ch04_05.htm

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

<h3 class="sect2">4.5.5. Autoincrement and Autodecrement</h3>

<p><a name="INDEX-489" /><a name="INDEX-490" /><a name="INDEX-491" /><a name="INDEX-492" /><a name="INDEX-493" /><a name="INDEX-494" />If placed before a
variable, the <tt class="literal">++</tt> and <tt class="literal">--</tt>
operators increment or decrement the variable before returning the
value. If placed after, they increment or decrement the variable
after returning the value.
</p>

</div>
<a name="perlnut2-CHP-4-SECT-5.6" /><div class="sect2">
<h3 class="sect2">4.5.6. Assignment Operators</h3>

<p><a name="INDEX-495" />Perl
recognizes the following operators for assigning a value to a
variable:
</p>

<blockquote><pre class="code">=    **=    +=    *=    &amp;=    &lt;&lt;=    &amp;&amp;=
-=    /=    |=    &gt;&gt;=   ||=    .=     %=
^=    x=</pre></blockquote>

<p>Each operator requires a variable on the left side and an expression
on the right side. For the simple <a name="INDEX-496" /><a name="INDEX-497" /><a name="INDEX-498" />
<a name="INDEX-499" /><a name="INDEX-500" /><a name="INDEX-501" /><a name="INDEX-502" /><a name="INDEX-503" /><a name="INDEX-504" /><a name="INDEX-505" /><a name="INDEX-506" /><a name="INDEX-507" /><a name="INDEX-508" /><a name="INDEX-509" /><a name="INDEX-510" /><a name="INDEX-511" /> <a name="INDEX-512" /><a name="INDEX-513" /><a name="INDEX-514" /><a name="INDEX-515" /><a name="INDEX-516" /><a name="INDEX-517" /><a name="INDEX-518" /><a name="INDEX-519" /><a name="INDEX-520" /><a name="INDEX-521" /><a name="INDEX-522" /><a name="INDEX-523" /><a name="INDEX-524" /><a name="INDEX-525" />assignment
operator, <tt class="literal">=</tt>, the value of the expression is stored
in the designated variable. For the other operators, Perl evaluates
the expression:
</p>

<blockquote><pre class="code">$var <em class="replaceable"><tt>OP</tt></em>= $value</pre></blockquote>

<p>as if it was written: </p>

<blockquote><pre class="code">$var = $var <em class="replaceable"><tt>OP</tt></em> $value</pre></blockquote>

<p>except that <tt class="literal">$var</tt> is evaluated only once. For
example:
</p>

<blockquote><pre class="code">$a += 2;    # Same as $a = $a + 2</pre></blockquote>

</div>
<a name="perlnut2-CHP-4-SECT-5.7" /><div class="sect2">
<h3 class="sect2">4.5.7. Pattern Match Operators</h3>

<p><a name="INDEX-526" /><a name="INDEX-527" /> <a name="INDEX-528" />Binary <tt class="literal">=~</tt>
binds a scalar expression to a pattern match, substitution, or
translation. These operations search or modify the string
<tt class="literal">$_</tt> by default.
</p>

<p><a name="INDEX-529" /> Binary
<tt class="literal">!~</tt> is just like <tt class="literal">=~</tt> except the
return value is negated in the logical sense. The following
expressions are functionally equivalent:
</p>

<blockquote><pre class="code">$string !~ /<em class="replaceable"><tt>pattern</tt></em>/
not $string =~ /<em class="replaceable"><tt>pattern</tt></em>/</pre></blockquote>

<p>see <a href="ch04_06.htm#perlnut2-CHP-4-SECT-6">Section 4.6, "Regular Expressions"</a> later
in this chapter.
</p>

</div>
<a name="perlnut2-CHP-4-SECT-5.8" /><div class="sect2">
<h3 class="sect2">4.5.8. File Test Operators</h3>

<p><a name="INDEX-530" /><a name="INDEX-531" /><a name="INDEX-532" /><a name="INDEX-533" /><a name="INDEX-534" /><a name="INDEX-535" /><a name="INDEX-536" /><a name="INDEX-537" /><a name="INDEX-538" /><a name="INDEX-539" /><a name="INDEX-540" /><a name="INDEX-541" /><a name="INDEX-542" /><a name="INDEX-543" /><a name="INDEX-544" /><a name="INDEX-545" /><a name="INDEX-546" /><a name="INDEX-547" /><a name="INDEX-548" /><a name="INDEX-549" /><a name="INDEX-550" /><a name="INDEX-551" /><a name="INDEX-552" /><a name="INDEX-553" /><a name="INDEX-554" /><a name="INDEX-555" /><a name="INDEX-556" /><a name="INDEX-557" />A file test
operator is a unary operator that tests a filename or a filehandle.
</p>

