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Programming Ruby

The Pragmatic Programmer's Guide

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Basic Input and Output



Ruby provides what at first sight looks like two separate sets of I/O routines. The first is the simple interface---we've been using it pretty much exclusively so far.

print "Enter your name: "
name = gets

There are a whole set of I/O-related methods implemented in the Kernel module---gets, open, print, printf, putc, puts, readline, readlines, and test---that make it simple and convenient to write straightforward Ruby programs. These methods typically do I/O to standard input and standard output, which makes them useful for writing filters. You'll find them documented starting on page 411.

The second way, which gives you a lot more control, is to use IO objects.

What Is an IO Object?

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

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

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 ?. }

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.......
.

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}" }

produces:

Got "This is line one\n"
Got "This is line two\n"
Got "This is line three\n"
Got "And so on...\n"

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

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"

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 }

produces:

This is line one
This is line two
This is line three
And so on...

Or, if you prefer, you can retrieve an entire file into an array of lines:

arr = IO.readlines("testfile")
arr.length 4
arr[0] "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 = "" ""
str << 1 << 2 << 3 "\001\002\003"
 
[ 4, 5, 6 ].pack("c*") "\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

produces:

99 red balloons

Again, the << method uses to_s to convert its arguments to strings before sending them on their merry way.

Talking to Networks

Ruby is fluent in most of the Internet's protocols, both low-level and high-level.

For those who enjoy groveling around at the network level, Ruby comes with a set of classes in the socket library (documented starting on page 469). These classes give you access to TCP, UDP, SOCKS, and Unix domain sockets, as well as any additional socket types supported on your architecture. The library also provides helper classes to make writing servers easier. Here's a simple program that gets information about the ``oracle'' user on our local machine using the finger protocol.

require 'socket'
client = TCPSocket.open('localhost', 'finger')
client.send("oracle\n", 0)    # 0 means standard packet
puts client.readlines
client.close

produces:

Login: oracle         			Name: Oracle installation
Directory: /home/oracle             	Shell: /bin/bash
Never logged in.
No Mail.
No Plan.

At a higher level, the lib/net set of library modules provides handlers for a set of application-level protocols (currently FTP, HTTP, POP, SMTP, and telnet). These are documented starting on page 482. For example, the following program lists the images that are displayed on the Pragmatic Programmer home page.

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

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.

  

 

 

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