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   </STYLE><table bgcolor="#a03030" cellpadding="3" border="0" cellspacing="0"><tr><td colspan="3"><table bgcolor="#902020" cellpadding="20"><tr><td><h1 class="header">Programming Ruby</h1><h3 class="subheader">The Pragmatic Programmer's Guide</h3></td></tr></table></td></tr><tr><td width="33%" align="left"><a class="subheader" href="tut_methods.html">Previous &lt;</a></td><td width="33%" align="center" valign="middle"><a class="subheader" href="index.html">Contents ^</a><br></td><td width="33%" align="right"><a class="subheader" href="tut_exceptions.html">Next ></a><br></td></tr></table></head><body bgcolor="white">
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<h1>Expressions</h1><hr><br>
<P></P>
So far we've been fairly cavalier in our use of expressions in Ruby.
After all, <code>a=b+c</code> is pretty standard
stuff.  You could write a whole heap of Ruby code without reading any
of this chapter.
<P></P>
But it wouldn't be as much fun <code>;-)</code>.
<P></P>
One of the first differences with Ruby is that anything that can
reasonably return a value does: just about everything is an
expression. What does this mean in practice?
<P></P>
Some obvious things include the ability to chain statements together.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="500">
<tr>
  <td valign="top"><code>a&nbsp;=&nbsp;b&nbsp;=&nbsp;c&nbsp;=&nbsp;0</code></td>
  <td valign="top"></td>
  <td valign="top"><code>0</code></td>
</tr>
<tr>
  <td valign="top"><code>[&nbsp;3,&nbsp;1,&nbsp;7,&nbsp;0&nbsp;].sort.reverse</code></td>
  <td valign="top"></td>
  <td valign="top"><code>[7,&nbsp;3,&nbsp;1,&nbsp;0]</code></td>
</tr>
</table>
<P></P>

<P></P>
Perhaps less obvious, things that are normally statements in C
or Java are expressions in Ruby. For example, the <code>if</code> and
<code>case</code> statements both return the value of the last expression
executed.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
songType&nbsp;=&nbsp;if&nbsp;song.mp3Type&nbsp;==&nbsp;MP3::Jazz
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if&nbsp;song.written&nbsp;&lt;&nbsp;Date.new(1935,&nbsp;1,&nbsp;1)
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Song::TradJazz
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;else
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Song::Jazz
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;end
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;else
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Song::Other
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;end
<P></P>
&nbsp;rating&nbsp;=&nbsp;case&nbsp;votesCast
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;when&nbsp;0...10&nbsp;&nbsp;&nbsp;&nbsp;then&nbsp;Rating::SkipThisOne
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;when&nbsp;10...50&nbsp;&nbsp;&nbsp;then&nbsp;Rating::CouldDoBetter
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;else&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Rating::Rave
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;end
</pre></td></tr></table>

<P></P>
We'll talk more about <code>if</code> and <code>case</code> starting
on page 81.
<h2>Operator Expressions</h2>
<P></P>
Ruby has the basic set of operators (+, -, *, /, and so on) as well
as a few surprises. A complete list of the operators, and their
precedences, is given in Table 18.4 on page 221.
<P></P>
In Ruby, many operators are actually method calls. When you write
<code>a*b+c</code>
you're actually asking the object referenced by <code>a</code> to execute the 
method ``<code>*</code>'', passing in the parameter <code>b</code>. You then ask the
object that results from that calculation to execute ``<code>+</code>'',
passing <code>c</code> as a parameter. This is exactly equivalent
to writing
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
(a.*(b)).+(c)
</pre></td></tr></table>

<P></P>
Because everything is an object, and because you can redefine
instance methods, you can always redefine basic arithmetic if you
don't like the answers you're getting.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="500">
<tr>
<td colspan="3" valign="top"><code>class&nbsp;Fixnum</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>&nbsp;&nbsp;alias&nbsp;oldPlus&nbsp;+</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>&nbsp;&nbsp;def&nbsp;+(other)</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>&nbsp;&nbsp;&nbsp;&nbsp;oldPlus(other).succ</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>&nbsp;&nbsp;end</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>end</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code></code></td>
</tr>
<tr>
  <td valign="top"><code>1&nbsp;+&nbsp;2</code></td>
  <td valign="top"></td>
  <td valign="top"><code>4</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>a&nbsp;=&nbsp;3</code></td>
</tr>
<tr>
  <td valign="top"><code>a&nbsp;+=&nbsp;4</code></td>
  <td valign="top"></td>
  <td valign="top"><code>8</code></td>
</tr>
</table>
<P></P>

<P></P>
More useful is the fact that classes that you write can participate in
operator expressions just as if they were built-in objects. For
example, we might want to be able to extract a number of seconds of
music from the middle of a song. We could using the indexing operator
``<code>[]</code>'' to specify the music to be extracted.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
class&nbsp;Song
&nbsp;&nbsp;def&nbsp;[](fromTime,&nbsp;toTime)
&nbsp;&nbsp;&nbsp;&nbsp;result&nbsp;=&nbsp;Song.new(self.title&nbsp;+&nbsp;"&nbsp;[extract]",
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;self.artist,
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;toTime&nbsp;-&nbsp;fromTime)
&nbsp;&nbsp;&nbsp;&nbsp;result.setStartTime(fromTime)
&nbsp;&nbsp;&nbsp;&nbsp;result
&nbsp;&nbsp;end
end
</pre></td></tr></table>

<P></P>
This code fragment extends class <code>Song</code> with the method
``<code>[]</code>'', which takes two parameters (a start time and an end
time).  It returns a new song, with the music clipped to the given
interval.  We could then play the introduction to a song with code
such as:
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
aSong[0,&nbsp;0.15].play
</pre></td></tr></table>

<h2>Miscellaneous Expressions</h2>
<P></P>
As well as the obvious operator expressions and method calls, and the
(perhaps) less obvious statement expressions (such as <code>if</code> and
<code>case</code>), Ruby has a few more things that you can use in
expressions.
<h3>Command Expansion</h3>
<P></P>
If you enclose a string in backquotes, or use the delimited form
prefixed by <code>%x</code>, it will (by default) be executed as a command by
your underlying operating system.
The value of the expression is the
standard output of that command. Newlines will not be stripped, so it is
likely that the value you get back will have a trailing return or
linefeed character. 
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="500">
<tr>
  <td valign="top"><code>`date`</code></td>
  <td valign="top"></td>
  <td valign="top"><code>"Sun&nbsp;Mar&nbsp;&nbsp;4&nbsp;23:23:52&nbsp;CST&nbsp;2001\n"</code></td>
</tr>
<tr>
  <td valign="top"><code>`dir`.split[34]</code></td>
  <td valign="top"></td>
  <td valign="top"><code>"lib_pstore.txi"</code></td>
</tr>
<tr>
  <td valign="top"><code>%x{echo&nbsp;"Hello&nbsp;there"}</code></td>
  <td valign="top"></td>
  <td valign="top"><code>"Hello&nbsp;there\n"</code></td>
</tr>
</table>
<P></P>

