[英]Change list, that you are iterating through, in that loop
In my case I have a for loop, looping through a list, but I want to change that list in said loop. 就我而言,我有一个for循环,遍历一个列表,但是我想在上述循环中更改该列表。 After that I want the for loop to iterate through the new list.
之后,我希望for循环遍历新列表。
li = [4,5,6,7,8,9]
for item in li:
#do something
if item == 5:
#now continue iterating through this loop and not the old one
li = [9,9,9,9,9,9]
How can I do something like that? 我该怎么做?
Allthough @BoarGules's comment is correct, you may solve your problem using enumerate. 尽管@BoarGules的评论是正确的,但您可以使用枚举解决问题。
li = [4,5,6,7,8,9]
for i, item in enumerate(li):
print(li[i])
if li[i] == 5:
li = [9,9,9,9,9,9]
This outputs: 输出:
>4
>5
>9
>9
>9
You shouldn't change the list through iteration. 您不应该通过迭代来更改列表。 I would use indexing:
我会使用索引:
for i in range(len(li)):
if li[i] == 5:
li = len(li) * [9]
To see why this doesn't work, a for
loop is equivalent to a while
loop like this: 要了解为什么这行不通,
for
循环等效于while
循环,如下所示:
# for x in y:
# ...
itr = iter(y)
while True:
try:
x = next(itr)
except StopIteration:
break
...
If you assign something new to y
, the loop isn't affected, because it only uses the iterator over the original value assigned to y
, not the name y
itself. 如果为
y
分配新的内容,则该循环不会受到影响,因为它仅在分配给y
的原始值(而不是名称y
本身)上使用迭代器。
However, if you do mutate the list in the body of the loop, the iterator might return a value you don't expect. 但是,如果您确实在循环体内更改了列表,则迭代器可能会返回您不期望的值。 If you need to change the iterable, it's best to get the iterator yourself.
如果需要更改迭代器,最好自己获取迭代器。
li = [4,5,6,7,8,9]
itr = iter(li)
while True:
try:
item = next(itr)
except StopIteration:
break
#do something
if item == 5:
#now continue iterating through this loop and not the old one
itr = iter([9,9,9,9,9])
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