Ruby中的File.open，open和IO.foreach有什么区别? [英] File.open, open and IO.foreach in Ruby, what is the difference?

问题描述

以下所有API都具有相同的作用:打开文件并为每一行调用一个块.我们应该优先使用一个优先选项吗?

All of the following API do the same thing: open a file and call a block for each line. Is there any preference we should use one than another?

File.open("file").each_line {|line| puts line}
open("file").each_line {|line| puts line}   
IO.foreach("file") {|line | puts line}

推荐答案

这三种选择之间存在重要区别.

There are important differences beetween those 3 choices.

• File.open打开本地文件并返回文件对象
• 文件保持打开状态，直到您在其上调用IO#close

• File.open opens a local file and returns a file object
• the file stays open until you call IO#close on it

Kernel.open查看字符串以确定如何处理它.

Kernel.open looks at the string to decide what to do with it.

open(".irbrc").class # => File
open("http://google.com/").class # => StringIO
File.open("http://google.com/") # => Errno::ENOENT: No such file or directory - http://google.com/

在第二种情况下，由Kernel#open返回的StringIO对象实际上保存了 http://google.com/的内容.如果Kernel#open返回一个File对象，则该对象将保持打开状态，直到您在其上调用IO#close.

In the second case the StringIO object returned by Kernel#open actually holds the content of http://google.com/. If Kernel#open returns a File object, it stays open untill you call IO#close on it.

• IO.foreach打开文件，为其读取的每一行调用给定的块，然后关闭文件.
• 您不必担心会关闭文件.

• IO.foreach opens a file, calls the given block for each line it reads, and closes the file afterwards.
• You don't have to worry about closing the file.

您没有提到这一选择，但这是我在大多数情况下都会使用的选择.

You didn't mention this choice, but this is the one I would use in most cases.

• File.read完全读取文件并将其作为字符串返回.
• 您不必担心会关闭文件.
• 与IO.foreach相比，这很清楚，您正在处理文件.
• 为此的内存复杂度为O(n).如果您知道要处理的是小文件，这不是缺点.但是，如果它可以是一个大文件，并且您知道自己的内存复杂度可以小于O(n)，则不要使用此选择.

• File.read reads a file completely and returns it as a string.
• You don't have to worry about closing the file.
• In comparison to IO.foreach this makes it clear, that you are dealing with a file.
• The memory complexity for this is O(n). If you know you are dealing with a small file, this is no drawback. But if it can be a big file and you know your memory complexity can be smaller than O(n), don't use this choice.

在这种情况下失败:

 s= File.read("/dev/zero") # => never terminates
 s.each …

ri

ri是一个向您显示ruby文档的工具.您可以在外壳上像这样使用它.

ri

ri is a tool which shows you the ruby documentation. You use it like this on your shell.

ri File.open
ri open
ri IO.foreach
ri File#each_line

有了这个，您几乎可以找到我在这里写的所有内容.

With this you can find almost everything I wrote here and much more.

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