<P></P>
You can use expression expansion and all the usual escape sequences in
the command string. 
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
for&nbsp;i&nbsp;in&nbsp;0..3
&nbsp;&nbsp;status&nbsp;=&nbsp;`dbmanager&nbsp;status&nbsp;id=#{i}`
&nbsp;&nbsp;#&nbsp;...
end
</pre></td></tr></table>

<P></P>
The exit status of the command is available in the global variable
<code>$?</code>.
<h3>Backquotes Are Soft</h3>
<P></P>
In the description of the command output expression, we said that the
string in backquotes would ``by default'' be executed as a command. In 
fact, the string is passed to the method called <a href="ref_m_kernel.html#_bq"><code>Kernel::`</code></a>
(a single backquote). If you want, you can override
this.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
alias&nbsp;oldBackquote&nbsp;`
def&nbsp;`(cmd)
&nbsp;&nbsp;result&nbsp;=&nbsp;oldBackquote(cmd)
&nbsp;&nbsp;if&nbsp;$?&nbsp;!=&nbsp;0
&nbsp;&nbsp;&nbsp;&nbsp;raise&nbsp;"Command&nbsp;#{cmd}&nbsp;failed"
&nbsp;&nbsp;end
&nbsp;&nbsp;result
end
print&nbsp;`date`
print&nbsp;`data`
</pre></td></tr></table>

<em>produces:</em>
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
Sun&nbsp;Mar&nbsp;&nbsp;4&nbsp;23:23:52&nbsp;CST&nbsp;2001
prog.rb:3:&nbsp;command&nbsp;not&nbsp;found:&nbsp;data
prog.rb:5:in&nbsp;``':&nbsp;Command&nbsp;data&nbsp;failed&nbsp;(RuntimeError)
	from&nbsp;prog.rb:10
</pre></td></tr></table>

<h2>Assignment</h2>
<P></P>
Just about every example we've given so far in this book has featured
assignment. Perhaps it's about time we said something about it.
<P></P>
An assignment statement sets the variable or attribute on its left
side (the <em>lvalue</em>) to refer to the value on the right (the
<em>rvalue</em>).
It then returns that value as the result of the
assignment expression.  This means that you can chain assignments and
that you can
perform assignments in some unexpected places.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="500">
<tr>
<td colspan="3" valign="top"><code>a&nbsp;=&nbsp;b&nbsp;=&nbsp;1&nbsp;+&nbsp;2&nbsp;+&nbsp;3</code></td>
</tr>
<tr>
  <td valign="top"><code>a</code></td>
  <td valign="top"></td>
  <td valign="top"><code>6</code></td>
</tr>
<tr>
  <td valign="top"><code>b</code></td>
  <td valign="top"></td>
  <td valign="top"><code>6</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>a&nbsp;=&nbsp;(b&nbsp;=&nbsp;1&nbsp;+&nbsp;2)&nbsp;+&nbsp;3</code></td>
</tr>
<tr>
  <td valign="top"><code>a</code></td>
  <td valign="top"></td>
  <td valign="top"><code>6</code></td>
</tr>
<tr>
  <td valign="top"><code>b</code></td>
  <td valign="top"></td>
  <td valign="top"><code>3</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>File.open(name&nbsp;=&nbsp;gets.chomp)</code></td>
</tr>
</table>
<P></P>

<P></P>
There are two basic forms of assignment in Ruby. The first assigns an
object reference to a variable or constant.  This form of assignment
is hard-wired into the language.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
instrument&nbsp;=&nbsp;"piano"
MIDDLE_A&nbsp;&nbsp;&nbsp;=&nbsp;440
</pre></td></tr></table>

<P></P>
The second form of assignment involves having an object attribute or
element reference on the left-hand side. 
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
aSong.duration&nbsp;&nbsp;&nbsp;&nbsp;=&nbsp;234
instrument["ano"]&nbsp;=&nbsp;"ccolo"
</pre></td></tr></table>

<P></P>
These forms are special, because they are implemented by calling
methods in the lvalues, which means you can override them.
<P></P>
We've already seen how to define a writable object attribute. Simply
define a method name ending in an equals sign. This method receives as 
its parameter the assignment's rvalue.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
class&nbsp;Song
&nbsp;&nbsp;def&nbsp;duration=(newDuration)
&nbsp;&nbsp;&nbsp;&nbsp;@duration&nbsp;=&nbsp;newDuration
&nbsp;&nbsp;end
end
</pre></td></tr></table>

<P></P>
There is no reason that these attribute setting methods must
correspond with internal instance variables, or that there has to be
an attribute reader for every attribute writer (or vice versa).
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
class&nbsp;Amplifier
&nbsp;&nbsp;def&nbsp;volume=(newVolume)
&nbsp;&nbsp;&nbsp;&nbsp;self.leftChannel&nbsp;=&nbsp;self.rightChannel&nbsp;=&nbsp;newVolume
&nbsp;&nbsp;end
&nbsp;&nbsp;#&nbsp;...
end
</pre></td></tr></table>

<P></P>
<p></p><table width="500" border="2" cellpadding="15" bgcolor="#ffe0e0" align="center"><tr><td align="center"><b>Sidebar: Using Accessors Within a Class</b></td></tr><tr><td>
<P></P>
  Why did we write <code>self.leftChannel</code> in the example on page
  76? Well, there's a hidden gotcha with writable
  attributes. Normally, methods within a class can invoke other
  methods in the same class and its superclasses in functional form
  (that is, with an implicit receiver of <code>self</code>). However, this
  doesn't work with attribute writers. Ruby sees the assignment and
  decides that the name on the left must be a local variable, not a
  method call to an attribute writer.    

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="500">
<tr>
<td colspan="3" valign="top"><code>class&nbsp;BrokenAmplifier</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>&nbsp;&nbsp;attr_accessor&nbsp;:leftChannel,&nbsp;:rightChannel</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>&nbsp;&nbsp;def&nbsp;volume=(vol)</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>&nbsp;&nbsp;&nbsp;&nbsp;leftChannel&nbsp;=&nbsp;self.rightChannel&nbsp;=&nbsp;vol</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>&nbsp;&nbsp;end</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>end</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code></code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>ba&nbsp;=&nbsp;BrokenAmplifier.new</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>ba.leftChannel&nbsp;=&nbsp;ba.rightChannel&nbsp;=&nbsp;99</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>ba.volume&nbsp;=&nbsp;5</code></td>
</tr>
<tr>
  <td valign="top"><code>ba.leftChannel</code></td>
  <td valign="top"></td>
  <td valign="top"><code>99</code></td>
</tr>
<tr>
  <td valign="top"><code>ba.rightChannel</code></td>
  <td valign="top"></td>
  <td valign="top"><code>5</code></td>
</tr>
</table>
<P></P>

