如何在python中向后循环？
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How to loop backwards in python?
提问by snakile
I'm talking about doing something like:
我说的是做类似的事情：
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 whileand --i, ...) but I wondered if there's a more elegant way to do it. Is there?
我可以想到在 python 中这样做的一些方法（创建一个列表range(1,n+1)并反转它，使用whileand --i，...）但我想知道是否有更优雅的方法来做到这一点。在那儿？
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.
编辑：有人建议我使用 xrange() 而不是 range() 因为 range 返回一个列表而 xrange 返回一个迭代器。但是在 Python 3（我碰巧使用）中， range() 返回一个迭代器，而 xrange 不存在。
采纳答案by Chinmay Kanchi
range()and xrange()take a third parameter that specifies a step. So you can do the following.
range()并xrange()采用指定步骤的第三个参数。因此，您可以执行以下操作。
range(10, 0, -1)
Which gives
这使
[10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
But for iteration, you should really be using xrangeinstead. So,
但是对于迭代，你真的应该使用它xrange。所以，
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.
Python 3 用户注意事项：Python 3 中没有单独的
range和xrange函数，只有range，它遵循 Python 2 的xrange.
回答by habnabit
for x in reversed(whatever):
do_something()
This works on basically everything that has a defined order, including xrangeobjects and lists.
这基本上适用于具有定义顺序的所有内容，包括xrange对象和列表。
回答by alex.hunter
To reverse a string without using reversedor [::-1], try something like:
要在不使用reversed或的情况下反转字符串[::-1]，请尝试以下操作：
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
回答by Michael Qin
def reverse(text):
reversed = ''
for i in range(len(text)-1, -1, -1):
reversed += text[i]
return reversed
print("reverse({}): {}".format("abcd", reverse("abcd")))
回答by chocolate codes
All of these three solutions give the same results if the input is a string:
如果输入是字符串，所有这三种解决方案都会给出相同的结果：
1.
1.
def reverse(text):
result = ""
for i in range(len(text),0,-1):
result += text[i-1]
return (result)
2.
2.
text[::-1]
3.
3.
"".join(reversed(text))