<a name="ch04-4-fm2xml" /><table border="1" cellpadding="3">



<tr>
<th>
<p>Operator</p>
</th>
<th>
<p>Meaning</p>
</th>
</tr>


<tr>
<td>
<p><tt class="literal">-r</tt></p>
</td>
<td>
<p>File is readable by effective uid/gid</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-w</tt></p>
</td>
<td>
<p>File is writable by effective uid/gid</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-x</tt></p>
</td>
<td>
<p>File is executable by effective uid/gid</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-o</tt></p>
</td>
<td>
<p>File is owned by effective uid</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-R</tt></p>
</td>
<td>
<p>File is readable by real uid/gid</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-W</tt></p>
</td>
<td>
<p>File is writable by real uid/gid</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-X</tt></p>
</td>
<td>
<p>File is executable by real uid/gid</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-O</tt></p>
</td>
<td>
<p>File is owned by real uid</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-e</tt></p>
</td>
<td>
<p>File exists</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-z</tt></p>
</td>
<td>
<p>File has zero size</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-s</tt></p>
</td>
<td>
<p>File has nonzero size (returns size)</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-f</tt></p>
</td>
<td>
<p>File is a plain file</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-d</tt></p>
</td>
<td>
<p>File is a directory</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-l</tt></p>
</td>
<td>
<p>File is a symbolic link</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-p</tt></p>
</td>
<td>
<p>File is a named pipe (FIFO)</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-S</tt></p>
</td>
<td>
<p>File is a socket</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-b</tt></p>
</td>
<td>
<p>File is a block special file</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-c</tt></p>
</td>
<td>
<p>File is a character special file</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-t</tt></p>
</td>
<td>
<p>Filehandle is opened to a tty</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-u</tt></p>
</td>
<td>
<p>File has setuid bit set</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-g</tt></p>
</td>
<td>
<p>File has setgid bit set</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-k</tt></p>
</td>
<td>
<p>File has sticky bit set</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-T</tt></p>
</td>
<td>
<p>File is a text file</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-B</tt></p>
</td>
<td>
<p>File is a binary file (opposite of <tt class="literal">-T</tt>)</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-M</tt></p>
</td>
<td>
<p>Age of file (at startup) in days since modification</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-A</tt></p>
</td>
<td>
<p>Age of file (at startup) in days since last access</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">-C</tt></p>
</td>
<td>
<p>Age of file (at startup) in days since inode change</p>
</td>
</tr>

</table><p>

</div>
<a name="perlnut2-CHP-4-SECT-5.9" /><div class="sect2">
<h3 class="sect2">4.5.9. Logical Operators</h3>

<p><a name="INDEX-558" /><a name="INDEX-559" /><a name="INDEX-560" /><a name="INDEX-561" /><a name="INDEX-562" /><a name="INDEX-563" /><a name="INDEX-564" /><a name="INDEX-565" />Perl provides the
<tt class="literal">&amp;&amp;</tt> (logical AND) and <tt class="literal">||</tt>
(logical OR) operators. They evaluate from left to right testing the
truth of the statement.
</p>

<a name="ch04-5-fm2xml" /><table border="1" cellpadding="3">




<tr>
<th>
<p>Example</p>
</th>
<th>
<p>Name</p>
</th>
<th>
<p>Result</p>
</th>
</tr>


<tr>
<td>
<p><tt class="literal">$a &amp;&amp; $b</tt></p>
</td>
<td>
<p>And</p>
</td>
<td>
<p><tt class="literal">$a</tt> if <tt class="literal">$a</tt> is false,
<tt class="literal">$b</tt> otherwise
</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">$a || $b</tt></p>
</td>
<td>
<p>Or</p>
</td>
<td>
<p><tt class="literal">$a</tt> if <tt class="literal">$a</tt> is true,
<tt class="literal">$b</tt> otherwise
</p>
</td>
</tr>

</table><p>

<p>For example, an oft-appearing idiom in Perl programs is: </p>

<blockquote><pre class="code">open(FILE, "somefile") || die "Cannot open somefile: $!\n";</pre></blockquote>

<p>In this case, Perl first evaluates the <tt class="literal">open</tt>
function. If the value is true (because <tt class="literal">somefile</tt>
was successfully opened), the execution of the <tt class="literal">die</tt>
function is unnecessary and is skipped.
</p>

<p>Perl also provides lower-precedence <tt class="literal">and</tt> and
<tt class="literal">or</tt> operators that are more readable.
</p>

</div>
<a name="perlnut2-CHP-4-SECT-5.10" /><div class="sect2">
<h3 class="sect2">4.5.10. Bitwise Operators</h3>