We forgot to put ``<code>self.</code>'' in front of the assignment to
<code>leftChannel</code>, so Ruby stored the new value in a local variable of
method <code>volume=</code>; the object's attribute never got updated.
This can be a tricky bug to track down.
<P></P>
</td></tr></table><p></p>
<h3>Parallel Assignment</h3>
<P></P>
During your first week in a programming course (or the second semester
if it was a party school), you may have had to write code to swap the
values in two variables:
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
int&nbsp;a&nbsp;=&nbsp;1;
int&nbsp;b&nbsp;=&nbsp;2;
int&nbsp;temp;
<P></P>
temp&nbsp;=&nbsp;a;
a&nbsp;=&nbsp;b;
b&nbsp;=&nbsp;temp;
</pre></td></tr></table>

<P></P>
You can do this much more cleanly in Ruby:
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
a,&nbsp;b&nbsp;=&nbsp;b,&nbsp;a
</pre></td></tr></table>

<P></P>
Ruby assignments are effectively performed in parallel, so the values
assigned are not affected by the assignment itself.  The values on the
right-hand side are evaluated in the order in which they appear before any
assignment is made to variables or attributes on the left. A somewhat
contrived example illustrates this. The second line assigns to the
variables <code>a</code>, <code>b</code>, and <code>c</code> the values of the expressions
<code>x</code>, <code>x+=1</code>, and <code>x+=1</code>, respectively.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="500">
<tr>
  <td valign="top"><code>x&nbsp;=&nbsp;0</code></td>
  <td valign="top"></td>
  <td valign="top"><code>0</code></td>
</tr>
<tr>
  <td valign="top"><code>a,&nbsp;b,&nbsp;c&nbsp;&nbsp;&nbsp;=&nbsp;&nbsp;&nbsp;x,&nbsp;(x&nbsp;+=&nbsp;1),&nbsp;(x&nbsp;+=&nbsp;1)</code></td>
  <td valign="top"></td>
  <td valign="top"><code>[0,&nbsp;1,&nbsp;2]</code></td>
</tr>
</table>
<P></P>

<P></P>
When an assignment has more than one lvalue, the assignment expression
returns an array of the rvalues.
If an assignment contains more lvalues than rvalues, the excess
lvalues are
set to <code>nil</code>. If a multiple assignment contains more rvalues than
lvalues, the extra rvalues are ignored. As of Ruby 1.6.2, if an
assignment has one lvalue and multiple rvalues, the rvalues are
converted to an array and assigned to the lvalue.
<P></P>
You can collapse and expand arrays using Ruby's parallel assignment
operator.  If the last lvalue is preceded by an asterisk, all the
remaining rvalues will be collected and assigned to that lvalue as an
array. Similarly, if the last rvalue is an array, you can prefix it
with an asterisk, which effectively expands it into its constituent
values in place.  (This is not necessary if the rvalue is the only
thing on the right-hand side---the array will be expanded
automatically.)
<P></P>

  <table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3">
<tr>
<td colspan="4" valign="top"><code>a = [1, 2, 3, 4]</code></td>
</tr>
<tr>
  <td valign="top">b,&nbsp;&nbsp;c&nbsp;=&nbsp;a</td>
  <td valign="top"> </td>
  <td valign="top">b == 1,</td>
  <td valign="top">c == 2</td>
</tr>
<tr>
  <td valign="top">b,&nbsp;*c&nbsp;=&nbsp;a</td>
  <td valign="top"> </td>
  <td valign="top">b == 1,</td>
  <td valign="top">c == [2, 3, 4]</td>
</tr>
<tr>
  <td valign="top">b,&nbsp;&nbsp;c&nbsp;=&nbsp;99,&nbsp;&nbsp;a</td>
  <td valign="top"> </td>
  <td valign="top">b == 99,</td>
  <td valign="top">c == [1, 2, 3, 4]</td>
</tr>
<tr>
  <td valign="top">b,&nbsp;*c&nbsp;=&nbsp;99,&nbsp;&nbsp;a</td>
  <td valign="top"> </td>
  <td valign="top">b == 99,</td>
  <td valign="top">c == [[1, 2, 3, 4]]</td>
</tr>
<tr>
  <td valign="top">b,&nbsp;&nbsp;c&nbsp;=&nbsp;99,&nbsp;*a</td>
  <td valign="top"> </td>
  <td valign="top">b == 99,</td>
  <td valign="top">c == 1</td>
</tr>
<tr>
  <td valign="top">b,&nbsp;*c&nbsp;=&nbsp;99,&nbsp;*a</td>
  <td valign="top"> </td>
  <td valign="top">b == 99,</td>
  <td valign="top">c == [1, 2, 3, 4]</td>
</tr>
</table>
<P></P>

<h3>Nested Assignments</h3>
<P></P>
Parallel assignments have one more feature worth mentioning.
The left-hand side of an assignment may contain a parenthesized list of
terms. Ruby treats these terms as if they were a nested assignment
statement. It extracts out the corresponding rvalue, assigning it to
the parenthesized terms, before continuing with the higher-level
assignment.
<P></P>

  <table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3">
<tr>
  <td valign="top">b,&nbsp;(c,&nbsp;d),&nbsp;e&nbsp;=&nbsp;1,2,3,4</td>
  <td valign="top"> </td>
  <td valign="top">b == 1,</td>
  <td valign="top">c == 2,</td>
  <td valign="top">d == nil,</td>
  <td valign="top">e == 3</td>
</tr>
<tr>
  <td valign="top">b,&nbsp;(c,&nbsp;d),&nbsp;e&nbsp;=&nbsp;[1,2,3,4]</td>
  <td valign="top"> </td>
  <td valign="top">b == 1,</td>
  <td valign="top">c == 2,</td>
  <td valign="top">d == nil,</td>
  <td valign="top">e == 3</td>
</tr>
<tr>
  <td valign="top">b,&nbsp;(c,&nbsp;d),&nbsp;e&nbsp;=&nbsp;1,[2,3],4</td>
  <td valign="top"> </td>
  <td valign="top">b == 1,</td>
  <td valign="top">c == 2,</td>
  <td valign="top">d == 3,</td>
  <td valign="top">e == 4</td>
</tr>
<tr>
  <td valign="top">b,&nbsp;(c,&nbsp;d),&nbsp;e&nbsp;=&nbsp;1,[2,3,4],5</td>
  <td valign="top"> </td>
  <td valign="top">b == 1,</td>
  <td valign="top">c == 2,</td>
  <td valign="top">d == 3,</td>
  <td valign="top">e == 5</td>
</tr>
<tr>
  <td valign="top">b,&nbsp;(c,*d),&nbsp;e&nbsp;=&nbsp;1,[2,3,4],5</td>
  <td valign="top"> </td>
  <td valign="top">b == 1,</td>
  <td valign="top">c == 2,</td>
  <td valign="top">d == [3, 4],</td>
  <td valign="top">e == 5</td>
</tr>
</table>
<P></P>

