import bench
_list = range(10 ** 6)
@ bench
def foo():
list(reversed(_list))
@ bench
def bar():
_list.reverse()
foo()
bar()
print foo.time
print bar.time
0.167278051376
0.0122621059418
So I'm building a game that requires me to loop through a list in reverse order.
It seems that there are multiple ways to do this same task, but I want to make sure I understand the differences between the alternative ways.
From what I have found online:
I can use list comprehension -
for _ in desired_list[::-1]
I can use reversed() -
for _ in revered(desired_list)
I can use a range() -
for _ in range(len(desired_list) -1, 0, -1)
Is there any functional difference between these?
Also for the range iteration, could you verify what each number stands for?
It seems to be:
range("number to start on", "number to end up", "direction and distance to travel in list")
Is that correct?
Thank you for any help!
Use the built-in reversed() function:
>>> a = ["foo", "bar", "baz"]
>>> for i in reversed(a):
... print(i)
...
baz
bar
foo
To also access the original index, use enumerate() on your list before passing it to reversed():
>>> for i, e in reversed(list(enumerate(a))):
... print(i, e)
...
2 baz
1 bar
0 foo
Since enumerate() returns a generator and generators can't be reversed, you need to convert it to a list first.
You can do:
for item in my_list[::-1]:
print item
(Or whatever you want to do in the for loop.)
The [::-1] slice reverses the list in the for loop (but won't actually modify your list "permanently").
I want to iterate over a string in reverse. I googled it and found this way
for i in range(len(k)-1, 0-1, -1):
but to be honest I don't understand it. can you please explain it to me?
also, I would love it if you guys can suggest other methods
Basically, I wanted the loop to start from the last index and work that way done. However, I am not getting anything.
listr = [10,20,30,50]
count = 0
for i in range(len(listr),-1):
count +=1
print(listr[i], count)
print('-----')To get a new reversed list, apply the reversed function and collect the items into a list:
>>> xs = [0, 10, 20, 40]
>>> list(reversed(xs))
[40, 20, 10, 0]
To iterate backwards through a list:
>>> xs = [0, 10, 20, 40]
>>> for x in reversed(xs):
... print(x)
40
20
10
0
>>> xs = [0, 10, 20, 40]
>>> xs[::-1]
[40, 20, 10, 0]
Extended slice syntax is explained here. See also, documentation.