ch02_03.htm

来自「by Randal L. Schwartz and Tom Phoenix I」· HTM 代码 · 共 415 行 · 第 1/2 页

<blockquote><pre class="code">\cC</pre></blockquote>
</td>
<td>
<p>A "control" character (here, Ctrl-C)</p>
</td>
</tr>
<tr>
<td>
<blockquote><pre class="code">\\</pre></blockquote>
</td>
<td>
<p>Backslash</p>
</td>
</tr>
<tr>
<td>
<blockquote><pre class="code">\"</pre></blockquote>
</td>
<td>
<p>Double quote</p>
</td>
</tr>
<tr>
<td>
<blockquote><pre class="code">\l</pre></blockquote>
</td>
<td>
<p>Lowercase next letter</p>
</td>
</tr>
<tr>
<td>
<blockquote><pre class="code">\L</pre></blockquote>
</td>
<td>
<p>Lowercase all following letters until <tt class="literal">\E</tt></p>
</td>
</tr>
<tr>
<td>
<blockquote><pre class="code">\u</pre></blockquote>
</td>
<td>
<p>Uppercase next letter</p>
</td>
</tr>
<tr>
<td>
<blockquote><pre class="code">\U</pre></blockquote>
</td>
<td>
<p>Uppercase all following letters until <tt class="literal">\E</tt></p>
</td>
</tr>
<tr>
<td>
<blockquote><pre class="code">\Q</pre></blockquote>
</td>
<td>
<p>Quote non-word characters by adding a backslash until
<tt class="literal">\E</tt>
</p>
</td>
</tr>
<tr>
<td>
<blockquote><pre class="code">\E</pre></blockquote>
</td>
<td>
<p>Terminate <tt class="literal">\L</tt>, <tt class="literal">\U</tt>, or
<tt class="literal">\Q</tt>
</p>
</td>
</tr>

</table><p>

<p>Another feature of double-quoted strings is that they are
<em class="emphasis">variable interpolated,</em><a name="INDEX-160" /><a name="INDEX-161" /><a name="INDEX-162" /> meaning that some variable names
within the string are replaced with their current values when the
strings are used. We haven't formally been introduced to what a
variable looks like yet, so we'll get back to this later in
this chapter.
</p>

</div>
<a name="lperl3-CHP-2-SECT-3.3" /><div class="sect2">
<h3 class="sect2">2.3.3. String Operators</h3>

<p><a name="INDEX-163" />String values can be concatenated with the
<tt class="literal">.</tt><a name="INDEX-164" /> operator. (Yes, that's a single
period.) This does not alter either string, any more than
<tt class="literal">2+3</tt> alters either <tt class="literal">2</tt> or
<tt class="literal">3</tt>. The resulting (longer) string is then available
for further computation or to be stored into a variable. For example:
</p>

<blockquote><pre class="code">"hello" . "world"       # same as "helloworld"
"hello" . ' ' . "world" # same as 'hello world'
'hello world' . "\n"    # same as "hello world\n"</pre></blockquote>

<p>Note that the concatenation must be explicitly requested with the
<tt class="literal">.</tt> operator, unlike in some other languages where
you merely have to stick the two values next to each other.
</p>

<p>A special string operator is the <em class="emphasis">string
repetition</em><a name="INDEX-165" /> <a name="INDEX-166" /> operator, consisting of
the single lowercase letter <tt class="literal">x</tt>. This operator takes
its left operand (a string) and makes as many concatenated copies of
that string as indicated by its right operand (a number). For
example:
</p>

<blockquote><pre class="code">"fred" x 3       # is "fredfredfred"
"barney" x (4+1) # is "barney" x 5, or "barneybarneybarneybarneybarney"
5 x 4            # is really "5" x 4, which is "5555"</pre></blockquote>

<p>That last example is worth spelling out slowly. The string repetition
operator wants a string for a left operand, so the number
<tt class="literal">5</tt> is converted to the string
<tt class="literal">"5"</tt> (using rules described in detail later),
giving a one-character string. This new string is then copied four
times, yielding the four-character string <tt class="literal">5555</tt>.
Note that if we had reversed the order of the operands, as <tt class="literal">4
x 5</tt>, we would have made five copies of the string
<tt class="literal">4</tt>, yielding <tt class="literal">44444</tt>. This shows
that string repetition is not commutative.
</p>

<p>The copy count (the right operand) is first truncated to an integer
value (4.8 becomes 4) before being used. A copy count of less than
one results in an empty (zero-length) string.
</p>

</div>
<a name="lperl3-CHP-2-SECT-3.4" /><div class="sect2">
<h3 class="sect2">2.3.4. Automatic Conversion Between Numbers and Strings</h3>