<h3>Other Forms of Assignment</h3>
<P></P>
In common with many other languages, Ruby has a syntactic shortcut:
<code>a=a+2</code> may be written as <code>a+=2</code>.
<P></P>
The second form is converted internally to the first. This means that
operators that you have defined as methods in your own classes work as
you'd expect.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="500">
<tr>
<td colspan="3" valign="top"><code>class&nbsp;Bowdlerize</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>&nbsp;&nbsp;def&nbsp;initialize(aString)</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>&nbsp;&nbsp;&nbsp;&nbsp;@value&nbsp;=&nbsp;aString.gsub(/[aeiou]/,&nbsp;'*')</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>&nbsp;&nbsp;end</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>&nbsp;&nbsp;def&nbsp;+(other)</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>&nbsp;&nbsp;&nbsp;&nbsp;Bowdlerize.new(self.to_s&nbsp;+&nbsp;other.to_s)</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>&nbsp;&nbsp;end</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>&nbsp;&nbsp;def&nbsp;to_s</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>&nbsp;&nbsp;&nbsp;&nbsp;@value</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>&nbsp;&nbsp;end</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>end</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code></code></td>
</tr>
<tr>
  <td valign="top"><code>a&nbsp;=&nbsp;Bowdlerize.new("damn&nbsp;")</code></td>
  <td valign="top"></td>
  <td valign="top"><code>d*mn</code></td>
</tr>
<tr>
  <td valign="top"><code>a&nbsp;+=&nbsp;"shame"</code></td>
  <td valign="top"></td>
  <td valign="top"><code>d*mn&nbsp;sh*m*</code></td>
</tr>
</table>
<P></P>

<h2>Conditional Execution</h2>
<P></P>
Ruby has several different mechanisms for conditional execution of
code; most of them should feel familiar, and many have some neat
twists. Before we get into them, though, we need to spend a short time 
looking at boolean expressions.
<h3>Boolean Expressions</h3>
<P></P>
Ruby has a simple definition of truth. Any value that is not <code>nil</code> or
the constant <code>false</code> is true. You'll find that the library
routines use this fact consistently. For example, <a href="ref_c_io.html#gets"><code>IO#gets</code></a>,
which returns the next line from a file, returns <code>nil</code> at end of
file, enabling you to write loops such as:
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
while&nbsp;line&nbsp;=&nbsp;gets
&nbsp;&nbsp;#&nbsp;process&nbsp;line
end
</pre></td></tr></table>

<P></P>
However, there's a trap here for C, C++, and Perl
programmers. The number zero is <em>not</em> interpreted as a false
value. Neither is a zero-length string. This can be a tough habit to
break.
<h3>Defined?, And, Or, and Not</h3>
<P></P>
Ruby supports all the standard boolean operators and introduces the
new operator <code>defined?</code>.
<P></P>
Both ``<code>and</code>'' and ``<code>&amp;&amp;</code>''
evaluate to true only if both operands are
true. They evaluate the second operand only if the first is true
(this is sometimes known as ``short-circuit evaluation'').  The only
difference in the two forms is precedence (``<code>and</code>'' binds lower than
``<code>&amp;&amp;</code>'').
<P></P>
Similarly, both ``<code>or</code>'' and ``<code>||</code>''
evaluate to true if either operand
is true.  They evaluate their second operand only if the first is
false.  As with ``<code>and</code>'', the only difference between ``<code>or</code>'' and
``<code>||</code>'' is their precedence.
<P></P>
Just to make life interesting, ``<code>and</code>'' and ``<code>or</code>'' have the
same precedence, while ``<code>&amp;&amp;</code>'' has a higher precedence than
``<code>||</code>''.
<P></P>
``<code>not</code>'' and ``<code>!</code>''
return the opposite of their operand (false if the
operand is true, and true if the operand is false). And, yes, ``<code>not</code>''
and ``<code>!</code>'' differ only in precedence.
<P></P>
All these precedence rules are summarized in Table
18.4 on page 221.
<P></P>
The <code>defined?</code>
operator returns <code>nil</code> if its argument (which can be
an arbitrary expression) is not defined, otherwise it returns a
description of that argument. If the argument is <code>yield</code>,
<code>defined?</code> returns the string ``yield'' if a code block is
associated with the current context.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="500">
<tr>
  <td valign="top"><code>defined?&nbsp;1</code></td>
  <td valign="top"></td>
  <td valign="top"><code>"expression"</code></td>
</tr>
<tr>
  <td valign="top"><code>defined?&nbsp;dummy</code></td>
  <td valign="top"></td>
  <td valign="top"><code>nil</code></td>
</tr>
<tr>
  <td valign="top"><code>defined?&nbsp;printf</code></td>
  <td valign="top"></td>
  <td valign="top"><code>"method"</code></td>
</tr>
<tr>
  <td valign="top"><code>defined?&nbsp;String</code></td>
  <td valign="top"></td>
  <td valign="top"><code>"constant"</code></td>
</tr>
<tr>
  <td valign="top"><code>defined?&nbsp;$&amp;</code></td>
  <td valign="top"></td>
  <td valign="top"><code>"$&amp;"</code></td>
</tr>
<tr>
  <td valign="top"><code>defined?&nbsp;$_</code></td>
  <td valign="top"></td>
  <td valign="top"><code>"global-variable"</code></td>
</tr>
<tr>
  <td valign="top"><code>defined?&nbsp;Math::PI</code></td>
  <td valign="top"></td>
  <td valign="top"><code>"constant"</code></td>
</tr>
<tr>
  <td valign="top"><code>defined?&nbsp;(&nbsp;c,d&nbsp;=&nbsp;1,2&nbsp;)</code></td>
  <td valign="top"></td>
  <td valign="top"><code>"assignment"</code></td>
</tr>
<tr>
  <td valign="top"><code>defined?&nbsp;42.abs</code></td>
  <td valign="top"></td>
  <td valign="top"><code>"method"</code></td>
</tr>
</table>
<P></P>

