在列表中切割字符串的结尾 [英] slicing the end of a string in a list

问题描述

这是我写的代码：


file = open（''C：\switches.txt''，''r''）

switches = file.readlines（）

i = 0

for line in switch：

line = switches [我] [： - 1]

i + = 1


打印开关

你可以告诉我在做什么。从文件中读取一个行列表，

然后我想从每一行切掉''\ n''字符。但是这个代码运行后，

，\ n仍然存在。我认为它可能有一些与字符串是不可变的事实有关的事情，但是测试

如：


开关[0] [： - 1]


确实切掉了\ n字符。所以我猜问题就在于

作业或其中某处。


另外，这是索引列表的最佳方法吗？

Here''s the code I wrote:

file = open(''C:\switches.txt'', ''r'')
switches = file.readlines()
i = 0

for line in switches:
line = switches[i][:-1]
i += 1

print switches
You can probably tell what I''m doing. Read a list of lines from a file,
and then I want to slice off the ''\n'' character from each line. But
after this code runs, the \n is still there. I thought it might have
something to do with the fact that strings are immutable, but a test
such as:

switches[0][:-1]

does slice off the \n character. So I guess the problem lies in the
assignment or somewhere in there.

Also, is this the best way to index the list?

推荐答案

John Salerno写道：

John Salerno wrote:

你可以告诉我在做什么。从文件中读取行列表，
然后我想从每行中切掉''\ n''字符。但是在这段代码运行之后，\ n仍然存在。我认为它可能与字符串是不可变的这一事实有关，但是测试
如：

开关[0] [： - 1]
切掉了\ n字符。


实际上，它创建了一个新的字符串实例，删除了\ n字符

，然后将其丢弃。原来的开关[0]字符串已经没有了.b $ b更改了。

You can probably tell what I''m doing. Read a list of lines from a file,
and then I want to slice off the ''\n'' character from each line. But
after this code runs, the \n is still there. I thought it might have
something to do with the fact that strings are immutable, but a test
such as:

switches[0][:-1]

does slice off the \n character.
Actually, it creates a new string instance with the \n character
removed, then discards it. The original switches[0] string hasn''t
changed.

foo =''Hello world！''
foo [： - 1]
''Hello world''foo

foo = ''Hello world!''
foo[:-1] ''Hello world'' foo

' 'Hello world！''

所以我猜问题就在于
任务或其中的某个地方。


是的。您将重复将新的字符串实例分配给line，然后再次引用
。如果你想更新开关

list，那么不要分配给line。在循环内，你需要：


开关[i] =开关[i] [： - 1]

另外，这是最好的索引方式列表？

''Hello world!''
So I guess the problem lies in the
assignment or somewhere in there.
Yes. You are repeated assigning a new string instance to "line", which
is then never referenced again. If you want to update the switches
list, then instead of assigning to "line" inside the loop, you need:

switches[i] = switches[i][:-1]
Also, is this the best way to index the list?

否，因为行变量未使用。这个：


i = 0

for line in switch：

line = switches [i] [： - 1]

i + = 1


写得更好：


for i in range（len（switches））：

开关[i] =开关[i] [： - 1]


对于Python中的大多数循环场景，你不必手动

增加一个计数器变量。


--Ben


PS - 实际上，你可以完成以上所有在一行

代码：

打印[line [： - 1] for line in open（''C：\\switches.txt''） ]

No, since the line variable is unused. This:

i = 0
for line in switches:
line = switches[i][:-1]
i += 1

Would be better written as:

for i in range(len(switches)):
switches[i] = switches[i][:-1]

For most looping scenarios in Python, you shouldn''t have to manually
increment a counter variable.

--Ben

PS - actually, you can accomplish all of the above in a single line of
code:
print [line[:-1] for line in open(''C:\\switches.txt'')]

Ben Cartwright写道：

Ben Cartwright wrote:

实际上，它创建了一个带有\ n字符的新字符串实例
删除，然后丢弃它。原来的开关[0]字符串没有改变。
是的。您将重复为line分配一个新的字符串实例，然后再从未引用它。


啊，谢谢！

PS - 实际上，你可以用一行
代码完成上述所有工作：
打印[line [： - 1] for line in open（''C：\\switches.txt''）]

Actually, it creates a new string instance with the \n character
removed, then discards it. The original switches[0] string hasn''t
changed.
Yes. You are repeated assigning a new string instance to "line", which
is then never referenced again.
Ah, thank you!
PS - actually, you can accomplish all of the above in a single line of
code:
print [line[:-1] for line in open(''C:\\switches.txt'')]

哇，刚刚更换7行代码！所以*这就是为什么Python是好的b
。 :)

Wow, that just replaced 7 lines of code! So *this* is why Python is
good. :)

Ben Cartwright写道：

Ben Cartwright wrote:

print [line [： - 1] for line in open（''C： \\switches.txt''）]

print [line[:-1] for line in open(''C:\\switches.txt'')]

嗯，我刚刚在原始代码中意识到我没有逃脱

反斜杠。为什么它仍能正常工作？


顺便说一下，整个''一行'的东西让我大吃一惊。当我开始研究列表理解时，我并没有想到列表理解，但

只是事实上它可以在一行中完成所有这一切都是惊人的。我在C＃中尝试了这个
，当然我必须先创建一个类，然后打开

文件流等等。:)


我是否在开放功能中不需要''r''参数？

Hmm, I just realized in my original code that I didn''t escape the
backslash. Why did it still work properly?

By the way, this whole ''one line'' thing has blown me away. I wasn''t
thinking about list comprehensions when I started working on this, but
just the fact that it can all be done in one line is amazing. I tried
this in C# and of course I had to create a class first, and open the
file streams, etc. :)

And do I not need the ''r'' parameter in the open function?

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