<p><a name="INDEX-566" /><a name="INDEX-567" /><a name="INDEX-568" /><a name="INDEX-569" /><a name="INDEX-570" /><a name="INDEX-571" /><a name="INDEX-572" /><a name="INDEX-573" /><a name="INDEX-574" /><a name="INDEX-575" />Perl has bitwise AND, OR, and XOR
(exclusive OR) operators: <tt class="literal">&amp;</tt>,
<tt class="literal">|</tt>, and <tt class="literal">^</tt>. These operators work
differently on numeric values than they do on strings. If either
operand is a number, then both operands are converted to integers,
and the bitwise operation is performed between the two integers. If
both operands are strings, these operators do bitwise operations
between corresponding bits from the two strings.
</p>

</div>
<a name="perlnut2-CHP-4-SECT-5.11" /><div class="sect2">
<h3 class="sect2">4.5.11. Miscellaneous Operators</h3>

<p>The following operators don't fit into any of the
previous categories.
</p>

<a name="perlnut2-CHP-4-SECT-5.11.1" /><div class="sect3">
<h3 class="sect3">4.5.11.1. Range operator</h3>

<p><a name="INDEX-576" /><a name="INDEX-577" /><a name="INDEX-578" />The <tt class="literal">..</tt> range
operator is really two different operators depending on the context.
In a list context, it returns a list of values counting (by ones)
from the left value to the right value.
</p>

<p>In a scalar context, <tt class="literal">..</tt> returns a Boolean value.
It is false as long as its left operand is false. Once the left
operand is true, the range operator stays true until the right
operand is true, after which the range operator becomes false again.
The right operand is not evaluated while the operator is in the false
state, and the left operand is not evaluated while the operator is in
the true state.
</p>

<p><a name="INDEX-579" /><a name="INDEX-580" /><a name="INDEX-581" />The alternate version of this
operator, <tt class="literal">...</tt>, does not test the right operand
immediately when the operator becomes true; it waits until the next
evaluation.
</p>

</div>

<a name="perlnut2-CHP-4-SECT-5.11.2" /><div class="sect3">
<h3 class="sect3">4.5.11.2. Conditional operator</h3>

<p><a name="INDEX-582" /><a name="INDEX-583" /><a name="INDEX-584" />Ternary <tt class="literal">?</tt>: is the
conditional operator. It works like an if-then-else statement but can
safely be embedded within other operations and functions:
</p>

<blockquote><pre class="code"><em class="replaceable"><tt>test_expr</em> ? <em class="replaceable">if_true_expr</em> : <em class="replaceable">if_false_expr</tt></em></pre></blockquote>

<p>If the <em class="replaceable"><tt>test_expr</tt></em> is true, only the
<em class="replaceable"><tt>if_true_expr</tt></em> is evaluated. Otherwise, only
the <em class="replaceable"><tt>if_false_expr</tt></em> is evaluated. Either
way, the value of the evaluated expression becomes the value of the
entire expression.
</p>

</div>

<a name="perlnut2-CHP-4-SECT-5.11.3" /><div class="sect3">
<h3 class="sect3">4.5.11.3. Comma operator</h3>

<p><a name="INDEX-585" /><a name="INDEX-586" />In a list
context, <tt class="literal">,</tt> is the list argument separator and
inserts both its arguments into the list. In scalar context,
<tt class="literal">,</tt> evaluates its left argument, throws that value
away, then evaluates its right argument and returns that value.
</p>

<p><a name="INDEX-587" /><a name="INDEX-588" />The
<tt class="literal">=&gt;</tt> operator is mostly just a synonym for the
comma operator. It's useful for documenting
arguments that come in pairs. It also forces any identifier to the
left of it to be interpreted as a string.
</p>

</div>

<a name="perlnut2-CHP-4-SECT-5.11.4" /><div class="sect3">
<h3 class="sect3">4.5.11.4. String operator</h3>

<p><a name="INDEX-589" /><a name="INDEX-590" /><a name="INDEX-591" /><a name="INDEX-592" />The concatenation operator
<tt class="literal">.</tt> is used to add strings together:
</p>

<blockquote><pre class="code">print 'abc' . 'def';        # Prints abcdef
print $a . $b;              # Concatenates the string values of $a and $b</pre></blockquote>

<p><a name="INDEX-593" /><a name="INDEX-594" />Binary <tt class="literal">x</tt> is the
string repetition operator. In scalar context, it returns a
concatenated string consisting of the left operand repeated the
number of times specified by the right operand:
</p>

<blockquote><pre class="code">print '-' x 80;                           # Prints row of dashes
print "\t" x ($tab/8), ' ' x ($tab%8);    # Tabs over</pre></blockquote>

<p>In list context, if the left operand is a list in parentheses, the
<tt class="literal">x</tt> works as a list replicator rather than a string
replicator. This is useful for initializing all the elements of an
array of indeterminate length to the same value:
</p>

<blockquote><pre class="code">@ones = (1) x 80;           # A list of 80 1s
@ones = (5) x @ones;        # Set all elements to 5</pre></blockquote>

</div>
</div>


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