<P></P>
In addition to the boolean operators, Ruby objects support comparison
using the methods <code>==</code>, <code>===</code>, <code>&lt;=></code>, <code>=~</code>, <code>eql?</code>,
and <code>equal?</code> (see Table 7.1 on page 81). All but <code>&lt;=></code>
are defined in class <code>Object</code> but are often overridden by
descendents to provide appropriate semantics. For example, class
<code>Array</code> redefines <code>==</code> so that two array objects are equal if
they have the same number of elements and corresponding elements are
equal.
<P></P>
<table border="2" width="500" bgcolor="#ffe0e0"><tr><td>
  <b>Common comparison operators</b>            
  <table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3">
<tr bgcolor="#ff9999">
  <td valign="top"><b>Operator</b></td>
  <td valign="top"><b>Meaning</b></td>
</tr>
<tr>
  <td valign="top"><code>==</code></td>
  <td valign="top">Test for equal value.</td>
</tr>
<tr>
  <td valign="top"><code>===</code></td>
  <td valign="top">Used to test equality within a <code>when</code> clause of a
                <code>case</code> statement.</td>
</tr>
<tr>
  <td valign="top"><code>&lt;=></code></td>
  <td valign="top">General comparison operator. Returns -1, 0, or +1,
                depending on whether its receiver is less than, equal to, or
                greater than its argument.</td>
</tr>
<tr>
  <td valign="top"><code>&lt;</code>, <code>&lt;=</code>, <code>>=</code>, <code>></code></td>
  <td valign="top">Comparison operators for less than, less than or
                equal, greater than or equal, and greater than.</td>
</tr>
<tr>
  <td valign="top"><code>=~</code></td>
  <td valign="top">Regular expression pattern match.</td>
</tr>
<tr>
  <td valign="top"><code>eql?</code></td>
  <td valign="top">True if the receiver and argument have both the same
                type and equal values. 1 == 1.0 returns <code>true</code>, 
                but 1.eql?(1.0) is <code>false</code>.</td>
</tr>
<tr>
  <td valign="top"><code>equal?</code></td>
  <td valign="top">True if the receiver and argument have the same
                object id.</td>
</tr>
<tr><td colspan="9" bgcolor="#ff9999" height="2"><img src="dot.gif" width="1" height="1"></td></tr></table>
<P></P>
</td></tr></table>
<P></P>
Both <code>==</code> and <code>=~</code> have negated forms, <code>!=</code> and <code>!~</code>.
However, these are converted by Ruby when your program is read.
<code>a!=b</code> is equivalent to <code>!(a==b)</code>,
and <code>a!~b</code> is the
same as <code>!(a=~b)</code>.  This means that if you write a class that
overrides <code>==</code> or <code>=~</code> you get a working <code>!=</code> and <code>!~</code>
for free. But on the flip side, this also means that you cannot define 
<code>!=</code> and <code>!~</code> independent of <code>==</code> and <code>=~</code>, respectively.
<P></P>
You can use a Ruby range as a boolean expression.
A
range such as <code>exp1..exp2</code> will evaluate as false
until <code>exp1</code> becomes true. The range will then evaluate as true
until <code>exp2</code> becomes true. Once this happens, the range resets,
ready to fire again. We show some examples of this
on page 84. 
<P></P>
Finally, you can use a bare regular expression as a boolean
expression. Ruby expands it to <code>$_=~/re/</code>.
<h3>If and Unless Expressions</h3>
<P></P>
An <code>if</code> expression in Ruby is pretty similar to ``if'' statements
in other languages.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
if&nbsp;aSong.artist&nbsp;==&nbsp;"Gillespie"&nbsp;then
&nbsp;&nbsp;handle&nbsp;=&nbsp;"Dizzy"
elsif&nbsp;aSong.artist&nbsp;==&nbsp;"Parker"&nbsp;then
&nbsp;&nbsp;handle&nbsp;=&nbsp;"Bird"
else
&nbsp;&nbsp;handle&nbsp;=&nbsp;"unknown"
end
</pre></td></tr></table>

<P></P>
If you lay out your <code>if</code> statements on multiple lines, you can
leave off the <code>then</code> keyword.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
if&nbsp;aSong.artist&nbsp;==&nbsp;"Gillespie"
&nbsp;&nbsp;handle&nbsp;=&nbsp;"Dizzy"
elsif&nbsp;aSong.artist&nbsp;==&nbsp;"Parker"
&nbsp;&nbsp;handle&nbsp;=&nbsp;"Bird"
else
&nbsp;&nbsp;handle&nbsp;=&nbsp;"unknown"
end
</pre></td></tr></table>

<P></P>
However, if you lay your code out more tightly, the <code>then</code> keyword
is necessary to separate the boolean expression from the following
statements.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
if&nbsp;aSong.artist&nbsp;==&nbsp;"Gillespie"&nbsp;then&nbsp;&nbsp;handle&nbsp;=&nbsp;"Dizzy"
elsif&nbsp;aSong.artist&nbsp;==&nbsp;"Parker"&nbsp;then&nbsp;&nbsp;handle&nbsp;=&nbsp;"Bird"
else&nbsp;&nbsp;handle&nbsp;=&nbsp;"unknown"
end
</pre></td></tr></table>

<P></P>
You can have zero or more <code>elsif</code> clauses and an optional
<code>else</code> clause.
<P></P>
As we've said before, <code>if</code> is
an expression, not a statement---it returns a value.  You don't have
to use the value of an <code>if</code> expression, but it can come in handy.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
handle&nbsp;=&nbsp;if&nbsp;aSong.artist&nbsp;==&nbsp;"Gillespie"&nbsp;then
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;"Dizzy"
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;elsif&nbsp;aSong.artist&nbsp;==&nbsp;"Parker"&nbsp;then
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;"Bird"
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;else
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;"unknown"
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;end
</pre></td></tr></table>

<P></P>
Ruby also has a negated form of the <code>if</code> statement:
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
unless&nbsp;aSong.duration&nbsp;>&nbsp;180&nbsp;then
&nbsp;&nbsp;cost&nbsp;=&nbsp;.25
else
&nbsp;&nbsp;cost&nbsp;=&nbsp;.35
end
</pre></td></tr></table>

<P></P>
Finally, for the C fans out there, Ruby also supports the C-style
conditional expression:
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
cost&nbsp;=&nbsp;aSong.duration&nbsp;>&nbsp;180&nbsp;?&nbsp;.35&nbsp;:&nbsp;.25
</pre></td></tr></table>

<P></P>
The conditional expression returns the value of either the expression
before or the expression after the colon, depending on whether the
boolean expression before the question mark evaluates to <code>true</code>
or <code>false</code>. In this case, if the song duration is greater than 3 
minutes, the expression returns .35. For shorter songs, it returns
.25. Whatever the result, it is then assigned to <code>cost</code>.
<h3>If and Unless Modifiers</h3>
<P></P>
Ruby shares a neat feature with Perl. Statement modifiers let you tack
conditional statements onto the end of a normal statement.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
mon,&nbsp;day,&nbsp;year&nbsp;=&nbsp;$1,&nbsp;$2,&nbsp;$3&nbsp;if&nbsp;/(\d\d)-(\d\d)-(\d\d)/
puts&nbsp;"a&nbsp;=&nbsp;#{a}"&nbsp;if&nbsp;fDebug
print&nbsp;total&nbsp;unless&nbsp;total&nbsp;==&nbsp;0
</pre></td></tr></table>

