ch02_06.htm

by Randal L. Schwartz and Tom Phoenix ISBN 0-596-00132-0 Third Edition, published July 2001. (See

<td>
&nbsp;
</td>
<td>
<p><tt class="literal">==</tt> <tt class="literal">!=</tt>
<tt class="literal">&lt;=&gt;</tt> <tt class="literal">eq</tt>
<tt class="literal">ne</tt> <tt class="literal">cmp</tt> (the "equal"
ones)
</p>
</td>
</tr>
<tr>
<td>
<p>left</p>
</td>
<td>
<p><tt class="literal">&amp;</tt></p>
</td>
</tr>
<tr>
<td>
<p>left</p>
</td>
<td>
<p><tt class="literal">|</tt> <tt class="literal">^</tt></p>
</td>
</tr>
<tr>
<td>
<p>left</p>
</td>
<td>
<p><tt class="literal">&amp;&amp;</tt></p>
</td>
</tr>
<tr>
<td>
<p>left</p>
</td>
<td>
<p><tt class="literal">||</tt></p>
</td>
</tr>
<tr>
<td>
&nbsp;
</td>
<td>
<p><tt class="literal">..</tt> <tt class="literal">...</tt></p>
</td>
</tr>
<tr>
<td>
<p>right</p>
</td>
<td>
<p><tt class="literal">?:</tt> (ternary)</p>
</td>
</tr>
<tr>
<td>
<p>right</p>
</td>
<td>
<p><tt class="literal">=</tt> <tt class="literal">+=</tt> <tt class="literal">-=</tt>
<tt class="literal">.=</tt> (and similar assignment operators)
</p>
</td>
</tr>
<tr>
<td>
<p>left</p>
</td>
<td>
<p><tt class="literal">,</tt> <tt class="literal">=&gt;</tt></p>
</td>
</tr>
<tr>
<td>
&nbsp;
</td>
<td>
<p>list operators (rightward)</p>
</td>
</tr>
<tr>
<td>
<p>right</p>
</td>
<td>
<p><tt class="literal">not</tt></p>
</td>
</tr>
<tr>
<td>
<p>left</p>
</td>
<td>
<p><tt class="literal">and</tt></p>
</td>
</tr>
<tr>
<td>
<p>left</p>
</td>
<td>
<p><tt class="literal">or</tt> <tt class="literal">xor</tt></p>
</td>
</tr>

</table><p>

<p>In the chart, any given operator has higher precedence than all of
the operators listed below it, and lower precedence than all of the
operators listed above it. Operators at the same precedence level
resolve according to rules of <em class="emphasis">associativity</em>
instead.
</p>

<p>Just like precedence, associativity resolves the order of operations
when two operators of the same precedence compete for three operands:
</p>

<blockquote><pre class="code">4 ** 3 ** 2 # 4 ** (3 ** 2), or 4 ** 9 (right associative)
72 / 12 / 3 # (72 / 12) / 3, or 6/3, or 2 (left associative)
36 / 6 * 3  # (36/6)*3, or 18</pre></blockquote>

<p>In the first case, the <tt class="literal">**</tt> operator has right
associativity, so the parentheses are implied on the right.
Comparatively, the <tt class="literal">*</tt> and <tt class="literal">/</tt>
operators have left associativity, yielding a set of implied
parentheses on the left.
</p>

<p>So should you just memorize the precedence chart? No! Nobody actually
does that. Instead, just use parentheses when you don't
remember the order of operations, or when you're too busy to
look in the chart. After all, if you can't remember it without
the parentheses, your <a name="INDEX-218" />maintenance programmer is going to have
the same trouble. So be nice to your maintenance
programmer.<a name="INDEX-219" /> <a name="INDEX-220" /> <a name="INDEX-221" /> <a name="INDEX-222" /> 
</p>

