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<a name="Strings-and-quotes-1"></a>
<h4 class="subsubsection">7.2.3.1 Strings and quotes</h4>
<a name="index-String_002c-definition-of"></a>
<a name="index-String_002c-quoted"></a>
<a name="index-String_002c-unquoted"></a>
<p>Several of the built-in functions expect or return a string argument.
Examples include <code>$STRING</code> (see <a href="_0024STRING.html#g_t_0024STRING">$STRING</a>), <code>$UNQUOTE</code>
(see <a href="_0024UNQUOTE.html#g_t_0024UNQUOTE">$UNQUOTE</a>), and <code>$UNSTRING</code> (see <a href="_0024UNSTRING.html#g_t_0024UNSTRING">$UNSTRING</a>).
In understanding the operation of those functions, it is important to
understand just what a string means in the <small>FWEB</small> context. As usual,
it is a vector of characters. However, <em>those need not be
delimited by quotes</em>, although they may be. Internally, a string is
represented by the construction <i>sqc...cqs</i>, where <i>s</i> is a special
string delimiter never seen by the user, <i>q</i> is an optional quote
character (either single or double quote depending on the language), and
<i>c</i> is an ordinary character. Whether or not the quotes are present,
the string delimiters inhibit macro expansion.
</p>
<p>The difference between <code>$UNQUOTE</code> and <code>$UNSTRING</code> can now be
stated as follows. Given a quoted string such as <code>"abc"</code> (in C),
</p>
<ul>
<li> ‘<samp>$UNQUOTE</samp>’ removes the quote characters <i>q</i>, leaving <i>sabcs</i> (still a
string).
</li><li> ‘<samp>$UNSTRING</samp>’ removes both the quote characters <i>q</i> and the string
delimiters <i>s</i>, leaving <i>abc</i> (a collection of three characters). This
collection is <em>not</em> tokenized; it does <em>not</em> represent the
single identifier name <code>abc</code> (and therefore is not very useful).
<code>$UNSTRING</code> is primarily used internally.
</li></ul>
<p>The built-ins <code>$P</code> (see <a href="_0024P.html#g_t_0024P">$P</a>) and <code>$PP</code> (see <a href="_0024PP.html#g_t_0024PP">$PP</a>), which
both generate the preprocessor character ‘<samp>#</samp>’, provide a good
illustration of the differences between <code>$UNQUOTE</code> and
<code>$UNSTRING</code>. Consider <small>FORTRAN</small> as an example. Essentially,
<code>$P</code> is defined as ‘<samp>$UNQUOTE('#')</samp>’, which is internally
<i>s<tt>#</tt>s</i>. When this single-character string is sent to the output, it is
treated like any other expression and therefore would appear in column 7
or greater even if the construction appeared at the very beginning of
the line. On the other hand, <code>$PP</code> is (essentially) defined as
‘<samp>$UNSTRING('#')</samp>’, which is internally the single character <tt>#</tt>.
Because this character is not a string, the <small>FORTRAN</small> output driver
treats it as a special control character, defined in this case to force
the character into the first column.
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