<P></P>
For an <code>if</code> modifier, the preceding expression will be evaluated only
if the condition is true. <code>unless</code> works the other way around.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
while&nbsp;gets
&nbsp;&nbsp;next&nbsp;if&nbsp;/^#/&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;#&nbsp;Skip&nbsp;comments
&nbsp;&nbsp;parseLine&nbsp;unless&nbsp;/^$/&nbsp;&nbsp;&nbsp;#&nbsp;Don't&nbsp;parse&nbsp;empty&nbsp;lines
end
</pre></td></tr></table>

<P></P>
Because <code>if</code> itself is an expression, you can get really obscure
with statements such as:
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
if&nbsp;artist&nbsp;==&nbsp;"John&nbsp;Coltrane"
&nbsp;&nbsp;artist&nbsp;=&nbsp;"'Trane"
end&nbsp;unless&nbsp;nicknames&nbsp;==&nbsp;"no"
</pre></td></tr></table>

<P></P>
This path leads to the gates of madness.
<h2>Case Expressions</h2>
<P></P>
The Ruby <code>case</code> expression is a powerful beast: a multiway <code>if</code>
on steroids.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
case&nbsp;inputLine
<P></P>
&nbsp;&nbsp;when&nbsp;"debug"
&nbsp;&nbsp;&nbsp;&nbsp;dumpDebugInfo
&nbsp;&nbsp;&nbsp;&nbsp;dumpSymbols
<P></P>
&nbsp;&nbsp;when&nbsp;/p\s+(\w+)/
&nbsp;&nbsp;&nbsp;&nbsp;dumpVariable($1)
<P></P>
&nbsp;&nbsp;when&nbsp;"quit",&nbsp;"exit"
&nbsp;&nbsp;&nbsp;&nbsp;exit
<P></P>
&nbsp;&nbsp;else
&nbsp;&nbsp;&nbsp;&nbsp;print&nbsp;"Illegal&nbsp;command:&nbsp;#{inputLine}"
end
</pre></td></tr></table>

<P></P>
As with <code>if</code>, <code>case</code> returns the value of the last expression
executed, and you also need a <code>then</code> keyword if the
expression is on the same line as the condition.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
kind&nbsp;=&nbsp;case&nbsp;year
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;when&nbsp;1850..1889&nbsp;then&nbsp;"Blues"
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;when&nbsp;1890..1909&nbsp;then&nbsp;"Ragtime"
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;when&nbsp;1910..1929&nbsp;then&nbsp;"New&nbsp;Orleans&nbsp;Jazz"
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;when&nbsp;1930..1939&nbsp;then&nbsp;"Swing"
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;when&nbsp;1940..1950&nbsp;then&nbsp;"Bebop"
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;else&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;"Jazz"
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;end
</pre></td></tr></table>

<P></P>
<code>case</code> operates by comparing the target (the expression after the
keyword <code>case</code>) with each of the comparison expressions after the
<code>when</code> keywords.  This test is done using
<em>comparison</em>&nbsp;<code>===</code>&nbsp;<em>target</em>. 
As long as a class defines
meaningful semantics for <code>===</code> (and all the built-in classes do),
objects of that class can be used in case expressions.
<P></P>
For example, regular expressions define <code>===</code> as a simple pattern match.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
case&nbsp;line
&nbsp;&nbsp;when&nbsp;/title=(.*)/
&nbsp;&nbsp;&nbsp;&nbsp;puts&nbsp;"Title&nbsp;is&nbsp;#$1"
&nbsp;&nbsp;when&nbsp;/track=(.*)/
&nbsp;&nbsp;&nbsp;&nbsp;puts&nbsp;"Track&nbsp;is&nbsp;#$1"
&nbsp;&nbsp;when&nbsp;/artist=(.*)/
&nbsp;&nbsp;&nbsp;&nbsp;puts&nbsp;"Artist&nbsp;is&nbsp;#$1"
end
</pre></td></tr></table>

<P></P>
Ruby classes are instances of class <code>Class</code>, which defines <code>===</code>
as a test to see if the argument is an instance of the class or one of
its superclasses. So (abandoning the benefits of polymorphism and
bringing the gods of refactoring down around your ears), you can test
the class of objects:
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
case&nbsp;shape
&nbsp;&nbsp;when&nbsp;Square,&nbsp;Rectangle
&nbsp;&nbsp;&nbsp;&nbsp;#&nbsp;...
&nbsp;&nbsp;when&nbsp;Circle
&nbsp;&nbsp;&nbsp;&nbsp;#&nbsp;...
&nbsp;&nbsp;when&nbsp;Triangle
&nbsp;&nbsp;&nbsp;&nbsp;#&nbsp;...
&nbsp;&nbsp;else
&nbsp;&nbsp;&nbsp;&nbsp;#&nbsp;...
end
</pre></td></tr></table>

<h2>Loops</h2>
<P></P>
Don't tell anyone, but Ruby has pretty primitive built-in looping
constructs.
<P></P>
The <code>while</code> loop executes its body zero or more times as long as
its condition is true. For example, this common idiom reads until
the input is exhausted.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
while&nbsp;gets
&nbsp;&nbsp;#&nbsp;...
end
</pre></td></tr></table>

<P></P>
There's also a negated form that executes the body until the
condition becomes true.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
until&nbsp;playList.duration&nbsp;>&nbsp;60
&nbsp;&nbsp;playList.add(songList.pop)
end
</pre></td></tr></table>

<P></P>
As with <code>if</code> and <code>unless</code>, both of the loops can also be used
as statement modifiers.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
a&nbsp;*=&nbsp;2&nbsp;while&nbsp;a&nbsp;&lt;&nbsp;100
a&nbsp;-=&nbsp;10&nbsp;until&nbsp;a&nbsp;&lt;&nbsp;100
</pre></td></tr></table>

<P></P>
On page 80 in the section on boolean
expressions,
we said that a range can be used as a kind of flip-flop, returning true
when some event happens and then staying true until a second event occurs.
This facility is normally used within loops. In the example that
follows, we read a text file containing the first ten ordinal numbers
(``first,'' ``second,'' and so on)
but only print the lines starting with the one that matches
``third'' and ending with the one that matches ``fifth.''
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
file&nbsp;=&nbsp;File.open("ordinal")
while&nbsp;file.gets
&nbsp;&nbsp;print&nbsp;&nbsp;if&nbsp;/third/&nbsp;..&nbsp;/fifth/
end
</pre></td></tr></table>

<em>produces:</em>
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
third
fourth
fifth
</pre></td></tr></table>

<P></P>
The elements of a range used in a boolean expression can themselves be
expressions. These are evaluated each time the overall boolean
expression is evaluated. For example, the following code uses the fact 
that the variable <code>$.</code> contains the current input line number to 
display line numbers one through three and those between a match of
<code>/eig/</code> and <code>/nin/</code>.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
file&nbsp;=&nbsp;File.open("ordinal")
while&nbsp;file.gets
&nbsp;&nbsp;print&nbsp;if&nbsp;($.&nbsp;==&nbsp;1)&nbsp;||&nbsp;/eig/&nbsp;..&nbsp;($.&nbsp;==&nbsp;3)&nbsp;||&nbsp;/nin/
end
</pre></td></tr></table>