<p>For the most part, Perl automatically converts between
<a name="INDEX-167" />numbers to
<a name="INDEX-168" />strings as needed.
How does it know whether a number or a string is needed? It all
depends upon the operator being used on the scalar value. If an
operator expects a number (like <tt class="literal">+</tt> does), Perl will
see the value as a number. If an operator expects a string (like
<tt class="literal">.</tt> does), Perl will see the value as a string. So
you don't need to worry about the difference between numbers
and strings; just use the proper operators, and Perl will make it all
work.
</p>

<p>When a string value is used where an operator needs a number (say,
for multiplication), Perl automatically converts the string to its
equivalent numeric value, as if it had been entered as a decimal
floating-point value.<a href="#FOOTNOTE-50">[50]</a> So
<tt class="literal">"12" * "3"</tt> gives the value <tt class="literal">36</tt>.
Trailing nonnumber stuff and leading whitespace are discarded, so
<tt class="literal">"12fred34" * " 3"</tt> will also give
<tt class="literal">36</tt> without any complaints.<a href="#FOOTNOTE-51">[51]</a> At the extreme end of this, something that isn't a
number at all converts to zero. This would happen if you used the
string <tt class="literal">"fred"</tt> as a number.
</p><blockquote class="footnote"> <a name="FOOTNOTE-50" /><p>[50]The trick of using a leading
zero to mean a nondecimal value works for literals, but never for
automatic conversion. Use <tt class="literal">hex( )</tt>or <tt class="literal">oct(
)</tt>to convert those kinds of strings.</p> </blockquote><blockquote class="footnote"> <a name="FOOTNOTE-51" /><p>[51]Unless
you request warnings, which we'll discuss in a moment.</p>
</blockquote>

<p>Likewise, if a numeric value is given when a string value is needed
(say, for string concatenation), the numeric value is expanded into
whatever string would have been printed for that number. For example,
if you want to concatenate the string <tt class="literal">Z</tt> followed
by the result of 5 multiplied by 7,<a href="#FOOTNOTE-52">[52]</a> you can
say this simply as:
</p><blockquote class="footnote"> <a name="FOOTNOTE-52" /><p>[52]We'll see
about precedence and parentheses shortly.</p> </blockquote>

<blockquote><pre class="code">"Z" . 5 * 7 # same as "Z" . 35, or "Z35"</pre></blockquote>

<p>In other words, you don't really have to worry about whether
you have a number or a string (most of the time). Perl performs all
the conversions for you.<a href="#FOOTNOTE-53">[53]</a> And if
you're worried about efficiency, don't be. Perl generally
remembers the result of a conversion so that it's done only
once.<a name="INDEX-169" />
</p><blockquote class="footnote"> <a name="FOOTNOTE-53" /><p>[53]It's usually not an
issue, but these conversions can cause small round-off errors. That
is, if you start with a number, convert it to a string, then convert
that string back to a number, the result may not be the same number
as you started with. It's not just Perl that does this;
it's a consequence of the conversion process, so it happens to
any powerful programming language.</p> </blockquote>

</div>


<hr width="684" align="left" />
<div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="ch02_02.htm"><img alt="Previous" border="0" src="../gifs/txtpreva.gif" /></a></td><td align="center" valign="top" width="228"><a href="index.htm"><img alt="Home" border="0" src="../gifs/txthome.gif" /></a></td><td align="right" valign="top" width="228"><a href="ch02_04.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr><tr><td align="left" valign="top" width="228">2.2. Numbers</td><td align="center" valign="top" width="228"><a href="index/index.htm"><img alt="Book Index" border="0" src="../gifs/index.gif" /></a></td><td align="right" valign="top" width="228">2.4. Perl's Built-in Warnings</td></tr></table></div>
<hr width="684" align="left" />

<img alt="Library Navigation Links" border="0" src="../gifs/navbar.gif" usemap="#library-map" />
<p><p><font size="-1"><a href="copyrght.htm">Copyright &copy; 2002</a> O'Reilly &amp; Associates. All rights reserved.</font></p>

<map name="library-map"><area shape="rect" coords="1,0,85,94" href="../index.htm"><area shape="rect" coords="86,1,178,103" href="../lwp/index.htm"><area shape="rect" coords="180,0,265,103" href="../lperl/index.htm"><area shape="rect" coords="267,0,353,105" href="../perlnut/index.htm"><area shape="rect" coords="354,1,446,115" href="../prog/index.htm"><area shape="rect" coords="448,0,526,132" href="../tk/index.htm"><area shape="rect" coords="528,1,615,119" href="../cookbook/index.htm"><area shape="rect" coords="617,0,690,135" href="../pxml/index.htm"></map>

</body></html>