</div>
<a name="lperl3-CHP-2-SECT-6.3" /><div class="sect2">
<h3 class="sect2">2.6.3. Comparison Operators</h3>

<p>For comparing <a name="INDEX-223" />
<a name="INDEX-224" />numbers, Perl has the logical
<a name="INDEX-225" />
<a name="INDEX-226" />comparison
operators that remind you of algebra: <tt class="literal">&lt; &lt;= == &gt;=
&gt; !=</tt>. Each of these returns a <em class="emphasis">true</em>
or <em class="emphasis">false</em> value. We'll find out more about
those return values in the next section. Some of these may be
different than you'd use in other languages. For example,
<tt class="literal">==</tt> is used for equality, not a single
<tt class="literal">=</tt> sign, because that's used for another
purpose in Perl. And <tt class="literal">!=</tt> is used for inequality
testing, because <tt class="literal">&lt;&gt;</tt> is used for another
purpose in Perl. And you'll need <tt class="literal">&gt;=</tt> and
not <tt class="literal">=&gt;</tt> for "greater than or equal
to", because the latter is used for another purpose in Perl. In
fact, nearly every sequence of punctuation is used for something in
Perl.
</p>

<p>For comparing strings, Perl has an equivalent set of
<a name="INDEX-227" />
<a name="INDEX-228" />string comparison operators which look like
funny little words: <tt class="literal">lt</tt> <tt class="literal">le</tt>
<tt class="literal">eq</tt> <tt class="literal">ge</tt> <tt class="literal">gt</tt>
<tt class="literal">ne</tt>. These compare two strings character by
character to see whether they're the same, or whether one comes
first in standard string sorting order. (In ASCII, the capital
letters come before the lowercase letters, so beware.)
</p>

<p>The comparison operators (for both numbers and strings) are given in
<a href="ch02_06.htm#lperl3-CHP-2-TABLE-3">Table 2-3</a>.
</p>

<a name="lperl3-CHP-2-TABLE-3" /><h4 class="objtitle">Table 2-3. Numeric and string comparison operators </h4><table border="1">




<tr>
<th>
<p>Comparison</p>
</th>
<th>
<p>Numeric</p>
</th>
<th>
<p>String</p>
</th>
</tr>


<tr>
<td>
<p>Equal</p>
</td>
<td>
<p><tt class="literal">==</tt></p>
</td>
<td>
<p><tt class="literal">eq</tt></p>
</td>
</tr>
<tr>
<td>
<p>Not equal</p>
</td>
<td>
<p><tt class="literal">!=</tt></p>
</td>
<td>
<p><tt class="literal">ne</tt></p>
</td>
</tr>
<tr>
<td>
<p>Less than</p>
</td>
<td>
<p><tt class="literal">&lt;</tt></p>
</td>
<td>
<p><tt class="literal">lt</tt></p>
</td>
</tr>
<tr>
<td>
<p>Greater than</p>
</td>
<td>
<p><tt class="literal">&gt;</tt></p>
</td>
<td>
<p><tt class="literal">gt</tt></p>
</td>
</tr>
<tr>
<td>
<p>Less than or equal to</p>
</td>
<td>
<p><tt class="literal">&lt;=</tt></p>
</td>
<td>
<p><tt class="literal">le</tt></p>
</td>
</tr>
<tr>
<td>
<p>Greater than or equal to</p>
</td>
<td>
<p><tt class="literal">&gt;=</tt></p>
</td>
<td>
<p><tt class="literal">ge</tt></p>
</td>
</tr>

</table><p>

<p>Here are some example expressions using these comparison operators:</p>

<blockquote><pre class="code">35 != 30 + 5         # false
35 == 35.0           # true
'35' eq '35.0'       # false (comparing as strings)
'fred' lt 'barney'   # false
'fred' lt 'free'     # true
'fred' eq "fred"     # true
'fred' eq 'Fred'     # false
' ' gt ''            # true</pre></blockquote>

</div>


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