<em>produces:</em>
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
first
second
third
eighth
ninth
</pre></td></tr></table>

<P></P>
There's one wrinkle when <code>while</code> and <code>until</code> are used as statement
modifiers. If the statement they are modifying is a
<code>begin</code>/<code>end</code> block,
the code in the block will always execute
at least one time, regardless of the value of the boolean expression.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
print&nbsp;"Hello\n"&nbsp;while&nbsp;false
begin
&nbsp;&nbsp;print&nbsp;"Goodbye\n"
end&nbsp;while&nbsp;false
</pre></td></tr></table>

<em>produces:</em>
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
Goodbye
</pre></td></tr></table>

<h3>Iterators</h3>
<P></P>
If you read the beginning of the previous section, you might have been
discouraged. ``Ruby has pretty primitive built-in looping
constructs,'' it said. Don't despair, gentle reader, for there's good
news. Ruby doesn't need any sophisticated built-in loops, because all
the fun stuff is implemented using Ruby iterators.
<P></P>
For example, Ruby doesn't have a ``for'' loop---at least not the kind
you'd find in C, C++, and Java. Instead, Ruby uses methods defined in
various built-in classes to provide equivalent, but less error-prone,
functionality.
<P></P>
Let's look at some examples.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
3.times&nbsp;do
&nbsp;&nbsp;print&nbsp;"Ho!&nbsp;"
end
</pre></td></tr></table>

<em>produces:</em>
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
Ho!&nbsp;Ho!&nbsp;Ho!
</pre></td></tr></table>

<P></P>
It's easy to avoid fencepost and off-by-1 errors; this loop will
execute three times, period.  In addition to <code>times</code>, integers
can loop over specific ranges by calling <code>downto</code>,
<code>upto</code>, and <code>step</code>. For instance, a traditional ``for''
loop that runs from 0 to 9 (something like <code>i=0; i &lt; 10; i++</code>)
is written as follows.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
0.upto(9)&nbsp;do&nbsp;|x|
&nbsp;&nbsp;print&nbsp;x,&nbsp;"&nbsp;"
end
</pre></td></tr></table>

<em>produces:</em>
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
0&nbsp;1&nbsp;2&nbsp;3&nbsp;4&nbsp;5&nbsp;6&nbsp;7&nbsp;8&nbsp;9
</pre></td></tr></table>

<P></P>
A loop from 0 to 12 by 3 can be written as follows.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
0.step(12,&nbsp;3)&nbsp;{|x|&nbsp;print&nbsp;x,&nbsp;"&nbsp;"&nbsp;}
</pre></td></tr></table>

<em>produces:</em>
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
0&nbsp;3&nbsp;6&nbsp;9&nbsp;12
</pre></td></tr></table>

<P></P>
Similarly, iterating over arrays and other containers is made easy
using their <code>each</code> method.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
[&nbsp;1,&nbsp;1,&nbsp;2,&nbsp;3,&nbsp;5&nbsp;].each&nbsp;{|val|&nbsp;print&nbsp;val,&nbsp;"&nbsp;"&nbsp;}
</pre></td></tr></table>

<em>produces:</em>
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
1&nbsp;1&nbsp;2&nbsp;3&nbsp;5
</pre></td></tr></table>

<P></P>
And once a class supports <code>each</code>, the additional methods in the
<code>Enumerable</code> module (documented beginning on page 407
and summarized on pages 104--105)
become available. For example, the <code>File</code> class provides an
<code>each</code> method, which returns each line of a file in turn. Using
the <code>grep</code> method in <code>Enumerable</code>, we could iterate over only
those lines that meet a certain condition.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
File.open("ordinal").grep&nbsp;/d$/&nbsp;do&nbsp;|line|
&nbsp;&nbsp;print&nbsp;line
end
</pre></td></tr></table>

<em>produces:</em>
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
second
third
</pre></td></tr></table>

<P></P>
Last, and probably least, is the most basic loop of all. Ruby provides 
a built-in iterator called <code>loop</code>.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
loop&nbsp;{
&nbsp;&nbsp;#&nbsp;block&nbsp;...
}
</pre></td></tr></table>

<P></P>
The <code>loop</code> iterator calls the associated block forever (or at
least until you break out of the loop, but you'll have to read ahead
to find out how to do that).
<h3>For ... In</h3>
<P></P>
Earlier we said that the only built-in Ruby looping primitives were
<code>while</code> and <code>until</code>. What's this ``<code>for</code>'' thing, then?
Well, <code>for</code> is almost a lump of syntactic sugar. When you write
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
for&nbsp;aSong&nbsp;in&nbsp;songList
&nbsp;&nbsp;aSong.play
end
</pre></td></tr></table>

<P></P>
Ruby translates it into something like:
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
songList.each&nbsp;do&nbsp;|aSong|
&nbsp;&nbsp;aSong.play
end
</pre></td></tr></table>

<P></P>
The only difference between the <code>for</code> loop and the <code>each</code>
form is the scope of local variables that are defined in the body.
This is discussed on page 89.
<P></P>
You can use <code>for</code>
to iterate over any object that responds to the method <code>each</code>, such
as an <code>Array</code> or a <code>Range</code>.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
for&nbsp;i&nbsp;in&nbsp;['fee',&nbsp;'fi',&nbsp;'fo',&nbsp;'fum']
&nbsp;&nbsp;print&nbsp;i,&nbsp;"&nbsp;"
end
for&nbsp;i&nbsp;in&nbsp;1..3
&nbsp;&nbsp;print&nbsp;i,&nbsp;"&nbsp;"
end
for&nbsp;i&nbsp;in&nbsp;File.open("ordinal").find_all&nbsp;{&nbsp;|l|&nbsp;l&nbsp;=~&nbsp;/d$/}
&nbsp;&nbsp;print&nbsp;i.chomp,&nbsp;"&nbsp;"
end
</pre></td></tr></table>

<em>produces:</em>
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
fee&nbsp;fi&nbsp;fo&nbsp;fum&nbsp;1&nbsp;2&nbsp;3&nbsp;second&nbsp;third
</pre></td></tr></table>

<P></P>
As long as your class defines a sensible <code>each</code> method, you can use 
a <code>for</code> loop to traverse it.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
class&nbsp;Periods
&nbsp;&nbsp;def&nbsp;each
&nbsp;&nbsp;&nbsp;&nbsp;yield&nbsp;"Classical"
&nbsp;&nbsp;&nbsp;&nbsp;yield&nbsp;"Jazz"
&nbsp;&nbsp;&nbsp;&nbsp;yield&nbsp;"Rock"
&nbsp;&nbsp;end
end
<P></P>
periods&nbsp;=&nbsp;Periods.new
for&nbsp;genre&nbsp;in&nbsp;periods
&nbsp;&nbsp;print&nbsp;genre,&nbsp;"&nbsp;"
end
</pre></td></tr></table>

