基本输入输出
Ruby提供了两种I/O例程，第一种是简单的接口，我们已经用了很多了。
print "Enter your name: "
name = gets
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Kernel模块提供了一整套和I/O相关的方法：gets, open, print, printf, putc, puts, readline, readlines, 和test等，这些方法能使你简单方便的进行Ruby编程。这些方法典型的对标准输入输出进行操作，很适合编写过滤器等。
另一方面，ruby提供了IO对象，让我们可以对io进行更多的控制。
什么是IO对象?
Ruby defines a single base class,
IO, to handle input and output. This base class is subclassed by classes
File and
BasicSocket to provide more specialized behavior, but the principles are the same throughout. An
IO object is a bidirectional channel between a Ruby program and some external resource.
[For those who just have to know the implementation details, this means that a single IO object can sometimes be managing more than one operating system file descriptor. For example, if you open a pair of pipes, a single IO object contains both a read pipe and a write pipe.] There may be more to an
IO object than meets the eye, but in the end you still simply write to it and read from it.
In this chapter, we'll be concentrating on class
IO and its most commonly used subclass, class
File. For more details on using the socket classes for networking, see the section beginning on page 469.
Opening and Closing Files
As you might expect, you can create a new file object using
File.new .
aFile = File.new("testfile", "r")
# ... process the file
aFile.close
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You can create a
File object that is open for reading, writing, or both, according to the mode string (here we opened ``
testfile'' for reading with an ``
r''). The full list of allowed modes appears on page 326. You can also optionally specify file permissions when creating a file; see the description of
File.new on page 303 for details. After opening the file, we can work with it, writing and/or reading data as needed. Finally, as responsible software citizens, we close the file, ensuring that all buffered data is written and that all related resources are freed.
But here Ruby can make life a little bit easier for you. The method
File.open also opens a file. In regular use, it behaves just like
File.new . However, if there's a block associated with the call,
open behaves differently. Instead of returning a new
File object, it invokes the block, passing the newly opened
File as a parameter. When the block exits, the file is automatically closed.
File.open("testfile", "r") do |aFile|
# ... process the file
end
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Reading and Writing Files
The same methods that we've been using for ``simple'' I/O are available for all file objects. So,
gets reads a line from standard input, and
aFile.gets reads a line from the file object
aFile.
However, I/O objects enjoy an additional set of access methods, all intended to make our lives easier.
Iterators for Reading
As well as using the usual loops to read data from an
IO stream, you can also use various Ruby iterators.
IO#each_byte invokes a block with the next 8-bit byte from the
IO object (in this case, an object of type
File).
aFile = File.new("testfile")
aFile.each_byte {|ch| putc ch; putc ?. }
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produces:
T.h.i.s. .i.s. .l.i.n.e. .o.n.e.
.T.h.i.s. .i.s. .l.i.n.e. .t.w.o.
.T.h.i.s. .i.s. .l.i.n.e. .t.h.r.e.e.
.A.n.d. .s.o. .o.n.......
.
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IO#each_line calls the block with the next line from the file. In the next example, we'll make the original newlines visible using
String#dump , so you can see that we're not cheating.
aFile.each_line {|line| puts "Got #{line.dump}" }
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produces:
Got "This is line one\n"
Got "This is line two\n"
Got "This is line three\n"
Got "And so on...\n"
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You can pass
each_line any sequence of characters as a line separator, and it will break up the input accordingly, returning the line ending at the end of each line of data. That's why you see the ``
\n'' characters in the output of the previous example. In the next example, we'll use ``
e'' as the line separator.
aFile.each_line("e") do |line|
puts "Got #{ line.dump }"
end
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produces:
Got "This is line"
Got " one"
Got "\nThis is line"
Got " two\nThis is line"
Got " thre"
Got "e"
Got "\nAnd so on...\n"
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If you combine the idea of an iterator with the auto-closing block feature, you get
IO.foreach . This method takes the name of an I/O source, opens it for reading, calls the iterator once for every line in the file, and then closes the file automatically.
