"典型"从列表中删除元素的方法 [英] "Canonical" way of deleting elements from lists
问题描述
您好，
从字符串列表中我想删除所有空字符串。这有效：
关键字中的''''：keywords.remove（''''）
但是，长期以来术语C程序员看起来像是一个尴尬的方式来实现一个简单的目标，因为列表必须在每次迭代中重新评估
。有没有办法只是走一次列表然后扔掉
不需要的元素？
当这些小东西真实时，我开始编程性能
问题，所以当看起来好像没什么问题时我会有某种畏缩反射。
罗伯特
Hello,
From a list of strings I want to delete all empty ones. This works:
while '''' in keywords: keywords.remove('''')
However, to a long-term C programmer this looks like an awkward way of
accomplishing a simple goal, because the list will have to be re-evaluated
in each iteration. Is there a way to just walk the list once and throw out
unwanted elements as one goes along?
I started programming back when such little things were real performance
issues, so I have some sort of cringe reflex when something looks
inefficient.
robert
推荐答案
Robert Latest写道：
Robert Latest wrote:
>>来自a我要删除所有空字符串的字符串列表。这适用：
>>From a list of strings I want to delete all empty ones. This works:
关键字中的''''：keywords.remove（''''）
但是，对于一个长期的C程序员来说，这看起来像是一个尴尬的方式来完成一个简单的目标，因为每次迭代都需要重新评估列表
。
while '''' in keywords: keywords.remove('''')
However, to a long-term C programmer this looks like an awkward way of
accomplishing a simple goal, because the list will have to be re-evaluated
in each iteration.
你正在使用二次算法（in是线性搜索，并删除
必须移动一切每次调用），你担心Python获取变量需要花费多少时间？
you''re using a quadratic algorihm ("in" is a linear search, and remove
has to move everything around for each call), and you''re worried about
the time it takes Python to fetch a variable?
有没有办法只需走一次列表就可以随意丢弃不需要的
元素？
Is there a way to just walk the list once and throw out unwanted
elements as one goes along?
创建一个新列表总是几乎正确的方式来做这样的事情，比如
。在这种特殊情况下，filter（）或列表推导是好的
选择：
keywords = filter（无，关键字）＃get" true"仅限商品
keywords = [k为关键字中的k如果k]
也可以看到：
http://effbot.org/zone/python-list.htm#modifying
< / F>
creating a new list is always almost the right way to do things like
this. in this specific case, filter() or list comprehensions are good
choices:
keywords = filter(None, keywords) # get "true" items only
keywords = [k for k in keywords if k]
also see:
http://effbot.org/zone/python-list.htm#modifying
</F>
Fredrik Lundh写道：
Fredrik Lundh wrote:
创建一个新列表总是几乎正确的方式来执行诸如
creating a new list is always almost the right way to do things like
message = message.replace（" always almost） ，几乎总是）
message = message.replace("always almost", "almost always")
Robert最新< bo ******* @ yahoo.comwrites：
Robert Latest <bo*******@yahoo.comwrites:
从字符串列表中我想删除所有空字符串。这有效：
而关键字中的''''关键字：keywords.remove（''''）
From a list of strings I want to delete all empty ones. This works:
while '''' in keywords: keywords.remove('''')
如果你'正在寻找一个快速（没有二次行为）和方便的
的方式，你可以这样做：
keywords = [s对于s中的关键字如果s！='''''
但这会创建一个新列表，对于长列表可能不需要
空元素（或共享列表）。它还迭代了Python循环中每个列表元素的
，这可能需要一些时间才能获得很长的
列表。
If you''re looking for a quick (no quadratic behavior) and convenient
way to do it, you can do it like this:
keywords = [s for s in keywords if s != '''']
But that creates a new list, which might not be wanted for long lists
with few empty elements (or for shared lists). It also iterates over
every list element in a Python loop, which can take some time for long
lists.
有没有办法只需走一次列表就扔出不需要的
元素？
Is there a way to just walk the list once and throw out unwanted
elements as one goes along?
我会这样做：
i = 0
而1：
尝试：
i = keywords.index（''''，i）
除了ValueError：
break
del keywords [i]
甚至：
试试：
i = 0
而1：
i = keywords.index（''''，i）＃throws ValueError并停止循环
del keywords [i]
除了ValueError：
pass
在这两种情况下，搜索空字符串都是在高效的C
代码，你只在实际匹配中循环Python。
I''d do it like this:
i = 0
while 1:
try:
i = keywords.index('''', i)
except ValueError:
break
del keywords[i]
Or even:
try:
i = 0
while 1:
i = keywords.index('''', i) # throws ValueError and stops the loop
del keywords[i]
except ValueError:
pass
In both cases the search for the empty string is done in efficient C
code, and you only loop in Python over the actual matches.
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