<em>produces:</em>
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
Classical&nbsp;Jazz&nbsp;Rock
</pre></td></tr></table>

<h3>Break, Redo, and Next</h3>
<P></P>
The loop control constructs <code>break</code>, <code>redo</code>, and <code>next</code>
let you alter the normal flow through a loop or iterator.
<P></P>
<code>break</code> terminates the immediately enclosing loop; control resumes
at the statement following the block.  <code>redo</code> repeats the loop from
the start, but without reevaluating the condition or fetching the
next element (in an iterator).  <code>next</code> skips to the end of the
loop, effectively starting the next iteration.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
while&nbsp;gets
&nbsp;&nbsp;next&nbsp;if&nbsp;/^\s*#/&nbsp;&nbsp;&nbsp;#&nbsp;skip&nbsp;comments
&nbsp;&nbsp;break&nbsp;if&nbsp;/^END/&nbsp;&nbsp;&nbsp;#&nbsp;stop&nbsp;at&nbsp;end
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;#&nbsp;substitute&nbsp;stuff&nbsp;in&nbsp;backticks&nbsp;and&nbsp;try&nbsp;again
&nbsp;&nbsp;redo&nbsp;if&nbsp;gsub!(/`(.*?)`/)&nbsp;{&nbsp;eval($1)&nbsp;}
&nbsp;&nbsp;#&nbsp;process&nbsp;line&nbsp;...
end
</pre></td></tr></table>

<P></P>
These keywords can also be used with any of the iterator-based
looping mechanisms:
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
i=0
loop&nbsp;do
&nbsp;&nbsp;i&nbsp;+=&nbsp;1
&nbsp;&nbsp;next&nbsp;if&nbsp;i&nbsp;&lt;&nbsp;3
&nbsp;&nbsp;print&nbsp;i
&nbsp;&nbsp;break&nbsp;if&nbsp;i&nbsp;>&nbsp;4
end
</pre></td></tr></table>

<em>produces:</em>
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
345
</pre></td></tr></table>

<h3>Retry</h3>
<P></P>
The <code>redo</code> statement causes a loop to repeat the current
iteration. Sometimes, though, you need to wind the loop right back to
the very beginning. The <code>retry</code> statement is just the
ticket. <code>retry</code> restarts any kind of iterator loop.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
for&nbsp;i&nbsp;in&nbsp;1..100
&nbsp;&nbsp;print&nbsp;"Now&nbsp;at&nbsp;#{i}.&nbsp;Restart?&nbsp;"
&nbsp;&nbsp;retry&nbsp;if&nbsp;gets&nbsp;=~&nbsp;/^y/i
end
</pre></td></tr></table>

<P></P>
Running this interactively, you might see
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
Now&nbsp;at&nbsp;1.&nbsp;Restart?&nbsp;n
Now&nbsp;at&nbsp;2.&nbsp;Restart?&nbsp;y
Now&nbsp;at&nbsp;1.&nbsp;Restart?&nbsp;n
&nbsp;.&nbsp;.&nbsp;.
</pre></td></tr></table>

<P></P>
<code>retry</code> will reevaluate any arguments to the iterator before
restarting it. The online Ruby documentation has the following example
of a do-it-yourself <em>until</em> loop.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
def&nbsp;doUntil(cond)
&nbsp;&nbsp;yield
&nbsp;&nbsp;retry&nbsp;unless&nbsp;cond
end
<P></P>
i&nbsp;=&nbsp;0
doUntil(i&nbsp;>&nbsp;3)&nbsp;{
&nbsp;&nbsp;print&nbsp;i,&nbsp;"&nbsp;"
&nbsp;&nbsp;i&nbsp;+=&nbsp;1
}
</pre></td></tr></table>

<em>produces:</em>
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
0&nbsp;1&nbsp;2&nbsp;3&nbsp;4
</pre></td></tr></table>

<h2>Variable Scope and Loops</h2>
<P></P>
The <code>while</code>, <code>until</code>, and <code>for</code> loops are built into the
language and do not introduce new scope; previously existing locals
can be used in the loop, and any new locals created will be available
afterward.
<P></P>
The blocks used by iterators (such as <code>loop</code> and <code>each</code>) are
a little different.  Normally, the local variables created in these
blocks are not accessible outside the block.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
[&nbsp;1,&nbsp;2,&nbsp;3&nbsp;].each&nbsp;do&nbsp;|x|
&nbsp;&nbsp;y&nbsp;=&nbsp;x&nbsp;+&nbsp;1
end
[&nbsp;x,&nbsp;y&nbsp;]
</pre></td></tr></table>

<em>produces:</em>
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
prog.rb:4: undefined local variable or method `x'<br>for #&lt;Object:0x4019ac90> (NameError)
</pre></td></tr></table>

<P></P>
However, if at the time the block executes a local variable
already exists with the same name as that of a
variable in the block, the existing local variable will be used in the 
block. Its value will therefore be available after the block finishes.
As the following example shows, this applies both to normal variables in the
block and to the block's parameters.
<P></P>

<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="500">
<tr>
<td colspan="3" valign="top"><code>x&nbsp;=&nbsp;nil</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>y&nbsp;=&nbsp;nil</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>[&nbsp;1,&nbsp;2,&nbsp;3&nbsp;].each&nbsp;do&nbsp;|x|</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>&nbsp;&nbsp;y&nbsp;=&nbsp;x&nbsp;+&nbsp;1</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>end</code></td>
</tr>
<tr>
  <td valign="top"><code>[&nbsp;x,&nbsp;y&nbsp;]</code></td>
  <td valign="top"></td>
  <td valign="top"><code>[3,&nbsp;4]</code></td>
</tr>
</table>
<P></P>

<P></P>

<p></p><hr><table bgcolor="#a03030" cellpadding="10" border="0" cellspacing="0"><tr><td width="33%" align="left"><a class="subheader" href="tut_methods.html">Previous &lt;</a></td><td width="33%" align="center" valign="middle"><a class="subheader" href="index.html">Contents ^</a><br></td><td width="33%" align="right"><a class="subheader" href="tut_exceptions.html">Next ></a><br></td></tr></table><p></p><font size="-1">Extracted from the book "Programming Ruby -
     The Pragmatic Programmer's Guide"</font><br><font size="-3">
      Copyright
      &#169;
      2000 Addison Wesley Longman, Inc. Released under the terms of the
      <a href="http://www.opencontent.org/openpub/">Open Publication License</a> V1.0.
        <br>
      This reference is available for
        <a href="http://www.pragmaticprogrammer.com/ruby/downloads/book.html">download</a>.
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