IO.foreach("testfile") { |line| puts line }
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produces:
This is line one
This is line two
This is line three
And so on...
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Or, if you prefer, you can retrieve an entire file into an array of lines:
arr = IO.readlines("testfile") |
arr.length |
?/td>
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4 |
arr[0] |
?/td>
|
"This is line one\n" |
Don't forget that I/O is never certain in an uncertain world---exceptions will be raised on most errors, and you should be ready to catch them and take appropriate action.
Writing to Files
So far, we've been merrily calling
puts and
print, passing in any old object and trusting that Ruby will do the right thing (which, of course, it does). But what exactly
is it doing?
The answer is pretty simple. With a couple of exceptions, every object you pass to
puts and
print is converted to a string by calling that object's
to_s method. If for some reason the
to_s method doesn't return a valid string, a string is created containing the object's class name and id, something like
<ClassName:0x123456>.
The exceptions are simple, too. The
nil object will print as the string ``nil,'' and an array passed to
puts will be written as if each of its elements in turn were passed separately to
puts.
What if you want to write binary data and don't want Ruby messing with it? Well, normally you can simply use
IO#print and pass in a string containing the bytes to be written. However, you can get at the low-level input and output routines if you really want---have a look at the documentation for
IO#sysread and
IO#syswrite on page 335.
And how do you get the binary data into a string in the first place? The two common ways are to poke it in byte by byte or to use
Array#pack .
str = "" |
?/td>
|
"" |
str << 1 << 2 << 3 |
?/td>
|
"\001\002\003" |
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[ 4, 5, 6 ].pack("c*") |
?/td>
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"\004\005\006" |
But I Miss My C++ Iostream
Sometimes there's just no accounting for taste...However, just as you can append an object to an
Array using the
<< operator, you can also append an object to an output
IO stream:
endl = "\n"
$stdout << 99 << " red balloons" << endl
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produces:
Again, the
<< method uses
to_s to convert its arguments to strings before sending them on their merry way.
与网络交互
Ruby支持很多网络协议，不管是高层的还是底层的。
ruby提供了一些基本类，让你可以使用TCP,UDP,SOCKS等很多协议交互，而不必拘泥在网络层。这些类也提供了辅助类，让你可以轻松的对服务器进行读写。这个例子利用finger协议查询用户oracle的信息：
require 'socket'
client = TCPSocket.open('localhost', 'finger')
client.send("oracle\n", 0) # 0 means standard packet
puts client.readlines
client.close
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结果:
Login: oracle Name: Oracle installation
Directory: /home/oracle Shell: /bin/bash
Never logged in.
No Mail.
No Plan.
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对于高层，lib/net里面提供了一些与应用层协议（FTP,HTTP,POP,SMTP,TELNET）等交互的库模块。比如，下面的例子列出了Pragmatic Programmer主页里的图像。
require 'net/http'
h = Net::HTTP.new('www.pragmaticprogrammer.com', 80)
resp, data = h.get('/index.html', nil)
if resp.message == "OK"
data.scan(/<img src="(.*?)"/) { |x| puts x }
end
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produces:
images/title_main.gif
images/dot.gif
images/dot.gif
images/dot.gif
images/aafounders_70.jpg
images/pp_cover_thumb.png
images/ruby_cover_thumb.png
images/dot.gif
images/dot.gif
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Extracted from the book "Programming Ruby - The Pragmatic Programmer's Guide" Copyright © 2001 by Addison Wesley Longman, Inc. This material may be distributed only subject to the terms and conditions set forth in the Open Publication License, v1.0 or later (the latest version is presently available at http://www.opencontent.org/openpub/)).
Distribution of substantively modified versions of this document is prohibited without the explicit permission of the copyright holder.
Distribution of the work or derivative of the work in any standard (paper) book form is prohibited unless prior permission is obtained from the copyright holder.
